Search

Purpose Relating dose-volume histogram (DVH) information to patient outcomes is critical for outcomes research in radiation oncology, but this is statistically challenging. We performed this focused review of DVH toxicity studies to characterize current statistical approaches and determine the need for updated reporting recommendations. Methods and materials We performed a focused MEDLINE search to identify studies published in 5 radiation oncology specialty journals that associated dosimetry with toxicity outcomes in humans receiving radiotherapy between 2015 and 2021. Elements abstracted from each manuscript included the study outcome, organs-at-risk (OARs) considered, DVH parameters analyzed, summary of the analytic approach, use of multivariable statistics, goodness-of-fit reporting, completeness of model reporting, assessment of multicollinearity, adjustment for multiple comparisons, and methods for dichotomizing variables. Each study was also assessed for sufficient reporting to allow for replication of results. Results The MEDLINE search returned 2,300 studies for review and 325 met the inclusion criteria for the analysis. DVH variables were dichotomized using cut points in 154 (47.4%) studies. Logistic regression (55.4% of studies) was the most common statistical method used to relate DVH to toxicity outcomes, followed by Cox regression (20.6%) and linear regression (12.0%). Multivariable statistical tests were performed in 226 (69.5%) studies; of these, the possibility of multicollinearity was addressed in 47.8% and model goodness-of-fit were reported in 32.6%. The threshold for statistical significance was adjusted to account for multiple comparisons in 41 of 196 (17.1%) studies that included multiple statistical comparisons. Twenty-eight (8.6%) studies were classified as missing details necessary to reproduce the study results. Conclusions Current practices of statistical reporting in DVH outcomes suggest that studies may be vulnerable to threats against internal and external validity. Recommendations for reporting are provided herein to guard against such threats and to promote cohesiveness among radiation oncology outcomes researchers.

PurposeOnline Adaptive Radiation Therapy(oART) follows a different treatment paradigm than conventional radiotherapy and, because of this, the resources, implementation, and workflows needed are unique. The purpose of this report is to outline our institution’s experience establishing, organizing, and implementing an oART program using the Ethos therapy system.MethodsWe include resources used; operational models utilized, program creation timelines, and our institutional experiences with implementation and operation of an oART program. Additionally, we provide a detailed summary of our first year’s clinical experience where we delivered over 1000 daily adaptive fractions. For all treatments, the different stages of online adaption, primary patient set-up, initial kV-CBCT acquisition, contouring review and edit of influencer structures, target review and edits, plan evaluation and selection, Mobius3D 2ndcheck and adaptive QA, 2ndkV-CBCT for positional verification, treatment delivery, and patient leaving the room, were analyzed.ResultsWe retrospectively analyzed data from ninety-seven patients treated from August 2021-August 2022. 1677 individual fractions were treated and analyzed, 632(38%) were non-adaptive and 1045(62%) were adaptive. 74 of the 97 patients (76%) were treated with standard fractionation and 23 (24%) received stereotactic treatments. For the adaptive treatments, the generated adaptive plan was selected in 92% of treatments. On average(±std), adaptive sessions took 34.52±11.42 minutes from start to finish. The entire adaptive process (from start of contour generation to verification CBCT), performed by the physicist (and physician on select days), was 19.84±8.21 minutes.ConclusionWe present our institution’s experience commissioning an oART program using the Ethos therapy system. It took us 12 months from project inception to treatment of our first patient and 12 months to treat 1000 adaptive fractions. Retrospective analysis of delivered fractions showed that average overall treatment time was approximately 35 minutes and average time for the adaptive component of treatment was approximately 20 minutes.

The purpose of this paper is to summarize treatment guidelines for the performance of single isocenter LINAC radiosurgery of multiple brain metastases developed and used by 3 experienced centers. This article is not meant to provide consensus guidelines. Rather, this is a practical, "how we do it" reference without substantial discussion. To serve as a treatment reference, the great majority of the information is presented in topic-specific tables.

The clinical introduction of dedicated treatment units for online adaptive radiation therapy (OART) has led to widespread adoption of daily adaptive radiotherapy. OART allows for rapid generation of treatment plans using daily patient anatomy, potentially leading to reduction of treatment margins and increased normal tissue sparing. However, the OART workflow does not allow for measurement of patient-specific quality assurance (PSQA) during treatment delivery sessions and instead relies on secondary dose calculations for verification of adapted plans. It remains unknown if independent dose verification is a sufficient surrogate for PSQA measurements.To evaluate the plan quality of previously treated adaptive plans through multiple standard PSQA measurements.This IRB-approved retrospective study included sixteen patients previously treated with OART at our institution. PSQA measurements were performed for each patient's scheduled and adaptive plans: five adaptive plans were randomly selected to perform ion chamber measurements and two adaptive plans were randomly selected for ArcCHECK measurements. The same ArcCHECK 3D dose distribution was also sent to Mobius3D to evaluate the second-check dosimetry system.All (n = 96) ion chamber measurements agreed with the planned dose within 3% with a mean of 1.4% (± 0.7%). All (n = 48) plans passed ArcCHECK measurements using a 95% gamma passing threshold and 3%/2 mm criteria with a mean of 99.1% (± 0.7%). All (n = 48) plans passed Mobius3D second-check performed with 95% gamma passing threshold and 5%/3 mm criteria with a mean of 99.0% (± 0.2%).Plan measurement for PSQA may not be necessary for every online-adaptive treatment verification. We recommend the establishment of a periodic PSQA check to better understand trends in passing rates for delivered adaptive treatments.

To validate surface imaging (SI)-reported offsets using a six degree-of-freedom couch and an anthropomorphic phantom for commissioning and routine quality assurance of an SI system used for stereotactic radiosurgery (SRS).An anthropomorphic phantom with a radiopaque ball bearing (BB) placed either anterior, midline, or posterior, was tracked with SI with a typical SRS region of interest. Couch motion in all six degrees of freedom was programmed and delivered on a linac. SI system logs were synchronized with linac trajectory logs. Ten random couch positions were selected at couch 0°, 45°, 90°, 270°, 315° with megavolt (MV) images taken to account for couch walkout. The SI residual error (ε), the difference between SI reported offset and MV or trajectory log position, was calculated. Residual errors were measured with and without one SI pod blocked.The median [range] of magnitude of translational ε was 0.13 [0.07, 0.21], 0.16 [0.11, 0.26], 0.61 [0.50, 0.68], 0.49 [0.42, 0.55], 0.55 [0.38, 0.72] mm for couch rotations of 0°, 45°, 90°, 270°, 315°, respectively, for the midline BB and no pod blocked. The range of all translational ε from all couch angles (with and without pod block) at different BB positions is [0.05, 0.96] mm. The absolute range of difference when changing BB position when no pod is blocked in median translational ε is [0.01, 0.40] mm with the maximum at BB posterior. The absolute range of difference when not changing BB positions with and without pod block in median translational ε is [0.01, 0.37] mm with the maximum at BB posterior and couch 315°.SI system and linac trajectory log analysis can be used to assess SI system performance with automated couch motion to validate SI accuracy.

Properly planned single isocenter volumetric modulated arc therapy (VMAT) radiosurgery plans exhibit high quality and efficiency. We report here the largest clinical experience to date, to our knowledge, comparing manual planning with a new automated platform designed to standardize and simplify radiosurgery planning and delivery processes.We treated 693 patients with single isocenter VMAT radiosurgical plans generated by either our conventional manual (mVMAT) or a recently implemented automated (HyperArcThe median Radiation Therapy Oncology Group (RTOG) conformity index was 1.14 and was not different between the 2 techniques. The median Paddick gradient index was 5.42 for HyperArc versus 7.09 for mVMAT (P.001). The median mean brain doses were 4.6% and 5.1% for HyperArc and mVMAT, respectively (P = .04). The PSQA for both techniques met clinical criteria, but 97% of the HyperArc plans satisfied the gamma tolerance limit recommended by the American Association of Physicists in Medicine Task Group No. 218, compared with 94% of the mVMAT plans (P = .02). The median treatment-planning times were not significantly different. The median treatment times were 10.5 and 11.4 minutes for HyperArc and mVMAT, respectively (P.001). The Kaplan-Meier estimate of local control was 90.1% at 1 year.HyperArc produces high-quality radiosurgical plans that are at least as good as mVMAT plans created by an expert manual planner with easier planning and more efficient delivery workflow. A less experienced planner can produce very high-quality radiosurgical plans even for patients with more than 10 targets. The use of a single-isocenter technique for multiple targets with no PTV margin did not compromise clinical outcomes, and 1-year local control for treated targets remained congruent with historical series.

Purpose To report on the use of surface guided imaging during frameless intracranial stereotactic radiotherapy with automated delivery via HyperArcTM (Varian Medical Systems, Palo Alto, CA). Methods All patients received intracranial radiotherapy with HyperArcTM and were monitored for intrafraction motion by the AlignRT® (VisionRT, London, UK) surface imaging (SI) system. Immobilization was with the EncompassTM (Qfix, Avondale, PA) aquaplast mask device. AlignRT® log files were correlated with trajectory log files to correlate treatment parameters with SI reported offsets. SI reported offsets were correlated with gantry angle and analyzed for performance issues at non‐zero couch angles and during camera‐pod blockage during gantry motion. Demographics in the treatment management system were used to identify race and determine if differences in SI reported offsets are due to skin tone settings. Results A total of 981 fractions were monitored over 14 months and 819 were analyzed. The median AlignRT® reported motion from beginning to the end of treatment was 0.24 mm. The median offset before beam on at non‐zero couch angles was 0.55 mm. During gantry motion when camera pods are blocked, the median magnitude was below 1 mm. Median magnitude of offsets at non‐zero couch angles was not found to be significantly different for patients stratified by race. Conclusions Surface image guidance is a viable alternative to scheduled mid‐treatment imaging for monitoring intrafraction motion during stereotactic radiosurgery with automated delivery.

Purpose To create automated tools within the treatment planning system (TPS) that eliminate the common error pathway of providing incorrect shift instructions to therapists. Materials/Methods Two scripts were created within the TPS using the Eclipse API (Varian Medical Systems, Palo Alto, CA). One script detects whether or not the user origin has been placed correctly at the intersection of the simulation markers while the other calculates a shift instruction sheet that can be printed for treatment. Results Analysis of our RO‐ILS database identified eight errors caused by improper setting of the user origin in the treatment planning system. The user origin script flagged all of the treatment plans for markers inconsistent with user origin. Automated calculation of shifts eliminated the error pathway of miscalculating or transcribing shift values. Conclusion Automation can eliminate the common error pathway of providing the wrong shifts to therapists. The scripts have been made available as open‐source software for implementation at other radiotherapy clinics.

The Calypso Beacon transponder has been modified by the addition of a nitinol anchor feature to allow for positional stability when implanted bronchoscopically into the lung. The purpose of this study was to confirm the feasibility and safety of anchored transponder placement and feasibility of lung target localization and tracking.This study enrolled patients with histologically confirmed cancer in the lung (primary or metastatic) who were scheduled to receive external beam radiation therapy. Three anchored transponders were implanted via flexible bronchoscopy into small (approximately 2- to 2.5-mm diameter) airways. Patient alignment at each radiation fraction was performed with the Calypso system, and anchored transponder position was tracked during radiation delivery. The primary endpoint was defined as the ability to localize at least 85% of the patients during the first week of treatment. Four follow-up visits were specified including a posttreatment assessment and every 3 months up to 1 year.A total of 69 patients underwent anchored transponder placement, and all 207 implanted anchored transponders were visible on the treatment-planning simulation computed tomography scan. Sixty-seven patients underwent radiation therapy, and localization was successful in 66 cases (98.5%). With 1 failure in 67 cases, the P value for rejecting the null hypothesis was.001 and the primary objective of the study met. Five adverse events in 5 patients were potentially attributed to the study device or implantation procedure, consisting of pneumonia (2 cases), pleural abscess (1 case), and pneumothorax (2 cases). Two serious events (cardiac arrest and acute hypotension) were attributed to anesthesia during the implantation procedure.This study strongly supports that anchored transponders are safe, positionally stable, and useful for lung tumor localization and monitoring.

We present a framework for collecting data which elucidates detailed clinical flow timelines for different treatment sites, modalities, planners, and physicians in radiotherapy treatments. A web based tool, the Whiteboard, was developed which allows communication between the physicians and staff about the current state of the radiotherapy treatment plan and provides detailed time data for each stage of the patient's journey from simulation to treatment. Detailed timestamped data was collected over a period of 6 years which included 22 discrete steps of the treatment planning process. Whiteboard data was combined with data in the treatment planning database Aria (Varian Medical Systems, Palo Alto, CA) using the Eclipse Scripting API to include treatment plan information. Over 6 years, 110,477 patient statuses were time-logged for 9683 courses of treatment using our Whiteboard software, which allowed determination of patient volumes per individual and the time to perform planning tasks. The mean planning volume per dosimetrist was 375.8 ± 142.4 plans and 71.03 ± 27.10 plan revisions per year. The mean planning volume per physician was 218.9 ± 110.8 plan approvals with 41.55 ± 43.73 plan revisions per year. Physician contour time was a mean of 79.59 ± 84.61 hours, but varied greatly by diagnosis category and physician. The longest median modality planning time of 119.6 hours was for the IMRT plans with 8 or more fields. The shortest median times were 48.25, 49.53, and 50.98 hours for plans with 5, 4, and 3 fields, respectively. We demonstrated a framework that could help facilities in staff planning and aid in workload distribution, and could be used to improve understanding of radiotherapy timelines for patients, payers, and other institutions involved in radiotherapy processes.

Purpose: Surgery is often used for large or symptomatic brain metastases but is associated with risk of developing leptomeningeal dissemination. Emerging data suggest that fractionated stereotactic radiation therapy (FSRT) is an effective management strategy in large brain metastases. We sought to retrospectively compare leptomeningeal disease (LMD) and local control (LC) rates for patients treated with surgical resection followed by radiosurgery (S + SRS) versus FSRT alone. Methods and Materials: We identified all patients with a brain metastasis ≥3 cm in diameter treated from 2004 to 2017 with S + SRS or FSRT alone (25 or 30 Gy in 5 fractions) who had follow-up imaging. LMD was defined as focal or diffuse leptomeningeal enhancement that was >5 mm from the index metastasis. Categorical baseline characteristics were compared with the χ2 test. LMD and LC rates were evaluated by the Kaplan-Meier (KM) method, with the log-rank test used to compare subgroups. Results: A total of 125 patients were identified, including 82 and 43 in the S + SRS and FSRT alone groups, respectively. Median pretreatment Graded Prognostic Assessment in the S + SRS and FSRT groups was 2.5 and 1.5, respectively (P

A multileaf collimator (MLC), virtual-cone treatment technique has been commissioned for trigeminal neuralgia (TGN) at Tri-Cities Cancer Center (TCCC). This novel technique was initially developed at the University of Alabama in Birmingham (UAB); it is designed to produce a spherical dose profile similar to a fixed, 5-mm conical collimator distribution. Treatment is delivered with a 10-MV flattening-filter-free (FFF) beam using a high-definition MLC on a Varian Edge linear accelerator. Absolute dose output and profile measurements were performed in a 20 × 20 × 14 cm

Purpose: Interest and application of stereotactic radiosurgery for multiple brain metastases continue to increase. Various planning systems are available for linear accelerator (linac)–based single-isocenter multiple metastasis radiosurgery. Two of the most advanced systems are BrainLAB Multiple Metastases Elements (MME), a dynamic conformal arc (DCA) approach, and Varian RapidArc (RA), a volumetric modulated arc therapy (VMAT) approach. In this work, we systematically compared plan quality between the 2 techniques. Methods and Materials: Thirty patients with 4 to 10 metastases (217 total; median 7.5; Vmin = 0.014 cm3; Vmax = 17.73 cm3) were planned with both Varian RA and MME at 2 different institutions with extensive experience in each respective technique. All plans had a single isocenter and used Varian linac equipped with high-definition multileaf collimator. RA plans used 2 to 4 noncoplanar VMAT arcs with 10 MV flattening filter-free beam. MME plans used 4 to 9 noncoplanar DCAs and 6 MV flattening filter-free beam, (minimum planning target volume [PTVmin] = 0.49 cm3; PTVmax = 27.32 cm3; PTVmedian = 7.05 cm3). Prescriptions were 14 to 24 Gy in a single fraction. Target coverage goal was 99% of volume receiving prescription dose (D99% ≥ 100%). Plans were evaluated by Radiation Therapy Oncology Group/Paddick conformity index (CI) score, 12 Gy volume (V12Gy), V8Gy, V5Gy, mean brain dose (Dmean), and beam-on time. Results: Conformity was favorable among RA plans (median: MME CIRTOG = 1.38; RA CIRTOG = 1.21; P < .0001). V12Gy and V8Gy were lower for RA plans (median: MME V12 = 23.7 cm3; RA V12 = 19.2 cm3; P = .0001; median: MME V8Gy = 53.6 cm3; RA V8Gy = 44.1 cm3; P = .024). V5Gy was lower for MME plans (median: MME V5Gy = 141.4 cm3; RA V5Gy = 142.8 cm3; P = .009). Mean brain was lower for MME plans (median: MME Dmean = 2.57 Gy; RA Dmean = 2.76 Gy; P < .0001). Conclusions: For linac-based multiple metastasis stereotactic radiosurgery, RapidArc VMAT facilitates favorable conformity and V12Gy/V8Gy volume compared with the MME DCA plan. MME planning facilitates reduced dose spill at levels ≤V5Gy.

Background Vaginal bleeding (VB) is common in women with gynecologic (GYN) malignancies. Radiation therapy (RT) is used for the definitive treatment of GYN cancers and palliation of bleeding. The historical dogma is that high dose-per-fraction radiation leads to more rapid bleeding cessation, yet there is scant data supporting this claim. We sought to examine the effect of RT fraction size on VB via retrospective analysis of patients receiving hypofractionated radiation (HFRT) compared to conventionally fractionated radiation (CFRT) for control of bleeding secondary to GYN malignancies. Methods We identified patients receiving external beam RT for continuous VB from GYN malignancy treated in our department from 2012 to 2020. RT was classified as HFRT (> 2.0 Gy/fx) or CFRT (1.8–2.0 Gy/fx). Demographic information, disease characteristics, and treatment details were collected. The primary endpoint was days from RT initiation until bleeding resolution. Characteristics between groups were compared via Fisher’s exact test. Time to bleeding cessation was assessed via Kaplan–Meier and log-rank test. Univariable and multivariable Cox-proportional hazards were used to identify factors associated with bleeding cessation. Results We identified 43 patients meeting inclusion criteria with 26 and 17 patients receiving CFRT and HFRT, respectively. Comparison of baseline characteristics revealed patients receiving HFRT were older (p = 0.001), more likely to be post-menopausal (p = 0.002), and less likely to receive concurrent chemotherapy (p = 0.004). Time to bleeding cessation was significantly shorter for patients receiving HFRT (log-rank p p = 0.008). Conclusions Patients with continuous VB from GYN tumors receiving HFRT experienced more rapid bleeding cessation than those receiving CFRT. For patients with severe VB, initiation of HFRT to control malignancy related bleeding quickly may be warranted.

Purpose To evaluate a two‐dimensional diode array for patient‐specific quality assurance of VMAT stereotactic radiosurgery (SRS) plans. Methods The diode array, an SRS MapCHECK (SRSMC), was composed of a 77 mm ×77 mm face‐centered array having a spacing of 2.47 mm. Sixty SRS plans were selected from our clinical database, 30 for treatment of a single target and 30 for multiple targets. The target sizes ranged from 2.4 mm to 44.7 mm equivalent diameter (median 8.7 mm). The plans were delivered to the diode array. For multiple target plans, two measurements were obtained at two locations, one corresponding to the largest target and the other to the smallest target. Gamma using a 3%/1 mm criteria and the dose to the center diode were compared with radiochromic film (RCF). Dose to selected regions of the detector electronics was calculated. Results The mean difference between the center diode and RCF was −1.2%. For a threshold of at least 95% of detectors/pixels having gamma

Purpose: The aim of this study was to report the treatment planning feasibility of dose escalation to suspicious lymph nodes (LNs) for a series of men who underwent pretreatment [18F]fluciclovine positron emission tomography (PET)/magnetic resonance imaging (MRI). Methods and Materials: Cases of men with prostate cancer who enrolled in a clinical trial of pretreatment [18F]fluciclovine PET who had suspicious LNs were selected. Pelvic LNs

Purpose To evaluate a scintillator detector for patient‐specific quality assurance of VMAT radiosurgery plans. Methods The detector was comprised of a 1 mm diameter, 1 mm high scintillator coupled to an acrylic optical fiber. Sixty VMAT SRS plans for treatment of single targets having sizes ranging from 3 mm to 30.2 mm equivalent diameter (median 16.3 mm) were selected. The plans were delivered to a 20 cm × 20 cm x 15 cm water equivalent plastic phantom having either the scintillator detector or radiochromic film at the center. Calibration films were obtained for each measurement session. The films were scanned and converted to dose using a 3‐channel technique. Results The mean difference between scintillator and film was ‒0.45% (95% confidence interval ‒0.1% to 0.8%). For target equivalent diameter smaller than the median, the mean difference was 1.1% (95% confidence interval 0.5% to 1.7%). For targets larger than the median, the mean difference was ‒0.2% (95% confidence interval ‒0.7% to 0.1%). Conclusions The scintillator detector response is independent of target size for targets as small as 3 mm and is well‐suited for patient‐specific quality assurance of VMAT SRS plans. Further work is needed to evaluate the accuracy for VMAT plans that treat multiple targets using a single isocenter.

Purpose To evaluate the accuracy of monitoring intrafraction motion during stereotactic radiotherapy with the optical surface monitoring system. Prior studies showing a false increase in the magnitude of translational offsets at non‐coplanar couch positions prompted the vendor to implement software changes. This study evaluated two software improvements intended to address false offsets. Methods The vendor implemented two software improvements: a volumetric (ACO) rather than planar calibration and, approximately 6 months later, an improved calibration workflow (CIB) designed to better compensate for thermal drift. Offsets relative to the reference position, obtained at table angle 0 following image‐guided setup, were recorded before beam‐on at each table position and at the end of treatment the table returned to 0° for patients receiving SRT. Results Prior to ACO, between ACO and CIB, and after CIB, 223, 155, and 436 fractions were observed respectively. The median magnitude of translational offsets at the end of treatment was similar for all three intervals: 0.29, 0.33, and 0.27 mm. Prior to ACO, the offset magnitude for non‐zero table positions had a median of 0.79 mm and was found to increase with increasing distance from isocenter to the anterior patient surface. After ACO, the median magnitude was 0.74 mm, but the dependence on surface‐to‐isocenter distance was eliminated. After CIB, the median magnitude for non‐zero table positions was reduced to 0.57 mm. Conclusion Ongoing improvements in software and calibration procedures have decreased reporting of false offsets at non‐zero table angles. However, the median magnitude for non‐zero table angles is larger than that observed at the end of treatment, indicating that accuracy remains better when the table is not rotated.

Purpose To measure dosimetric and spatial accuracy of stereotactic radiosurgery (SRS) delivered to targets as small as the trigeminal nerve (TN) using a standard external beam treatment planning system (TPS) and multileaf collimator‐(MLC) equipped linear accelerator without cones or other special attachments or modifications. Methods Dosimetric performance was assessed by comparing computed dose distributions to film measurements. Comparisons included the γ‐index, beam profiles, isodose lines, maximum dose, and spatial accuracy. Initially, single static 360° arcs of MLC‐shaped fields ranging from 1.6 × 5 to 30 × 30 mm2 were planned and delivered to an in‐house built block phantom having approximate dimensions of a human head. The phantom was equipped with markings that allowed accurate setup using planar kV images. Couch walkout during multiple‐arc treatments was investigated by tracking a ball pointer, initially positioned at cone beam computed tomography (CBCT) isocenter, as the couch was rotated. Tracks were mapped with no load and a 90 kg stack of plastic plates simulating patient treatment. The dosimetric effect of walkout was assessed computationally by comparing test plans that corrected for walkout to plans that neglected walkout. The plans involved nine 160° arcs of 2.4 × 5 mm2 fields applied at six different couch angles. For end‐to‐end tests that included CT simulation, target contouring, planning, and delivery, a cylindrical phantom mimicking a 3 mm lesion was constructed and irradiated with the nine‐arc regimen. The phantom, lacking markings as setup aids was positioned under CBCT guidance by registering its surface and internal structures with CTs from simulation. Radiochromic film passing through the target center was inserted parallel to the coronal and the sagittal plane for assessment of spatial and dosimetric accuracy. Results In the single‐arc block phantom tests computed maximum doses of all field sizes agreed with measurements within 2.4 ± 2.0%. Profile widths at 50% maximum agreed within 0.2 mm. The largest targeting error was 0.33 mm. The γ‐index (3%, 1 mm) averaged over 10 experiments was >1 in only 1% of pixels for field sizes up to 10 × 10 mm2 and rose to 4.4% as field size increased to 20 × 20 mm2. Table walkout was not affected by load. Walkout shifted the target up to 0.6 mm from CBCT isocenter but, according to computations shifted the dose cloud of the nine‐arc plan by only 0.16 mm. Film measurements verified the small dosimetric effect of walkout, allowing walkout to be neglected during planning and treatment. In the end‐to‐end tests average and maximum targeting errors were 0.30 ± 0.10 and 0.43 mm, respectively. Gamma analysis of coronal and sagittal dose distributions based on a 3%/0.3 mm agreement remained

Purpose To assess the feasibility of using portal dosimetry (PD) for pre‐treatment quality assurance of single target, flattening filter free (FFF), volumetric arc therapy intracranial radiosurgery plans. Methods A PD algorithm was created for a 10X FFF beam on a Varian Edge linear accelerator (Varian Inc, Palo Alto, CA, USA). Treatment plans that were previously evaluated with Gafchromic EBT‐XD (Ashland, Bridgewater, NJ, USA) film were measured via PD and analyzed with the ARIA Portal Dosimetry workspace. Absolute dose evaluation for film and PD was done by computing the mean dose in the region receiving greater than or equal to 90% of the max dose and comparing to the mean dose in the same region calculated by the treatment planning system (TPS). Gamma analysis with 10% threshold and 3%/2 mm passing criteria was performed on film and portal images. Results Thirty‐six PD verification plans were delivered and analyzed. The average PD to TPS dose was 0.989 ± 0.01 while film to TPS dose was 1.026 ± 0.01. All PD plans passed the gamma analysis with 100% of points having gamma

Purpose This study aimed to report the early toxicity results of a prospective clinical trial of prostate stereotactic body radiation therapy (SBRT) to the entire prostate with a simultaneous integrated boost (SIB) to magnetic resonance imaging (MRI)-defined focal lesions. Methods and materials Eligible patients included men with biopsy-proven prostate stage T1c to T2c adenocarcinoma, a Gleason score ≤7, and prostate-specific antigen values of ≤20 ng/mL, who had at least 1 focal lesion visible on MRI and a total prostate volume no greater than 120 cm3. SBRT consisted of a dose of 36.25 Gy to the entire prostate with an SIB of 40 Gy to the MRI-defined lesions, delivered in 5 fractions. The primary purpose of the study was to confirm the feasibility of treatment planning/delivery and to estimate the rate of urinary retention requiring placement of a Foley catheter within 90 days of treatment. This study was to be considered successful if urinary retention occurred in no more than 15% of cases, with a planned enrollment of at least 25 patients. Results A total of 26 men were enrolled, and all underwent SBRT as planned. Twenty patients (77%) had intermediate-risk features, and the remainder were low risk. A treatment plan that met the protocol-defined goals for all cases was developed. Two patients (7.7%) developed acute urinary symptoms that required the temporary placement of a Foley catheter. No grade 3+ toxicity events were observed. Conclusions Planning and delivery of prostate SBRT with a whole prostate dose of 36.25 Gy and a focal 40 Gy SIB is feasible. Early follow-up suggests that this treatment is not associated with undue morbidity.

Purpose The task of software development has become an increasing part of the medical physicist's role. Many physicists who are untrained in the best practices of software development have begun creating scripts for clinical use. There is an increasing need for guidance for both developers and medical physicists to code wisely in the clinic. Materials and Methods We created a novel model for assessing risk for custom clinical software analogous to failure modes and effects analysis and propose minimum best practices that should be followed to mitigate the risks. Using this risk model, we integrated a literature review and institutional experience to form a practical guide for risk mitigation. Results Using this new risk assessment model, we outlined several risk mitigation techniques including unit testing, code review, source control, end‐user testing, and commissioning from the literature while sharing our institutional guidelines for evaluating software for risk and implementing these strategies. Conclusion We found very little literature for custom software development guidelines targeted at medical physicists. We have shared our institutional experience and guidelines to help facilitate safe software development for the evolving role of the medical physicist.

Purpose The study aimed to develop and demonstrate a standardized linear accelerator multileaf collimator-based method of delivering small, spherical dose distributions suitable for radiosurgical treatment of small targets such as the trigeminal nerve. Methods and materials The virtual cone is composed of a multileaf collimator–defined field with the central 2 leaves set to a small gap. For 5 table positions, clockwise and counter-clockwise arcs were used with collimator angles of 45 and 135 degrees, respectively. The dose per degree was proportional to the sine of the gantry angle. The dose distribution was calculated by the treatment planning system and measured using radiochromic film in a skull phantom for leaf gaps of 1.6, 2.1, and 2.6 mm. Cones with a diameter of 4 mm and 5 mm were measured for comparison. Output factor constancy was investigated using a parallel-plate chamber. Results The mean ratio of the measured-to-calculated dose was 0.99, 1.03, and 1.05 for 1.6, 2.1, and 2.6 mm leaf gaps, respectively. The diameter of the measured (calculated) 50% isodose line was 4.9 (4.6) mm, 5.2 (5.1) mm, and 5.5 (5.5) mm for the 1.6, 2.1, and 2.6 mm leaf gap, respectively. The measured diameter of the 50% isodose line was 4.5 and 5.7 mm for the 4 mm and 5 mm cones, respectively. The standard deviation of the parallel-plate chamber signal relative to a 10 cm × 10 cm field was less than 0.4%. The relative signal changed 32% per millimeter change in leaf gap, indicating that the parallel-plate chamber is sensitive to changes in gap width. Conclusions The virtual cone is an efficient technique for treatment of small spherical targets. Patient-specific quality assurance measurements will not be necessary in routine clinical use. Integration directly into the treatment planning system will make planning using this technique extremely efficient.

A substantial barrier to the single-and multi-institutional aggregation of data to supporting clinical trials, practice quality improvement efforts, and development of big data analytics resource systems is the lack of standardized nomenclatures for expressing dosimetric data. To address this issue, the American Association of Physicists in Medicine (AAPM) Task Group 263 was charged with providing nomenclature guidelines and values in radiation oncology for use in clinical trials, data-pooling initiatives, population-based studies, and routine clinical care by standardizing: (1) structure names across image processing and treatment planning system platforms; (2) nomenclature for dosimetric data (eg, doseevolume histogram [DVH]-based metrics); (3) templates for clinical trial groups and users of an initial subset of software platforms to facilitate adoption of the standards; (4) formalism for nomenclature schema, which can accommodate the addition of other structures defined in the future. A multisociety, multidisciplinary, multinational group of 57 members representing stake holders ranging from large academic centers to community clinics and vendors was assembled, including physicists, physicians, dosimetrists, and vendors. The stakeholder groups represented in the membership included the AAPM, American Society for Radiation Oncology (ASTRO), NRG Oncology, European Society for Radiation Oncology (ESTRO), Radiation Therapy Oncology Group (RTOG), Children's Oncology Group (COG), Integrating Healthcare Enterprise in Radiation Oncology (IHE-RO), and Digital Imaging and Communications in Medicine working group (DICOM WG); A nomenclature system for target and organ at risk volumes and DVH nomenclature was developed and piloted to demonstrate viability across a range of clinics and within the framework of clinical trials. The final report was approved by AAPM in October 2017. The approval process included review by 8 AAPM committees, with additional review by ASTRO, European Society for Radiation Oncology (ESTRO), and American Association of Medical Dosimetrists (AAMD). This Executive Summary of the report highlights the key recommendations for clinical practice, research, and trials. (C) 2017 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

PURPOSE/OBJECTIVE(S) Stereotactic Body Radiation Therapy (SBRT) is increasingly utilized as a definitive treatment option for patients with prostate adenocarcinoma, especially in the low or intermediate risk setting. The aim of this study is to assess the late toxicity and biochemical recurrence rates of a single institution prospective clinical trial evaluating prostate SBRT with simultaneous integrated boost (SIB) targeting focal lesions as defined by magnetic resonance imaging (MRI). MATERIALS/METHODS Patients were eligible if they had biopsy-proven low or intermediate risk prostate adenocarcinoma with initial prostate specific antigen (PSA) values ≤20 ng/mL, clinical stage from T1c to T2c, and Gleason score ≤7. Patients also had to have one or more focal lesions on MRI and an MRI-defined total prostate volume of < 120 cm3. All patients received SBRT delivered to the entire prostate to a dose of 36.25 Gy in 5 fractions with an SIB to the lesions seen on MRI to 40 Gy in 5 fractions. Late toxicity was defined as any possible treatment-related adverse event occurring after three months from the completion of SBRT and graded according to the Common Terminology Criteria for Adverse Events version 4.0 (CTCAE v4.0). Biochemical recurrence was defined by the Phoenix definition of PSA increase of 2 ng/mL above the nadir. RESULTS A total of 26 patients were enrolled, and all enrolled patients underwent planned SBRT. Six patients (23.1%) had low risk disease and twenty patients had intermediate risk disease (76.9%). Eight patients (30.8%) received androgen deprivation therapy. Median follow up was 40.5 months (range 12-64 months). No biochemical failures have yet been observed. Three patients (11.5%) experienced late grade 2 genitourinary (GU) toxicity requiring cystoscopy; two of these three patients experienced hematuria requiring cystoscopy and one patient experienced dysuria requiring cystoscopy. Six patients (23.1%) had late grade 2 GU toxicity requiring medications such as tamsulosin or oxybutynin. Two patients (7.7%) had late grade 2 gastrointestinal (GI) toxicity; both patients experienced hematochezia requiring colonoscopy and were found to have proctitis requiring steroids per rectum. There was no grade 3 or higher toxicity events observed. CONCLUSION The results of this study support that SBRT to the entire prostate to a dose of 36.25 Gy in 5 fractions with focal SIB to 40 Gy in 5 fractions is not associated with undue late GI or GU toxicity. Thus far, this regimen has excellent biochemical control. Focal dose escalation with an SIB planning approach may be an opportunity to improve biochemical control while limiting dose to nearby organs at risk.

Purpose: Limited data exist on fractionated stereotactic radiation therapy (FSRT) for brain metastases. We sought to evaluate the safety and efficacy of FSRT and further define its role in brain metastasis management. Methods and materials: A total of 72 patients were treated with linear accelerator–based FSRT to 182 previously untreated, intact brain metastases. Targets received 25 or 30 Gy in 5 fractions. All targets within the same course received the same prescription regardless of size. Toxicity was recorded per Radiation Therapy Oncology Group central nervous system toxicity criteria. Results: The median follow-up was 5 months (range, 1-71 months). The Kaplan-Meier estimate of 12-month local control was 86%. Tumors

Purpose The purpose of this study was to evaluate dose prescription and recording compliance to international standard (International Commission on Radiation Units & Measurements [ICRU]-83) in patients treated with intensity modulated radiation therapy (IMRT) among academic institutions. Methods and materials Ten institutions participated in this study to collect IMRT data to evaluate compliance to ICRU-83. Under institutional review board clearance, data from 5094 patients—including treatment site, technique, planner, physician, prescribed dose, target volume, monitor units, planning system, and dose calculation algorithm—were collected anonymously. The dose-volume histogram of each patient, as well as dose points, doses delivered to 100% (D100), 98% (D98), 95% (D95), 50% (D50), and 2% (D2), of sites was collected and sent to a central location for analysis. Homogeneity index (HI) as a measure of the steepness of target and is a measure of the shape of the dose-volume histogram was calculated for every patient and analyzed. Results In general, ICRU recommendations for naming the target, reporting dose prescription, and achieving desired levels of dose to target were relatively poor. The nomenclature for the target in the dose prescription had large variations, having every permutation of name and number contrary to ICRU recommendations. There was statistically significant variability in D95, D50, and HI among institutions, tumor site, and technique with P values Conclusions Nearly 95% of patient treatments deviated from the ICRU-83 recommended D50 prescription dose delivery. This variability is significant (P

Purpose Compliance with TG‐263 nomenclature standards can be challenging. We introduce an open source solution to this problem and evaluate its impact on compliance within our institution. Materials/methods The TG‐236 nomenclature standards were implemented in our clinic in two phases. In phase 1, we deployed TG‐263 compliant templates for each disease site. In phase 2, we developed and deployed a script for evaluating compliance which presented errors to the user. After each phase the compliance was recorded. Results Mean compliance errors prior to phase 1 was 31.8% ± 17.4%. Error rates dropped to 8.1% ± 12.2% across phase 1 and dropped further to 2.2% ± 6.9% during the automation system deployed in phase 2. Conclusion Both structure templates and automation scripts are very useful for increasing compliance with structure naming standards. Our software solution is made available on GitHub for other institutions to implement.

This report details our institutional workflow and technique for use of the Calypso electromagnetic transponder system with respiratory gating for localization and tracking of lung tumors during stereotactic radiation therapy for early stage thoracic malignancies.Sixteen patients underwent bronchoscopic fiducial placement of 3 transponders in small airways in proximity to the primary tumor. Transponders were placed19 cm from the most anterior skin location of the patient for appropriate tracking functionality. Patients underwent simulation with 4-dimensional assessment and were treated with transponder based positional gating if tumors moved5 mm in any direction. Tumor motion5 mm was not gated and treated using an internal target volume approach. A 5 mm uniform planning target volume was used. Before treatment, fiducial placement and tumor location were verified by daily kilovoltage (kV) and cone beam computed tomography image guidance. Tracking limits were placed based on the movement of the transponders from the centroid of the structures on the maximum intensity projection image. The Calypso treatment system paused treatment automatically if beacons shifted beyond the predefined tracking limits.All 16 patients underwent successful implantation of the electromagnetic transponders. Eight patients exhibited tumor motion sufficient to require respiratory gating, and the other 8 patients were treated using a free breathing internal target volume technique. Difficulty with transponder sensing was experienced in 3 patients as a result of anatomic interference with the placement of the sensing arrays; each of these cases was successfully treated after making setup modifications. Triggered imaging of fiducials during treatment was consistent with real-time positioning determined by the Calypso tracking system.Respiratory gated electromagnetic based transponder guided stereotactic body radiation therapy using the workflow described is feasible and well tolerated in selected patients with early stage lung malignancies.

Stereotactic radiosurgery (SRS) places great demands on spatial accuracy. Steel BBs used as markers in quality assurance (QA) phantoms are clearly visible in MV and planar kV images, but artifacts compromise cone‐beam CT (CBCT) isocenter localization. The purpose of this work was to develop a QA phantom for measuring with sub‐mm accuracy isocenter congruence of planar kV, MV, and CBCT imaging systems and to design a practical QA procedure that includes daily Winston‐Lutz (WL) tests and does not require computer aid. The salient feature of the phantom (Universal Alignment Ball (UAB)) is a novel marker for precisely localizing isocenters of CBCT, planar kV, and MV beams. It consists of a 25.4 mm diameter sphere of polymethylmetacrylate (PMMA) containing a concentric 6.35 mm diameter tungsten carbide ball. The large density difference between PMMA and the polystyrene foam in which the PMMA sphere is embedded yields a sharp image of the sphere for accurate CBCT registration. The tungsten carbide ball serves in finding isocenter in planar kV and MV images and in doing WL tests. With the aid of the UAB, CBCT isocenter was located within 0.10±0.05 mm of its true positon, and MV isocenter was pinpointed in planar images to within 0.06±0.04 mm. In clinical morning QA tests extending over an 18 months period the UAB consistently yielded measurements with sub‐mm accuracy. The average distance between isocenter defined by orthogonal kV images and CBCT measured 0.16±0.12 mm. In WL tests the central ray of anterior beams defined by a 1.5×1.5 cm2 MLC field agreed with CBCT isocenter within 0.03±0.14 mm in the lateral direction and within 0.10±0.19 mm in the longitudinal direction. Lateral MV beams approached CBCT isocenter within 0.00±0.11 mm in the vertical direction and within ‐0.14±0.15 mm longitudinally. It took therapists about 10 min to do the tests. The novel QA phantom allows pinpointing CBCT and MV isocenter positions to better than 0.2 mm, using visual image registration. Under CBCT guidance, MLC‐defined beams are deliverable with sub‐mm spatial accuracy. The QA procedure is practical for daily tests by therapists. PACS number(s): 87.53.Ly, 87.56.Fc

The purpose of this study was to evaluate the accuracy and calculation speed of electron dose distributions calculated by the Eclipse electron Monte Carlo (eMC) algorithm for use with bolus electron conformal therapy (ECT). The recent commercial availability of bolus ECT technology requires further validation of the eMC dose calculation algorithm. eMC‐calculated electron dose distributions for bolus ECT have been compared to previously measured TLD‐dose points throughout patient‐based cylindrical phantoms (retromolar trigone and nose), whose axial cross sections were based on the mid‐PTV (planning treatment volume) CT anatomy. The phantoms consisted of SR4 muscle substitute, SR4 bone substitute, and air. The treatment plans were imported into the Eclipse treatment planning system, and electron dose distributions calculated using 1% and

Aim To report outcomes for patients with para-aortic lymph node positive cervical cancer treated with a dynamic field-matching technique. Background PET staging of cervical cancer has increased identification of patients with para-aortic lymph node metastasis. IMRT enables dose escalation in this area, but matching IMRT fields with traditional whole pelvis fields presents a challenge. Materials and methods From 2003 to 2012, 20 patients with cervical cancer and para-aortic lymph node metastasis were treated utilizing the dynamic field-matching technique. As opposed to single-isocenter half-beam junction techniques, this technique employs wedge-shaped dose junctions for the abutment of fields. We reviewed the records of all patients who completed treatment with the technique and abstracted treatment, toxicity, and disease-related outcome data for analysis. Results Median prescribed dose to the whole pelvis field was 45 Gy and para-aortic IMRT field 50.4 Gy. All but 3 patients underwent HDR (13 pts) or LDR (4 pts) brachytherapy. All patients developed lower GI toxicity; 10 grade 1, 9 grade 2, and 1 grade 4 (enterovaginal fistula). Median DFS was 12.4 months with 1 and 2-year DFS 60.0% and 38.1%. One-year OS was 83.7% and 2-year OS, 64.4%. A total of 10 patients developed recurrence; none occurred at the matched junction. Conclusions The dynamic field-matching technique provides a means for joining conventional whole pelvis fields and para-aortic IMRT fields that substantially reduces dose deviations at the junction due to field mismatch. Treatment with the dynamic matching technique is simple, effective, and tolerated with no apparent increase in toxicity.

Purpose To assess the long‐term stability of the anchored radiofrequency transponders and compare displacement rates with other commercially available lung fiducial markers. We also sought to describe late toxicity attributable to fiducial implantation or migration. Materials and methods The transponder cohort was comprised of 17 patients at our institution who enrolled in a multisite prospective clinical trial and underwent bronchoscopic implantation of three anchored transponders into small (2–2.5 mm) airways. We generated a comparison cohort of 34 patients by selecting patients from our institutional lung SBRT database and matching 2:1 based on the lobe containing tumor and proximity to the bronchial tree. Assessment of migration was performed by rigidly registering the most recent follow‐up CT scan to the simulation scan, and assessing whether the relative geometry of the fiducial markers had changed by more than 5 mm. Toxicity outcomes of interest were hemoptysis and pneumothorax. Results The median follow‐up of patients in the transponder cohort was 25.3 months and the median follow‐up in the comparison cohort was 21.7 months. When assessing the most recent CT, all fiducial markers were within 5 mm of their position at CT simulation in 11 (65%) patients in the transponder group as compared to 23 (68%) in the comparison group (P = 0.28). One case of hemoptysis was identified in the transponder cohort, and bronchoscopy confirmed bleeding from recurrent tumor; no cases of hemoptysis were noted in the comparison cohort. No case of pneumothorax was noted in either group. Conclusion No significant difference in the rates of fiducial marker retention and migration were noted when comparing patients who had anchored transponders placed into small airways and a 2:1 matched cohort of patients who had other commercially available lung fiducial markers placed. In both groups, no late or chronic toxicity appeared to be related to the implanted fiducial markers.

Purpose Intensity modulated radiation therapy (IMRT) is widely used in the treatment of head and neck cancers (HNC). There is not enough evidence to suggest that some radiation oncologists (ROs) are associated with better outcomes in patients with HNC. We conducted a cross-sectional analysis to evaluate the effect of ROs' characteristics on outcomes in patients with HNC treated with IMRT. Methods and materials The study used the Surveillance Epidemiology and End Results-Medicare linked database to identify patient characteristics. Physician records were obtained from the American Medical Association. Logistic regression models with propensity scores were analyzed to look for an association between RO characteristics and patient outcomes. Results RO characteristics showed that approximately 30% of ROS completed their training in or after the year 2000 (recently trained), and 17% were in top decile of treatment volume (high volume). Less than 3% of ROs work in academic settings. We found that ROs who were recently trained have higher odds (odds ratio [OR]: 1.10; 95% confidence interval [CI], 1.011-1.191) compared with those who were not. In addition, ROs who were treating high volumes of patients have higher odds (OR: 1.08; 95% CI, 1.010-1.165) compared with those treating low volumes of an event of adverse effect of IMRT or death among patients. ROs who work in academic settings have a protective effect (OR: 0.72; 95% CI, 0.569-0.925). Conclusions ROs who were recently trained and had a high treatment volume are associated with poorer outcomes among patients with HNC who receive IMRT treatment.

Purpose The purpose of this study was to determine whether radiation dose inhomogeneity within the prostate predisposes to late urinary strictures after moderately hypofractionated definitive external beam radiation therapy for prostate cancer. Methods and materials One hundred seventy-three men with clinically localized prostate cancer met the inclusion criteria for this analysis. All patients received 70 Gy to the prostate delivered over 28 fractions, had at least 2 years of clinical follow-up, and had dose-volume histogram information available for review. The endpoint of this study was the development of a urethral stricture that required a procedural intervention such as urethral dilation or suprapubic catheterization. Dosimetric parameters were evaluated for effect on the rate of urethral stricture formation by univariate Cox proportional hazards modeling. Results The median follow-up was 49.5 months (range, 24.6-108 months). At 5 years, the actuarial rate of intervention for urethral strictures across all patients was 4.9%. The maximum point dose within the prostate (P = .034, hazard ratio = 1.006) and the mean prostate dose (P = .039, hazard ratio = 1.004) were the only parameters predictive of urethral stricture formation. All patients who developed a urethral stricture were treated by a plan with a maximum prostate dose of > 75 Gy (median, 77.67 Gy). Conclusions For patients receiving moderately hypofractionated prostate radiation therapy over 28 fractions, a maximum point dose of 75 Gy within the prostate was associated with an increased probability of developing a urethral stricture that required procedural intervention. The hypothesis that hypofractionation increases susceptibility to toxicity from heterogeneity within the prostate should be confirmed by analyzing data from randomized trials with a conventionally fractionated control arm for comparison.

This report describes the current state of flattening filter‐free (FFF) radiotherapy beams implemented on conventional linear accelerators, and is aimed primarily at practicing medical physicists. The Therapy Emerging Technology Assessment Work Group of the American Association of Physicists in Medicine (AAPM) formed a writing group to assess FFF technology. The published literature on FFF technology was reviewed, along with technical specifications provided by vendors. Based on this information, supplemented by the clinical experience of the group members, consensus guidelines and recommendations for implementation of FFF technology were developed. Areas in need of further investigation were identified. Removing the flattening filter increases beam intensity, especially near the central axis. Increased intensity reduces treatment time, especially for high‐dose stereotactic radiotherapy/radiosurgery (SRT/SRS). Furthermore, removing the flattening filter reduces out‐of‐field dose and improves beam modeling accuracy. FFF beams are advantageous for small field (e.g., SRS) treatments and are appropriate for intensity‐modulated radiotherapy (IMRT). For conventional 3D radiotherapy of large targets, FFF beams may be disadvantageous compared to flattened beams because of the heterogeneity of FFF beam across the target (unless modulation is employed). For any application, the nonflat beam characteristics and substantially higher dose rates require consideration during the commissioning and quality assurance processes relative to flattened beams, and the appropriate clinical use of the technology needs to be identified. Consideration also needs to be given to these unique characteristics when undertaking facility planning. Several areas still warrant further research and development. Recommendations pertinent to FFF technology, including acceptance testing, commissioning, quality assurance, radiation safety, and facility planning, are presented. Examples of clinical applications are provided. Several of the areas in which future research and development are needed are also indicated. PACS number: 87.53.‐j, 87.53.Bn, 87.53.Ly, 87.55.‐x, 87.55.N‐, 87.56.bc

Emergency‐off systems (EOS) are essential to the safe operation of medical accelerators and other high‐risk equipment. To assure reliable functioning, some states require weekly tests; others permit monthly, tri‐monthly or even six‐monthly tests, while some do not specify test intervals. We investigate the relative safety of the various test schedules by computing the fraction of time during which a nonfunctional state of the EOS may remain undetected. Special attention is given to the effect of flexibility (i.e., to regulations that specify the number of tests that have to be done in any given time interval, but allow a range within the interval during which a test can be done). Compared to strict test intervals, a schedule that provides flexibility increases risk only marginally. Performing tests on any arbitrary day of the week when weekly tests are required increases the time span during which a nonfunctionality goes undetected by only 17%, compared to an exact one‐week schedule. The same ratio applies for monthly tests. For a three‐month schedule, the relative risk increases by only 2% if tests are done on an arbitrarily chosen day during each due‐month, compared to tests done on an exact three‐month schedule. The most irregular time intervals possible in a three‐calendar month schedule increase the relative risk by 11%. For the six‐month and the 12‐month schedule the ratio of risks is even smaller. The relative risk is virtually independent of the mean time between failures of the EOS, but the absolute risk decreases in proportion the mean time between failures. Adherence to strict, resource‐intensive test intervals provides little extra safety compared to flexible intervals that require the same number of tests per year. Regulations should be changed to provide the practicality offered by flexible test schedules. Any additional increase in patient safety could be achieved by strict regulations concerning reliability of emergency‐stop (e‐stop) systems. PACS numbers: 87.55.N‐, 87.55.Qr, 87.56.bd

Flattening filter‐free (FFF) beams are available on an increasing number of commercial linear accelerators. FFF beams have higher dose rates than flattened beams of equivalent energy which can lead to increased efficiency of treatment delivery, especially in conjunction with increased FFF beam energy and arc‐based delivery configurations. The purpose of this study is to quantify and assess the implications of improved treatment efficiency for several FFF delivery options on common types of linac applicable radiotherapy. Eleven characteristic cases representative of a variety of clinical treatment sites and prescription doses were selected from our patient population. Treatment plans were generated for a Varian TrueBeam linear accelerator. For each case, a reference plan was created using DMLC IMRT with 6 MV flat beams. From the same initial objectives, plans were generated using DMLC IMRT and volumetric‐modulated arc therapy (VMAT) with 6 MV FFF and 10 MV FFF beams (max. dose rates of 1400 and 2400 MU/min, respectively). The plans were delivered to a phantom; beam‐on time, total treatment delivery time, monitor units (MUs), and integral dose were recorded. For plans with low dose fractionations (1.8–2.0 & 3.85 Gy/fraction), mean beam‐on time difference between reference plan and most efficient FFF plan was 0.56 min (41.09% decrease); mean treatment delivery time difference between the reference plan and most efficient FFF plan was 1.54 min (range: 0.31–3.56 min), a relative improvement of 46.1% (range: 29.2%‐59.2%). For plans with high dose fractionations (16–20 Gy/fraction), mean beam‐on time difference was 6.79 min (74.9% decrease); mean treatment delivery time difference was 8.99 min (range: 5.40–13.05 min), a relative improvement of 71.1% (range: 53.4%‐82.4%). 10 MV FFF VMAT beams generated the most efficient plan, except in the spine SBRT case. The distribution of monitor unit counts did not vary by plan type. In cases where respiratory motion management would be applicable, 10 MV FFF DMLC IMRT reduced beam‐on time/field to less than 12 sec. FFF beams significantly reduced treatment delivery time. For radiosurgical doses, the efficiency improvement for FFF beams was clinically significant. For conventional fractionation, a large improvement in relative treatment delivery time was observed, but the absolute time savings were not likely to be of clinical value. In cases that benefit from respiratory motion management, beamon/field was reduced to a time for which most patients can comfortably maintain deep inspiratory breath hold. PACS numbers: 87.55.D‐, 87.55.de, 87.56.bd, 87.56.N‐

Ion recombination is approximately corrected for in the Task Group 51 protocol by Pion, which is calculated by a two‐voltage measurement. This measurement approach may be a poor estimate of the true recombination, particularly if Pion is large (greater than 1.05). Concern exists that Pion in high‐dose‐per‐pulse beams, such as flattening filter free (FFF) beams, may be unacceptably high, rendering the two‐voltage measurement technique inappropriate. Therefore, Pion was measured for flattened beams of 6, 10, 15, and 18 MV and for FFF beams of 6 and 10 MV. The values for the FFF beams were verified with 1/V versus 1/Q curves (Jaffé plots). Pion was also measured for electron beams of 6, 12, 16, 18, and 20 MeV on a traditional accelerator, as well as on the high‐dose‐rate Varian TrueBeam accelerator. The measurements were made at a range of depths and with PTW, NEL, and Exradin Farmer‐type chambers. Consistent with the increased dose per pulse, Pion was higher for FFF beams than for flattening filter beams. However, for all beams, measurement locations, and chambers examined, Pion never exceeded 1.018. Additionally, Pion was always within 0.3% of the recombination calculated from the Jaffé plots. We conclude that ion recombination can be adequately accounted for in high‐dose‐rate FFF beams using Pion determined with the standard two‐voltage technique. PACS numbers: 87.56.‐v, 87.56.Da