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To evaluate the image quality, dosimetric properties, setup reproducibility, and planar cine motion detection of a high‐resolution brain coil and integrated stereotactic brain immobilization system that constitute a new brain treatment package (BTP) on a low‐field magnetic resonance imaging (MRI) linear accelerator (MR‐linac). Image quality of the high‐resolution brain coil was evaluated with the 17 cm diameter spherical phantom and the American College of Radiology (ACR) Large MRI Phantom. Patient imaging studies approved by the institutional review board (IRB) assisted in selecting image acquisition parameters. Radiographic and dosimetric evaluation of the high‐resolution brain coil and the associated immobilization devices was performed using dose calculations and ion chamber measurements. End‐to‐end testing was performed simulating a cranial lesion in a phantom. Inter‐fraction setup variability and motion detection tests were evaluated on four healthy volunteers. Inter‐fraction variability was assessed based on three repeat setups for each volunteer. Motion detection was evaluated using three‐plane (axial, coronal, and sagittal) MR‐cine imaging sessions, where volunteers were asked to perform a set of specific motions. The images were post‐processed and evaluated using an in‐house program. Contrast resolution of the high‐resolution brain coil is superior to the head/neck and torso coils. The BTP receiver coils have an average HU value of 525 HU. The most significant radiation attenuation (3.14%) of the BTP, occurs through the lateral portion of the overlay board where the high‐precision lateral‐profile mask clips attach to the overlay. The greatest inter‐fraction setup variability occurred in the pitch (average 1.08 degree) and translationally in the superior/inferior direction (average 4.88 mm). Three plane cine imaging with the BTP was able to detect large and small motions. Small voluntary motions, sub‐millimeter in magnitude (maximum 0.9 mm), from motion of external limbs were detected. Imaging tests, inter‐fraction setup variability, attenuation, and end‐to‐end measurements were quantified and performed for the BTP. Results demonstrate better contrast resolution and low contrast detectability that allows for better visualization of soft tissue anatomical changes relative to head/neck and torso coil systems. [ABSTRACT FROM AUTHOR]

In head-and-neck IMPT, trigger-based offline plan adaptation (Offline trigger-based) is often used. Our goal was to compare this to four alternative adaptive strategies for dosimetry, workload and treatment time, considering also foreseen further technological advancements, including anticipated automation. Alternative strategies included weekly offline re-planning (Offline weekly), daily plan selection from a library (Library static and Library progsressive) and a fast, approximate daily online re-optimization approach (Online re-opt). Impact on CTV coverage and NTCPs was assessed by simulations based on repeat-CTs from 15 patients. Full daily re-planning was used as dosimetric benchmark. Increases in workload and treatment time were estimated. Both for coverage and NTCPs, fast Online re-opt performed as well as full re-planning. Compared to current practice, Online re -opt showed enhanced probabilities for high coverage, and resulted in reductions in grade ≥ II NTCPs of 4.6 ± 1.7 %-point for xerostomia and 4.2 ± 2.3 %-point for dysphagia. Offline weekly and library strategies did not show coverage enhancements and resulted in smaller NTCP improvements. Further automation can largely limit workload and treatment time increases. With anticipated further automation, adaptation-related workload of Offline weekly , Library static , Library progressive , and Online re-opt was expected to increase by 3, 8, 21, and 66 h for 35 fraction treatment courses compared to Offline trigger-based. The corresponding adaptation-related prolonged treatment times were estimated to be 0, 4, 6, and 29 min/fraction. Online adaptive strategies could approach dosimetric quality of full re-planning at the cost of additional workload and prolonged treatment time compared to the current offline adaptive strategy. Automation needs to play a key role in making more complex adaptive approaches feasible. [ABSTRACT FROM AUTHOR]

• The plan library (PL) for H&N IMPT was compared to conventional treatments. • The PL resulted in overall reduced NTCPs at limited cost in target coverage. • If NTCP was worse, adherence to target constraints could sometimes be improved. • The PL is a relatively simple online adaptive strategy to implement in clinics. In intensity modulated proton therapy (IMPT), the impact of setup errors and anatomical changes is commonly mitigated by robust optimization with population-based setup robustness (SR) settings and offline replanning. In this study we propose and evaluate an alternative approach based on daily plan selection from patient-specific pre-treatment established plan libraries (PLs). Clinical implementation of the PL strategy would be rather straightforward compared to daily online re-planning. For 15 head-and-neck cancer patients, the planning CT was used to generate a PL with 5 plans, robustly optimized for increasing SR: 0, 1, 2, 3, 5 mm, and 3% range robustness. Repeat CTs (rCTs) and realistic setup and range uncertainty distributions were used for simulation of treatment courses for the PL approach, treatments with fixed SR (fSR 3) and a trigger-based offline adaptive schedule for 3 mm SR (fSR 3 OfA). Daily plan selection in the PL approach was based only on recomputed dose to the CTV on the rCT. Compared to using fSR 3 and fSR 3 OfA, the risk of xerostomia grade ≥ II & III and dysphagia ≥ grade III were significantly reduced with the PL. For 6/15 patients the risk of xerostomia and/or dysphagia ≥ grade II could be reduced by > 2% by using PL. For the other patients, adherence to target coverage constraints was often improved. fSR 3 OfA resulted in significantly improved coverage compared to PL for selected patients. The proposed PL approach resulted in overall reduced NTCPs compared to fSR 3 and fSR 3 OfA at limited cost in target coverage. [ABSTRACT FROM AUTHOR]

Background and purpose: The more localized dose deposition of proton therapy (PT) compared to photon therapy might allow a reduction in treatment-related side effects but induces additional challenges to address. The aim of this study was to evaluate the impact of interfractional motion on the target and organs at risk (OARs) in cervical cancer patients treated with spot scanning PT using an internal target volume (ITV) strategy. Methods and materials: For ten locally advanced cervical cancer patients, empty and full bladder planning computed tomography (pCT) as well as 25 daily cone beam CTs (CBCTs) were available. The Clinical Target Volume (CTV), the High Risk CTV (CTVHR) (gross tumor volume and whole cervix), the non-involved uterus as well as the OARs (bowel, bladder and rectum) were contoured on the daily CBCTs and transferred to the pCT through rigid bony match. Using synthetic CTs derived from pCTs, four-beam spot scanning PT plans were generated to target the patient-specific ITV with 45 Gy(RBE) in 25 fractions. This structure was defined based on pre-treatment MRI and CT to anticipate potential target motion throughout the treatment. D98% of the targets and V40Gy(RBE) of the OARs were extracted from the daily anatomies, accumulated and analyzed. In addition, the impact of bladder volume deviations from planning values on target and bowel dose was investigated. Results: The ITV strategy ensured a total accumulated dose >42.75 Gy(RBE) to the CTVHR for all ten patients. Two patients with large bladder-related uterus motion had accumulated dose to the non-involved uterus of 35.7 Gy(RBE) and 41.1 Gy(RBE). Variations in bowel V40Gy(RBE) were found to be correlated (Pearson r = −0.55; p-value

Background: A magnetic resonance linear accelerator (MR-Linac) provides superior soft tissue contrast to evaluate inter- and intra-fraction motion and facilitate online adaptive radiation therapy (ART). We present here an unusual case of locally advanced castrate-resistant prostate cancer treated with high-dose palliative ultra-hypofractionated radiation therapy on the MR-Linac with significant inter-fraction tumor regression. Case Presentation: The patient was a 65-year-old man diagnosed with metastatic prostate cancer to bone and pelvic lymph nodes 7 years prior. At diagnosis, he presented with a PSA of 23 ng/ml and was commenced on a luteinizing hormone-releasing hormone agonist, achieving a PSA nadir of 4.68 ng/ml at 12 months. The patient subsequently had progressive lower urinary tract symptoms, his PSA increased to 47 ng/ml, and there was a markedly enlarged pelvic mass involving the prostate with gross extra-capsular disease and invasion into the posterior bladder wall. The patient was referred for palliative radiation to the pelvic mass due to urinary symptoms, pain, and lower limb paraesthesia. Treatment was planned to be delivered on the MR-Linac with a schedule of 36 Gy over 6 weekly factions allowing for maximal target dose delivery while minimizing surrounding organs at risk (OARs) radiation exposure. Unexpectedly, the target volume had a marked 49% (453 cc to 233 cc) reduction that was accounted for in the online adaptive process. A new reference plan was generated after 3 fractions to add sacral plexus as an OAR, previously not visible due to mass encroachment. The patient reported ongoing reduction in urinary symptoms, pelvic pain, and lower limb paresthesia by the end of treatment. Conclusion: Using daily MR-guided ART, improved visualization of the changing target and OARs ensured safe dose escalation. The unexpected positive response of the target and improved patient outcomes demonstrated the added value of the MR-Linac for online adaptive radiotherapy in this setting. [ABSTRACT FROM AUTHOR]

BackgroundA magnetic resonance linear accelerator (MR-Linac) provides superior soft tissue contrast to evaluate inter- and intra-fraction motion and facilitate online adaptive radiation therapy (ART). We present here an unusual case of locally advanced castrate-resistant prostate cancer treated with high-dose palliative ultra-hypofractionated radiation therapy on the MR-Linac with significant inter-fraction tumor regression.Case PresentationThe patient was a 65-year-old man diagnosed with metastatic prostate cancer to bone and pelvic lymph nodes 7 years prior. At diagnosis, he presented with a PSA of 23 ng/ml and was commenced on a luteinizing hormone-releasing hormone agonist, achieving a PSA nadir of 4.68 ng/ml at 12 months. The patient subsequently had progressive lower urinary tract symptoms, his PSA increased to 47 ng/ml, and there was a markedly enlarged pelvic mass involving the prostate with gross extra-capsular disease and invasion into the posterior bladder wall. The patient was referred for palliative radiation to the pelvic mass due to urinary symptoms, pain, and lower limb paraesthesia. Treatment was planned to be delivered on the MR-Linac with a schedule of 36 Gy over 6 weekly factions allowing for maximal target dose delivery while minimizing surrounding organs at risk (OARs) radiation exposure. Unexpectedly, the target volume had a marked 49% (453 cc to 233 cc) reduction that was accounted for in the online adaptive process. A new reference plan was generated after 3 fractions to add sacral plexus as an OAR, previously not visible due to mass encroachment. The patient reported ongoing reduction in urinary symptoms, pelvic pain, and lower limb paresthesia by the end of treatment.ConclusionUsing daily MR-guided ART, improved visualization of the changing target and OARs ensured safe dose escalation. The unexpected positive response of the target and improved patient outcomes demonstrated the added value of the MR-Linac for online adaptive radiotherapy in this setting.

Introduction: Temporary breast tissue expanders contain a metal port that varies in position throughout the course of radiation treatments. The purpose of this study was to quantify the robustness of the three most common external beam treatment techniques (tangential three‐dimensional conformal radiation therapy [3DCRT], volumetric modulated arc therapy [VMAT], and helical tomotherapy) against our measured inter‐fractional positional variations of the port. Methods: For eight breast cases, a clinical plan was created for each of the three techniques. The dosimetric effect of our previously measured inter‐fractional port errors was evaluated for two classes of error: internal port errors (IPEs) and patient registration errors (PREs). For both classes of error, daily variable and systematic errors were modeled, and their cumulative effects were compared against the originally planned doses. Results: For systematic IPE, the 1%–99% range in point dose differences inside a 5‐mm target abutting the implant was the highest for tangential 3DCRT, and it was within 6% and 9% when calculated with Monte Carlo and collapsed cone calculation engines, respectively. Daily variable PRE resulted in mean changes of −3.0% and −3.5% to V100%Rx of the target for VMAT and tomotherapy, respectively. For nearby organs, daily variable PRE resulted in changes to V20Gy of the ipsilateral lung of less than 2% in all three techniques, while V5Gy of the heart increased by as much as 6% in VMAT and 10% in tomotherapy. Conclusions: When IPEs were modeled, dose variability was the largest in tangential 3DCRT, leading to areas of underdosage in the shadow of the port. When PREs were modeled, the target coverage and nearby organs were affected the most in VMAT and helical tomotherapy. In reality, port positional errors result from a combination of IPE and PRE, suggesting that VMAT and tomotherapy are more robust when patient registration errors are minimized, despite the presence of IPE. [ABSTRACT FROM AUTHOR]

[Display omitted] • Proton therapy can be as robust as photon therapy to inter-fraction motion. • Proton therapy significantly reduces organ at risk doses compared to photon therapy. • Two ITV-based treatment planning techniques for PBS-PT presented. Large-field photon radiotherapy is current standard in the treatment of cervical cancer patients. However, with the increasing availability of Pencil Beam Scanning Proton Therapy (PBS-PT) and robust treatment planning techniques, protons may have significant advantages for cervical cancer patients in the reduction of toxicity. In this study, PBS-PT and photon Volumetric Modulated Arc Therapy (VMAT) were compared, examining target coverage and organ at risk (OAR) dose, taking inter- and intra-fraction motion into account. Twelve cervical cancer patients were included in this in-silico planning study. In all cases, a planning CT scan, five weekly repeat CT scans (reCTs) and an additional reCT 10 min after the first reCT were available. Two-arc VMAT and robustly optimised two- and four-field (2F and 4F) PBS-PT plans were robustly evaluated on planCTs and reCTs using set-up and range uncertainty. Nominal OAR doses and voxel-wise minimum target coverage robustness were compared. Average voxel-wise minimum accumulated doses for pelvic target structures over all patients were adequate for both photon and proton treatment techniques (D98 > 95%, [91.7–99.3%]). Average accumulated dose of the para-aortic region was lower than the required 95%, D98 > 94.4% [91.1–98.2%]. With PBS-PT 4F, dose to all OARs was significantly lower than with VMAT. Major differences were observed for mean bowel bag V 15Gy : 60% [39–70%] for VMAT vs 30% [10–52%] and 32% [9–54%] for PBS-PT 2F and 4F and for mean bone marrow V 10Gy : 88% [82–97%] for VMAT vs 66% [60–73%] and 67% [60–75%] for PBS-PT 2F and 4F. Robustly optimised PBS-PT for cervical cancer patients shows equivalent target robustness against inter- and intra-fraction variability compared to VMAT, and offers significantly better OAR sparing. [ABSTRACT FROM AUTHOR]

AimDeep inspiration breath hold (DIBH) is a method of motion management used in stereotactic ablative body radiotherapy (SABR) for lung tumours. An external gating block marker can be used as a tumour motion surrogate, however, inter-fraction gross target volume (GTV) displacement within DIBH occurs. This study measured this displacement during a reproducible breath hold regime. In addition, factors such as position of the gating block marker were analysed.Methods and materialsA total of 121 cone beam computed tomography scans (CBCTs) from 22 patients who received DIBH SABR were retrospectively evaluated and the magnitude of inter-fraction GTV displacement was calculated for each fraction. This data was analysed to assess if any correlation existed between tumour displacement and variation in the gating block marker position on the patient, the amplitude of breath hold (BH) at computed tomography (CT), the amplitude of BH at treatment and the tumour location.The measured tumour displacement was applied to the original planning CT to evaluate the dosimetric effect on surrounding organs at risk (OARs) using cumulative dose volume histograms (DVHs).ResultsBH amplitude was reproducible within 0·13±0·1 cm (mean±standard deviation). The magnitude of tumour displacement within BH ranged from 0 to 1·52 cm (0·41±0·28 cm). Displacement in the superior-inferior, anterior-posterior and left-right planes were 0·31±0·26 cm, 0·16±0·18 cm and 0·07±0·12 cm, respectively. No statistically significant correlation was detected between tumour displacement within DIBH and the factors investigated. The range of variation in OAR dose was −7·0 to +3·6 Gy with one statistically significant increase in OAR dose observed (oesophagus mean dose increasing by 0·16 Gy).FindingsReproducible BH was achievable across a range of patients. Inter-fraction GTV displacement measured 0·41±0·28 cm. Due to this low level of motion, the correction of soft tissue moves did not adversely affect OAR dose. [ABSTRACT FROM PUBLISHER]

This work measured time taken to integrate image-guidance with CBCTs in routine clinical practice for patients treated between 2007 and 2010 across eight linear accelerators. 117,301 CBCTs from 4592 patients across thirteen disease sites were included. The mean image assessment decision time was 79 s. Decision time was correlated with setup displacement magnitude. [ABSTRACT FROM AUTHOR]

The purpose of this study was to quantify the impact of inter-fraction modifications of bladder during RT of prostate cancer on bladder dose surface maps (DSM). Eighteen patients treated with daily image-guided Tomotherapy and moderate hypofractionation (70–72.8 Gy at 2.5–2.6 Gy/fr in 28 fractions and full bladder) were considered. Bladder contours were delineated on co-registered daily Megavoltage CT (MVCT) by a single observer and copied on the planning CT to generate dose–volume/surface histograms (DVH/DSH) and bladder DSMs. Discrepancies between planned and daily absorbed doses were analyzed through the average of individual systematic errors, the population systematic errors and the population random errors for the DVH/DSHs and DSMs. In total, 477 DVH/DSH and 472 DSM were available. DSH and DVH showed small population systematic errors of absolute surfaces (<3.4 cm 2 ) and volumes (<8.4 cm 3 ) at the highest doses. The dose to the posterior bladder base assessed on DSMs showed a mean systematic error below 1 Gy, with population systematic and random errors within 4 and 3 Gy, respectively. The region surrounding this area shows higher mean systematic errors (1–3 Gy), population systematic (8–11 Gy) and random (5–7 Gy) errors. In conclusion, DVH/DSH and DSMs are quite stable with respect to inter-fraction variations in the high-dose region, within about 2 cm from bladder base. Larger systematic variations occur in the anterior portion and cranially 2.5–3.5 cm from the base. Results suggest that dose predictors related to the high dose area (including the trigone dose) are likely to be sufficiently reliable with respect to the expected variations due to variable bladder filling. [ABSTRACT FROM AUTHOR]

Objective . Fractionated radiotherapy typically delivers the same dose in each fraction. Adaptive fractionation (AF) is an approach to exploit inter-fraction motion by increasing the dose on days when the distance of tumor and dose-limiting organs at risk (OAR) is large and decreasing the dose on unfavorable days. We develop an AF algorithm and evaluate the concept for patients with abdominal tumors previously treated at the MR-linac in 5 fractions. Approach . Given daily adapted treatment plans, inter-fractional changes are quantified by sparing factors δ t defined as the OAR-to-tumor dose ratio. The key problem of AF is to decide on the dose to deliver in fraction t , given δ t and the dose delivered in previous fractions, but not knowing future δ t s. Optimal doses that maximize the expected biologically effective dose in the tumor (BED 10 ) while staying below a maximum OAR BED 3 constraint are computed using dynamic programming, assuming a normal distribution over δ with mean and variance estimated from previously observed patient-specific δ t s. The algorithm is evaluated for 16 MR-linac patients in whom tumor dose was compromised due to proximity of bowel, stomach, or duodenum. Main Results . In 14 out of the 16 patients, AF increased the tumor BED 10 compared to the reference treatment that delivers the same OAR dose in each fraction. However, in 11 of these 14 patients, the increase in BED 10 was below 1 Gy. Two patients with large sparing factor variation had a benefit of more than 10 Gy BED 10 increase. For one patient, AF led to a 5 Gy BED 10 decrease due to an unfavorable order of sparing factors. Significance . On average, AF provided only a small increase in tumor BED. However, AF may yield substantial benefits for individual patients with large variations in the geometry., (Creative Commons Attribution license.)

Purpose: To perform a comparative study assessing potential benefits of endorectal-balloons (ERB) in post-prostatectomy patients. Method and materials: Ten retrospective post-prostatectomy patients treated without ERB and ten prospective patients treated with the ERB in situ were recruited. All patients received IMRT and IGRT using kilovoltage cone-beam computed tomography (kVCBCT). kVCBCT datasets were registered to the planning dataset, recontoured and the original plan recalculated on the kVCBCTs to recreate anatomical conditions during treatment. The imaging, structure and dose data were imported into in-house software for the assessment of geometric variation and cumulative equivalent uniform dose (EUD) in the two groups. Results: The difference in location (ΔCOV) for the bladder between planning and each CBCT was similar for each group. The range of mean ΔCOV for the rectum was 0.15–0.58cm and 0.15–0.59cm for the non-ERB and ERB groups. For superior-CTV and inferior-CTV the difference between planned and delivered D 95% (mean±SD) for the non-ERB group was 2.1±6.0Gy and −0.04±0.20Gy. While for the ERB group the difference in D 95% was 8.7±12.6Gy and 0.003±0.104Gy. Conclusions: The use of ERBs in the post-prostatectomy setting did improve geometric reproducibility of the target and surrounding normal tissues, however no improvement in dosimetric stability was observed for the margins employed. [ABSTRACT FROM AUTHOR]

Abstract: Background and purpose: Intensity-modulated radiotherapy (IMRT) can improve dose homogeneity within the breast planned target volume (PTV), but may be more susceptible to patient/organ motion than standard tangential radiotherapy (RT). We used daily cone-beam CT (CBCT) imaging to assess inter-fraction motion during breast IMRT and its subsequent impact on IMRT and standard RT dose homogeneity. Materials and methods: Ten breast cancer patients selected for IMRT were studied. CBCT images were acquired immediately after daily treatment. Automatic image co-registration was used to determine patient positioning variations. Daily PTV contours were used to calculate PTV variations and daily delivered IMRT and theoretically planned tangential RT dose. Results: Group systematic (and random) setup errors detected by CBCT were 5.7 (3.9)mm laterally, 2.8 (3.5)mm vertically and 2.3 (3.2)mm longitudinally. Rotations >2° in any axis occurred on 53/106 (50%) occasions. Daily PTV volume varied up to 23%. IMRT dose homogeneity was superior at planning and throughout the treatment compared with standard RT (1.8% vs. 15.8% PTV received >105% planned mean dose), despite increased motion sensitivity. Conclusions: CBCT revealed inadequacies of current patient positioning and verification procedures during breast RT and confirmed improved dose homogeneity using IMRT for the patients studied. [Copyright &y& Elsevier]

Abstract: Purpose: To investigate motion and shape change when using an endorectal balloon (ERB) in patients receiving radiotherapy for prostate cancer. Methods: In nine patients treated for prostate cancer using an ERB, the anterior wall of the ERB was contoured on right lateral images taken immediately before irradiation, and on left lateral images taken immediately after irradiation. Changes in the contours were used to calculate inter-fraction shape change and inter-imaging motion and shape change. Inter-imaging motion describes changes that occur after the right lateral image is taken that are seen in the left lateral image. Results: Eighty-six percent of all inter-imaging shifts of the anterior wall of the ERB were in the posterior direction (mean: 1.8mm, 1SD: 1.8mm, maximum posterior shift: 2.8–7.2mm). The inter-fraction shape change (1SD) of the anterior wall was equivalent to a change in the angle of the balloon of 2.5–5.7°, with a range of 8–20°, depending on the patient. Inter-imaging shape changes were similar in size. Conclusions: The inter-imaging motion and shape changes may be explained by the patient relaxing some time after insertion of the ERB, indicating that it could be reduced by a waiting period after insertion before irradiation. Development of image-guided localization strategies should consider intra-fraction motion and also inter- and intra-fraction shape change. [Copyright &y& Elsevier]

In this overview, we discuss some major issues related to the management of mobile tumors and gating in radiotherapy. For most types of organ motion, there are both interfraction and intrafraction components. For respiratory motion, the magnitudes of these 2 components can be comparable and therefore both should be handled carefully. The motion artifacts in computed tomography (CT) simulation are discussed and the 4-dimensional CT scan technique is recommended for treatment simulation of patients with mobile tumors. There are various methods for handling organ motion in treatment delivery. Caution should be exercised when using patient-specific motion information for treatment planning because motion characteristics may vary from the treatment simulation time to the treatment delivery sessions. Respiratory gating is potentially accurate, easy to implement, and may be widely adopted in clinical practice in the near future, if existing technical problems can be resolved. [Copyright &y& Elsevier]

In terms of dose distribution, protons are more sensitive to range variations than photons due to their unique properties. The aim of this study was to develop a method to identify patient-specific robust proton beam angles for lung tumor irradiation by investigating the association between water equivalent thickness (WET) variation and inter-fraction motion-induced target dose degradation. Using 3-dimensional computed tomography (3D-CT) images, the impact of WET variations on the target dose coverage of a series of coplanar proton beams was evaluated for 4 patients with lung cancer. Using ray tracing, WET maps, or WET baseline, were estimated for the internal target volume (ITV) at every 5° gantry interval in the axial plane. After calculating the WET baseline, the planning CT was shifted 5 mm in each anterior-posterior (AP), superior-inferior (SI), and left-right (LR) direction, yielding a total of 6 shifted CTs, and differential WET maps between the planning CT and each shifted CT were calculated. Target dose differences were associated with the average WET change between the original planning CT and the shifted CTs for all 360° gantry rotation beams. Target and OAR dose metrics in the ΔWET-guided plans were compared with those of the clinical plans. The WET variation maps showed areas of both high and low WET variations, with overall similar patterns yet individual differences reflecting tumor position differences. For all 4 patients investigated in this study, the coplanar plans demonstrated a strong correlation between WET changes and ITV dose reductions. Target dose coverage was more stable with the ΔWET-guided plan while OAR doses were comparable to the clinical plan. The WET variation maps have been used in this pilot study to identify proton beam angles that are either sensitive or robust to WET changes in proton passive scattering. This work demonstrates the feasibility of using WET variation maps to assist the planner in inter-fraction motion-robust proton beam angle selection.

Purpose: Substantial, unanticipated anatomic variances during cone-beam CT (CBCT)-guided radiotherapy can potentially impact treatment accuracy and clinical outcomes. This study assessed patterns of practice of CBCT variances reported by RTTs and subsequent interventions for multiple-disease sites., Methods: A chart review was conducted at a large cancer centre for patients treated with daily online CBCT-guided radiotherapy. Patients selected for review were identified via RTT-reported variances that then triggered offline multi-disciplinary assessment. Cases were categorized by the type of anatomic variance observed on CBCT and any further interventions recorded such as un-scheduled adaptive re-planning., Results: Over a 1-year period, 287 variances from 261 patients were identified (6.2% of the 4207 patients treated with daily CBCT-guided radiotherapy), most often occurring within the first 5 fractions of the treatment course. Of these variances, 21% (59/287) were re-planned and 3.5% (10/287) discontinued treatment altogether. Lung was the most frequent disease-site (27% of 287 variances) reported with IGRT-related variances although head and neck and sarcoma were most frequently re-planned (19% of 59 re-plans for each site). Technical or clinical rationales for re-planning were not routinely documented in patient medical records. All disease-sites had numerous categories of variances. Three of the four most frequent categories were for tumor-related changes on CBCT, and the re-planning rate was highest for tumor progression at 25%. Normal tissue variances were the second most frequency category, and re-planned in 14% of those cases., Conclusion: RTTs identified a wide range of anatomic variances during CBCT-guided radiotherapy. In a minority of cases, these substantially altered the care plan including ad hoc adaptive re-planning or treatment discontinuation. Improved understanding of the clinical decisions in these cases would aid in developing more routine, systematic adaptive strategies., Competing Interests: None., (© 2019 The Authors.)