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PURPOSE: Tilting of the posterior plaque margin during eye plaque brachytherapy can lead to tumor underdosing and increased risk of local recurrence. We performed a quantitative analysis of the dosimetric effects of plaque tilt as a function of tumor position, basal dimension, height and plaque type using 3D treatment planning software. MATERIALS AND METHODS: Posterior and anterior tumors with largest basal dimensions of 6, 12 and 18 mm and heights of 4, 7 and 10 mm were modeled. Both Eye Physics and COMS plaques were simulated and uniformly loaded. Plans were normalized to 85 Gy at the tumor apex. Posterior plaque tilts of 1, 2, 3 and 4 mm were simulated. RESULTS: Volumetric coverage is more sensitive to tilt than the area coverage. Wide, flat tumors are more susceptible to tilt. Apical dose changed significantly as a function of tumor height and diameter. No other parameter exhibited significant differences. Posterior tumors are slightly more susceptible to tilt due to the use of notched plaques. Plaque type does not significantly alter the effect of plaque tilt. CONCLUSIONS: Wide, flat tumors are the most susceptible to plaque tilt. Tumor location or plaque type does not have a significant effect on dosimetry changes from plaque tilt. Robust clinical procedures such as the use of mattress sutures, pre- and post-implant ultrasound and post-implant dosimetry can all mitigate the risk associated with plaque tilt.

Background Newer technology for stereotactic radiosurgery (SRS) should be assessed for different multi-leaf collimators (MLC). Objective Assess plan quality of an automated, frameless, linear accelerator based (linac) planning and delivery system (HyperArc) for SRS using both standard MLC (SMLC) and high definition MLC (HDMLC) compared to a cobalt-60 based SRS system (Gamma Knife, GK). Methods We re-planned twenty GK Perfexion-treated SRS patients (27 lesions) for HyperArc using SMLC and HDMLC. We assessed plan quality using the following metrics: gradient index (GI), Paddick and RTOG conformity indices (CIPaddick, CIRTOG), volume receiving half of prescription isodose (PIVhalf) and maximum dose to 0.03 cc for brainstem, optic chiasm and optic nerves, and V12Gy for brain-GTV. Results Linac plans had better conformity with HDMLC being most conformal. GK exhibited better GI. PIVhalf demonstrated no statistically significant difference between HDMLC and GK, and SMLC was nominally worse than GK. Mean PIVhalf was generally 0.85 cc larger for SMLC than HDMLC. For TV > 1.0 cc, the relative differences in CIRTOG, GI, and PIVhalf for SMLC vs. HDMLC were less than 21%. For TV less than Conclusions New linac-based plans positively compare to GK plans overall. HDMLC should be strongly considered for treatment of lesions

PurposeTo assess the performance of a proton-specific knowledge-based planning (KBP) model in the creation of robustly optimized intensity-modulated proton therapy (IMPT) plans for treatment of advanced head and neck (HN) cancer patients.MethodsSeventy-three patients diagnosed with advanced HN cancer previously treated with volumetric modulated arc therapy (VMAT) were selected and replanned with robustly optimized IMPT. A proton-specific KBP model, RapidPlanPT (RPP), was generated using 53 patients (20 unilateral cases and 33 bilateral cases). The remaining 20 patients (10 unilateral and 10 bilateral cases) were used for model validation. The model was validated by comparing the target coverage and organ at risk (OAR) sparing in the RPP-generated IMPT plans with those in the expert plans. To account for the robustness of the plan, all uncertainty scenarios were included in the analysis.ResultsAll the RPP plans generated were clinically acceptable. For unilateral cases, RPP plans had higher CTV_primary V100 (1.59% ± 1.24%) but higher homogeneity index (HI) (0.7 ± 0.73) than had the expert plans. In addition, the RPP plans had better ipsilateral cochlea Dmean (−5.76 ± 6.11 Gy), with marginal to no significant difference between RPP plans and expert plans for all other OAR dosimetric indices. For the bilateral cases, the V100 for all clinical target volumes (CTVs) was higher for the RPP plans than for the expert plans, especially the CTV_primary V100 (5.08% ± 3.02%), with no significant difference in the HI. With respect to OAR sparing, RPP plans had a lower spinal cord Dmax (−5.74 ± 5.72 Gy), lower cochlea Dmean (left, −6.05 ± 4.33 Gy; right, −4.84 ± 4.66 Gy), lower left and right parotid V20Gy (left, −6.45% ± 5.32%; right, −6.92% ± 3.45%), and a lower integral dose (−0.19 ± 0.19 Gy). However, RPP plans increased the Dmax in the body outside of CTV (body-CTV) (1.2 ± 1.43 Gy), indicating a slightly higher hotspot produced by the RPP plans.ConclusionIMPT plans generated by a broad-scope RPP model have a quality that is, at minimum, comparable with, and at times superior to, that of the expert plans. The RPP plans demonstrated a greater robustness for CTV coverage and better sparing for several OARs.

To evaluate intra-fraction target shift during automated mono-isocentric linac-based stereotactic radiosurgery with open-face mask system and optical real-time tracking.Ninety-five patients were treated using automated linac-based stereotactic radiosurgery in 1-5 fractions with single isocenter for a total of 195 fractions. During treatment, patient positioning was tracked real-time with optical surface guidance and immobilized with a rigid open-face mask. Patients were re-positioned if optical surface guidance error exceeded 1 mm magnitude or 1°. Translational and rotational intra-fractional changes were determined by post-treatment CBCT matched to the planning CT. Target specific error was calculated by translation and rotation matrices applied to isocenter and target spatial coordinates.For 132 fractions with isocenter within a single target, the median shift magnitude was 0.40 mm with a maximum shift of 1.17 mm. A total of 398 targets treated for plans having multiple or single targets that lied outside isocenter, resulted in a median shift magnitude of 0.46 mm, with median translational shifts of 0.20 mm and 0.20° rotational shifts. A 1 mm PTV margin was insufficient in 18% of targets at a distance greater than 6 cm away from isocenter, but sufficient for 96% of targets within 6 cm.The findings of this study support 1 mm PTV expansion due to intra-fraction motion to ensure target coverage for plans with isocenter placement less than 6 cm away from the targets.

OBJECTIVE: Shot-within-shot (SWS) optimization is a new planning technique that relies on various combinations of shot weighting and prescription isodose line (IDL) to reduce beam-on time. The method differs from other planning techniques that incorporate mixed collimation, multiple stereotactic coordinates, and traditionally low prescription IDLs (50%. Planning metrics were then compared among the original plan and the 2 replans. RESULTS: More than a third (39%) of the brain metastases were eligible for the SWS technique. For these lesions, the differences between the original plan and reference SWS plan were as follows: ΔV(12Gy) < 0.5 cc in 93% of cases, ΔV(12Gy) < 0.5 cc in 100% of cases, Δselectivity < 0.1 in 79% of cases. Negligible differences were seen between the 2 replans in terms of Δselectivity and ΔV(12Gy); ΔGI < 5% in 99% of cases. After optimization, beam-on time was reduced by 25%–30% in approximately 40%–50% of eligible lesions when compared with the reference SWS plan (ΔT(max) = 42%). In comparison with the original plan, beam-on time was reduced even further, ΔT > 50% in 20% of cases (ΔT(max) = 70%). CONCLUSIONS: This work demonstrates clinically that optimization using the shot-within-shot technique can reduce beam-on time without degrading treatment plan quality.

The tables included in this article will allow the user to implement shot within shot optimization for Gamma Knife radiosurgery planning and delivery. The method is intended to reduce treatment time when treating small to medium sized brain metastasis. The tables were previously developed by extracting profiles from Gamma Plan for three collimator settings and modeling their behavior when combined or prescribed at different isodose lines. For a given target size, the tables represent the optimal selection of shot weighting and prescription isodose line to reduce beam on time while maintaining an acceptable dose gradient. The method was recently validated in a large patient cohort and the data is this study is related to the research article titled “Clinical evaluation of shot within shot optimization for Gamma Knife radiosurgery planning and delivery” (Johnson et al., in press).

Purpose To assess the performance of a proton-specific knowledge based planning (KBPP) model in creation of robustly optimized intensity-modulated proton therapy (IMPT) plans for treatment of patients with prostate cancer. Materials and Methods Forty-five patients with localized prostate cancer, who had previously been treated with volumetric modulated arc therapy, were selected and replanned with robustly optimized IMPT. A KBPP model was generated from the results of 30 of the patients, and the remaining 15 patient results were used for validation. The KBPP model quality and accuracy were evaluated with the model-provided organ-at-risk regression plots and metrics. The KBPP quality was also assessed through comparison of expert and KBPP-generated IMPT plans for target coverage and organ-at-risk sparing. Results The resulting R2 (mean ± SD, 0.87 ± 0.07) between dosimetric and geometric features, as well as the χ2 test (1.17 ± 0.07) between the original and estimated data, showed the model had good quality. All the KBPP plans were clinically acceptable. Compared with the expert plans, the KBPP plans had marginally higher dose-volume indices for the rectum V65Gy (0.8% ± 2.94%), but delivered a lower dose to the bladder (−1.06% ± 2.9% for bladder V65Gy). In addition, KBPP plans achieved lower hotspot (−0.67Gy ± 2.17Gy) and lower integral dose (−0.09Gy ± 0.3Gy) than the expert plans did. Moreover, the KBPP generated better plans that demonstrated slightly greater clinical target volume V95 (0.1% ± 0.68%) and lower homogeneity index (−1.13 ± 2.34). Conclusions The results demonstrated that robustly optimized IMPT plans created by the KBPP model are of high quality and are comparable to expert plans. Furthermore, the KBPP model can generate more-robust and more-homogenous plans compared with those of expert plans. More studies need to be done for the validation of the proton KBPP model at more-complicated treatment sites.

Purpose/objective Online Adaptive Radiotherapy (ART) with daily MR-imaging has the potential to improve dosimetric accuracy by accounting for inter-fractional anatomical changes. This study provides an assessment for the feasibility and potential benefits of online adaptive MRI-Guided Stereotactic Body Radiotherapy (SBRT) for treatment of liver cancer. Materials/methods Ten patients with liver cancer treated with MR-Guided SBRT were included. Prescription doses ranged between 27 and 50 Gy in 3–5 fx. All SBRT fractions employed daily MR-guided setup while utilizing cine-MR gating. Organs-at-risk (OARs) included duodenum, bowel, stomach, kidneys and spinal cord. Daily MRIs and contours were utilized to create each adapted plan. Adapted plans used the beam-parameters and optimization-objectives from the initial plan. Planning target volume (PTV) coverage and OAR constraints were used to compare non-adaptive and adaptive plans. Results PTV coverage for non-adapted treatment plans was below the prescribed coverage for 32/47 fractions (68%), with 11 fractions failing by more than 10%. All 47 adapted fractions met prescribed coverage. OAR constraint violations were also compared for several organs. The duodenum exceeded tolerance for 5/23 non-adapted and 0/23 for adapted fractions. The bowel exceeded tolerance for 5/34 non-adaptive and 1/34 adaptive fractions. The stomach exceeded tolerance for 4/19 non-adapted and 1/19 for adaptive fractions. Accumulated dose volume histograms were also generated for each patient. Conclusion Online adaptive MR-Guided SBRT of liver cancer using daily re-optimization resulted in better target conformality, coverage and OAR sparing compared with non-adaptive SBRT. Daily adaptive planning may allow for PTV dose escalation without compromising OAR sparing.

Background: Adaptive radiotherapy is being used in few institutions in patients with head and neck cancer having bulky disease using periodic computed tomography imaging accounting for volumetric changes in tumor volume and/or weight loss. Limited data are available on ART in the postoperative setting. We aim to identify parameters that would predict the need for ART in patients with head and neck cancer and whether ART should be applied in postoperative setting. Materials and Methods: Twenty patients with stage III–IV head and neck cancer were prospectively accrued. A computed tomography simulation was done prior to treatment and repeated at weeks 3 and 6 of concurrent intensity-modulated radiotherapy and chemotherapy. The final plan was coregistered with the subsequent computed tomography images, and dosimetric/volumetric changes at weeks 1 (baseline), 3, and 6 were quantified in high-risk clinical target volumes, low-risk clinical target volumes , right parotid , left parotid , and spinal cord . An event to trigger ART was defined as spinal cord maximum dose >45 Gy, parotid mean dose >26 Gy, and clinical target volume coverage

Purpose A phase I clinical trial was designed to test the feasibility and toxicity of administering high-dose spatially fractionated radiation therapy to magnetic resonance imaging (MRI)–defined prostate tumor volumes, in addition to standard treatment. Methods and Materials We enrolled 25 men with favorable to high-risk prostate cancer and 1 to 3 suspicious multiparametric MRI (mpMRI) gross tumor volumes (GTVs). The mpMRI-GTVs were treated on day 1 with 12 to 14 Gy via dose cylinders using a lattice extreme ablative dose technique. The entire prostate, along with the proximal seminal vesicles, was then treated to 76 Gy at 2 Gy/fraction. For some high-risk patients, the distal seminal vesicles and pelvic lymph nodes received 56 Gy at 1.47 Gy/fraction concurrently in 38 fractions. The total dose to the lattice extreme ablative dose cylinder volume(s) was 88 to 90 Gy (112-123 Gy in 2.0 Gy equivalents, assuming an α-to-β ratio of 3). Results Dosimetric parameters were satisfactorily met. Median follow-up was 66 months. There were no grade 3 acute/subacute genitourinary or gastrointestinal adverse events. Maximum late genitourinary toxicity was grade 1 in 15 (60%), grade 2 in 4 (16%), and grade 4 in 1 (4%; sepsis after a posttreatment transurethral resection). Maximum late gastrointestinal toxicity was grade 1 in 11 (44%) and grade 2 in 4 (16%). Two patients experienced biochemical failure. Conclusions External beam radiation therapy delivered with an upfront spatially fractionated, stereotactic high-dose mpMRI-GTV boost is feasible and was not associated with any unexpected events. The technique is now part of a follow-up phase II randomized trial.

Purpose To compare dosimetric characteristics with multiparametric magnetic resonance imaging–identified imaging tumor volume (gross tumor volume, GTV), prostate clinical target volume and planning target volume, and organs at risk (OARs) for 2 treatment techniques representing 2 arms of an institutional phase 3 randomized trial of hypofractionated external beam image guided highly targeted radiation therapy. Methods and Materials Group 1 (n=20) patients were treated before the trial inception with the standard dose prescription. Each patient had an additional treatment plan generated per the experimental arm. A total of 40 treatment plans were compared (20 plans for each technique). Group 2 (n=15) consists of patients currently accrued to the hypofractionated external beam image guided highly targeted radiation therapy trial. Plans were created as per the treatment arm, with additional plans for 5 of the group 2 experimental arm with a 3-mm expansion in the imaging GTV. Results For all plans in both patient groups, planning target volume coverage ranged from 95% to 100%; GTV coverage of 89.3 Gy for the experimental treatment plans ranged from 95.2% to 99.8%. For both groups 1 and 2, the percent volumes of rectum/anus and bladder receiving 40 Gy, 65 Gy, and 80 Gy were smaller in the experimental plans than in the standard plans. The percent volume at 1 Gy per fraction and 1.625 Gy per fraction were compared between the standard and the experimental arms, and these were found to be equivalent. Conclusions The dose per fraction to the OARs can be made equal even when giving a large simultaneous integrated boost to the GTV. The data suggest that a GTV margin may be added without significant dose effects on the OARs.

Advances in magnetic resonance imaging (MRI) sequences allow physicians to define the dominant intraprostatic lesion (IPL) in prostate radiation therapy treatments allowing for dose escalation and potentially increased tumor control. This work quantifies the margin required around the MRI‐defined IPL accounting for both prostate motion and deformation. Ten patients treated with a simultaneous integrated intraprostatic boost (SIIB) were retrospectively selected and replanned with incremental 1 mm margins from 0‐5 mm around the IPL to determine if there were any significant differences in dosimetric parameters. Sensitivity analysis was then performed accounting for random and systematic uncertainties in both prostate motion and deformation to ensure adequate dose was delivered to the IPL. Prostate deformation was assessed using daily CBCT imaging and implanted fiducial markers. The average IPL volume without margin was 2.3% of the PTV volume and increased to 11.8% with a 5 mm margin. Despite these changes in volume, the only statistically significant dosimetric difference was found for the PTV maximum dose, which increased with increasing margin. The sensitivity analysis demonstrated that a 3.0 mm margin ensures >95% IPL coverage accounting for both motion and deformation. We found that a margin of 3.0 mm around the MRI defined IPL is sufficient to account for random and systematic errors in IPL position for the majority of cases. PACS number(s): 87.55.de

Knowledge‐based planning (KBP) can be used to improve plan quality, planning speed, and reduce the inter‐patient plan variability. KPB may also identify and reduce systematic variations in VMAT plans, something very important in multi‐institutional clinical trials. Training of a KBP library is a complex and difficult process, and models must be validated prior to their clinical use. The purpose of this work is to assess the quality of the treatment plans generated using a specific versus combined purpose model KBP library for prostate cancer. Seven KBP model libraries were created from a set of patients treated on various Institutional Review Board (IRB) approved protocols. All KBP libraries were validated using an independent set of twenty patients (half treated Pr: Prostate alone half treated PLN: prostate plus pelvic lymph nodes). Two models were tested on the Pr patients only, four tested on PLN patients only, and one tested on all patients. All plans were normalized such that at least 95% of the prostate planning target volume received 100% of the planned dose. The plans based on different model libraries were compared to each other and the expert clinical plan. For Pr plans there were almost no statistically significant differences (P

The major errors in HDR procedures were failures to enter the correct treatment distance, which could be caused by either entering wrong transmission lengths or imprecisely digitizing the dwelling positions. Most of those errors were not easily avoidable by enhancing the HDR management level because they were caused by implementations of nonstandardized applicators utilizing transmission tubes of different lengths in standard HDR procedures. We performed this comprehensive study to include all possible situations with different nonstandardized applicators that frequently occurred in HDR procedures, provide corresponding situations with standard applicator as comparisons, list all possible errors and in planning, clarify the confusions in offsets setting, and provide mathematical and quantitative solutions for each given scenarios. Training on HDR procedures with nonstandardized applicators are normally not included in most residential program for medical physics, thus this study could be meaningful in both clinical and educational purpose. At precision of 1 mm, our study could be used as the essential and practical reference for finding the correct treatment length as well as locating the accurate dwelling positions in any HDR procedure with nonstandardized applicators.

Purpose: To determine the sensitivity of dose-mass-histogram (DMH) due to anatomical changes of head-and-neck squamous cell carcinoma (HNSCC) radiotherapy (RT). Methods: Eight patients undergoing RT treatment for HNSCC were scanned during the third and sixth week of RT. These second (CT2) and third (CT3) CTs were co-registered to the planning CT (CT1). Contours were propagated via deformable registration from CT1 and doses were re-calculated. DMHs were extracted for each CT set. DMH sensitivity was assessed by dose-mass indices (DMIs), which represent the dose delivered to a certain mass of and anatomical structure. DMIs included: dose to 98%, 95% and 2% of the target masses (PTV1, PTV2, and PTV3) and organs-at-risk (OARs): cord DMI2%, brainstem DMI2%, left_ and right_parotid DMI2% and DMI50%, and mandible DMI2%. A two-tailed paired t-test was used to compare changes to DMIs in CT2 and CT3 with respect to CT1 (CT2/CT1 and CT3/CT1). Results: Changes to DMHs were found for all OARs and PTVs, but they were significant only for the PTVs. Maximum dose to PTVs increased significantly for CT2/CT1 in all three PTVs, but CT3/CT1 changes were only significantly different for PTV1 and PTV2. Dose coverage to the three PTVs was also significantly different, DMI98% was lower for both CT2/CT1 and CT3/CT1. DMI95% was significantly lower for PTV1 for CT2/CT1, PTV2 for CT2/CT1 and CT3/CT1, and PTV3 for CT3/CT1. Conclusion: Changes in anatomy significantly change dose-mass coverage for the planning targets, making it necessary to re-plan in order to maintain the therapeutic goals. Maximum dose to the PTVs increase significantly as RT progresses, which may not be problematic as long as the high dose remains in the gross tumor volume. Doses to OARs were minimally affected and the differences were not significant.

Purpose: Stereotactic body radiotherapy (SBRT) is used for spine treatments as it precisely delivers high radiation dose to tumors in close proximity to organs-at-risk (OARs). The goal of this work is to evaluate dosimetric properties of SBRT for spinal treatments with linear accelerators and CyberKnife (CK). Materials and methods: Plans of 27 patients, treated with CK for spine tumors, were also retrospectively optimized for linac-based (LB) intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). One nine-field IMRT plan and five VMAT plans were generated for each patient. The LB target volumes were uniformly expanded by 0.1 cm to accommodate for the uncertainty in patient positioning. All plans were optimized to cover 90% of the target volumes with a prescription dose of 27 Gy in three fractions. If dose constraints to OARs were not met, the prescription dose was decreased to 24 Gy. Target dose conformity and falloff were evaluated with Paddick's conformity (CI) and gradient (GI) indices. Results: PTV expansion resulted in a 31.5% volume increase in the LB plans. The three full-arcs VMAT (VMAT_3full) plans resulted in the best average CI(0.820) compared to CK(0.758) with worst average from one half-arcs VMAT (VMAT_1half) plans (0.747). Dose falloff was also superior with the VMAT_3full plans with an average GI value of 3.596, in comparison to CK(3.786) and IMRT(4.447). In 6 cases CK plans were unable to meet OAR constraints and the prescription dose was decreased to 24 Gy, compared to only 2 for VMAT_3full. Conclusion: Regardless of the larger target volumes, LB plans were comparable to CK plans. Conformity of target doses of the VMAT_3full plans were better than CK in all cases and dose fall-off was better 23 of 27 plans. Dose to OARs were lower for CK, but constraints met for all plans. The use of VMAT would reduce the treatment time.

PURPOSE The aims of this study were to determine if image-guided robotic stereotactic radiosurgery by CyberKnife® Radiosurgery System using ablative radiation doses achieves acceptable local control in medically inoperable patients with early non–small-cell lung cancer (NSCLC) and to evaluate disease-free survival, toxicity, and failure. CyberKnife® can deliver the prescribed dose by using many different angles converging on the target, with real-time target tracking through a combined orthogonal radiograph imaging and optic motion tracking system (Synchrony®). MATERIALS AND METHODS A review of treatment details and outcomes for 59 patients, ranging in age from 51 years to 96 years, with 61 tumors with histologically proven cancers treated by image-guided robotic stereotactic radiosurgery at the CyberKnife Center of Miami between March 2004 and March 2007 is presented. Target localization and respiratory movement compensation were accomplished using a single fiducial marker placed within the tumor, and the X-Sight TM and Synchrony® systems. Total doses ranged from 15 Gy to 67.5 Gy delivered in 1-5 fractions with an equivalent dose range of 24-110 Gy normalized treatment dose in 2 Gy fractions (α/β = 20 Gy). RESULTS Four patients with stage 1A NSCLC and 2 patients with stage 1B NSCLC had persistent or recurrent disease. All patients tolerated the radiosurgery well, fatigue being the main side effect. Of the 59 patients treated, 51 (86%) were still alive at 1-33–month follow-up. Eight patients have died, 2 of diseases other than cancer progression. CONCLUSION The results indicate that the delivery of precisely targeted ablative radiation doses with surgical precision to limited treatment volumes of lung tumors in a hypofractionated fashion is feasible and safe. Image-guided robotic stereotactic radiosurgery of lung tumors with CyberKnife® achieves excellent rates of local disease control with limited toxicity to surrounding tissues and, in many cases, might be curative for patients for whom surgery is not an option.

When using the CyberKnife® (Accuray Incorporated, Sunnyvale, CA) Image-Guided Stereotactic Radiosurgery (SRS) System to treat soft-tissue tumors in anatomic sites other than intracranial or spinal locations — such as in the lung, liver, kidney, prostate, and pancreas — fiducial placement in or close to the tumors is necessary to assist patient alignment and target tracking for precise treatment delivery. Under the assumption of rigid transformation, at least three fiducial markers are required to obtain six-degreesof-freedom transformation parameters, i.e., three translations and three rotations. However, in most cases, soft tissue is highly deformable and non-rigid. This results in three possible scenarios: 1) the rigid body criteria fail and the rotational transformation cannot be obtained, 2) the tumor deformation results in unreliable computed rotational information, and 3) even when the fiducial array meets the rigid body criteria, the orientation of the tumor often has poor correlation with the global body orientation and thus results in dosimetric deviation.

Purpose/Objective(s): Radiation therapy (RT) is frequently employed to palliate spine metastases. There are multiple randomized trials of single fraction (SF) versus multiple fraction (MF) palliative RT with equivalent pain outcomes in uncomplicated bone (including spine) metastases. However, there are little data to inform incidence and predictors of adverse outcomes after RT for uncomplicated spine metastases, including the possible role of spine instability and RT fractionation. Materials/Methods: In total 337 spine metastases in 258 patients treated with RT at a single institution from 2008 to 2013 were retrospectively reviewed. Previously irradiated sites, complicated spine lesions due to spinal cord compression (SCC) or requiring surgery were excluded. The primary outcome was the time to first adverse event (AE) at the treated site, including: symptomatic vertebral body (VB) collapse, hospitalization for site-related pain, salvage spine surgery, vertebroplasty or other interventional procedure for pain, and SCC. A competing risks survival model was used to simultaneously assess potential predictors, including demographic, disease [e.g., Spinal Instability Neoplastic Score, SINS (0-18)], and treatment (e.g., dose fractionation) factors. Death was the competing risk. Clustering was adjusted for with a random-effects model. Results: Median survival was 160 days; 260 days for those receiving post-RT systemic therapies, 40 days for those who did not; 30Gy/10 was given to 49% of sites, 20Gy/5 to 22%, and 8Gy SF to 9%. Median SINS was 7 (IQR 5 to 9). AEs developed in 46 sites (14%), including 12 symptomatic VB collapses, 32 hospitalizations, 4 salvage spinal surgeries, 14 interventional procedures, and 16 SCCs. Median time to first AE was 121 days (IQR 50-256 days), 75 days for SF RT and 139 days for MF RT. In competing risks MVA, SINS 11 [hazard ratio (HR) Z 2.5 (95% CI 1.3-4.8) p Z 0.006], SF RT [HR Z 6.1 (2.5-15.4), p 2-3 months, 8Gy x 1 SF RT for uncomplicated spine metastases should be avoided; further studies are required. Author Disclosure: T. Lam: None. H. Uno: None. M.S. Krishnan: None. M. Cheney: None. T.A. Balboni: None.

Purpose: Dosimetrically compare CyberKnife (CK) and linac-based (LB) stereotactic body radiotherapy (SBRT) plans for lumbar spine. Methods: Ten patient plans with lumbar spine tumors treated with CK were selected and retrospectively optimized using three techniques: CK, volumetric modulated arc (VMAT, three arcs), and 9-field-intensity modulated radiotherapy (IMRT). For the LB plans, the target volume was expanded by 1mm to accommodate additional uncertainty in patient positioning. All plans were optimized to a prescription dose of 27Gy in 3 fractions covering 90% of the PTV. If the dose constraints to the cauda equina (cauda) were not met, the prescription dose was lowered to 24Gy. Parameters evaluated included Paddick Conformity-Index (CI) and Gradient-Index (GI). A two-tailed paired t-test was used to establish statistically significant differences in cauda doses. Results: Target volumes for LB plans were on average 38% larger. In terms of the indices, the closer the index values to unity the steeper the dose falloff and the higher the dose conformity to the target. The results showed that LB plans were in general statistically superior to CK plans. The IMRT plan showed the best average gradient index of 2.995, with VMAT and CK GI values of 3.699 and 5.476, respectively. Similarly, themore » same trend occurs with the average CI results: 0.821, 0.814, and 0.758, corresponding to IMRT, VMAT, and CK. Notably, in one CK plan the target dose was reduced to 24Gy to meet cauda constraints. Additionally, there was a statistically significant dose difference for the cauda between the CK and LB plans. Conclusion: This study demonstrates that LB plans for lumbar spine SBRT can be as effective or even better than CK plans. Despite the expansion of the target volume, the LB plans did not demonstrate dosimetric inferiority. The LB plans Resultin 2-to-3 fold decrease of treatment time.« less

Absolute PT-mediastinal PT-peripheral LN C RL 1.3 1.0 1.6 0.9 1.0 0.8 1.5 0.7 1.0 0.7 1.8 0.7 1.2 0.9 1.4 1.2 AP 1.4 0.5 1.4 0.6 1.8 1.1 1.6 1.3 1.5 0.4 1.4 0.5 1.1 0.6 1.1 0.7 SI 4.9 2.8 5.7 2.5 3.4 3.2 5.6 3.6 2.9 1.7 4.1 0.7 4.4 1.5 4.7 2.0 Relative to carina PT-mediastinal PT-peripheral LN RL 0.6 0.4 0.6 0.4 0.8 0.5 0.8 1.2 0.4 0.0 0.7 0.6 AP 0.8 0.5 0.8 0.7 0.7 1.0 0.8 0.9 0.6 0.0 0.5 0.4 SI 1.3 1.0 1.7 1.4 2.7 0.2 4.9 3.0 1.2 0.1 1.4 1.6

Purpose: From its early inception, DMLC‐based IMRT has differed from Binary MLC‐based helical IMRT by delivering the treatment volumetrically. As the field of IMRT evolved, the demands on its performance converge to three main criteria: quality of dose plans, speed of treatment delivery, and amount of leakage exposure. Many have ventured the approach of DMLC‐based aperture‐modulated arc (AMAT) therapies, indicating their potential to satisfy these three criteria. The traditional arc therapy is delivered with a constant gantry rotation speed and a constant dose rate. It is logical to expect that if the outputs at any transient gantry position can vary, the capacity or flexibility of dose optimization would increase, regardless of the type of aperture modulation algorithms. The technology of arc delivery with varying gantry rotating speed and dose‐rate has recently become available. We conducted a study to investigate the impact of these new arcing parameters. Method and Materials: The study is conducted on the following equipments: a Varian Trilogy linear accelerator, the Varian Eclipse planning system and the Pinnacle SmartArc module. The linac can operate in constant dose‐rate/gantry rotation mode or varying dose‐rate/gantry rotation mode. Plans were derived with constant dose‐rate/gantry rotation and varying dose‐rate/gantry rotation. The comparisons were carried out as objectively as possible based on the three criteria with set benchmarks. Results: From a limited range of examples studied, it is observed that while AMAT with constant dose rate and gantry rotation is capable of producing good dose plans, dose‐rate and gantry rotation variation allows further improvement of the dose plan and the delivery efficiency. MLC inter‐digitation also increases the flexibility of planning and the delivery efficiency. Conclusion: The quality of plan and the treatment delivery efficiency are inter‐related. While plan evaluation is often subjective, a greater technical flexibility undoubtedly increases planning capacity and improves delivery efficiency.

Purpose: Unequivocally, the experience of Gamma knife has set a solid notion pertaining to the definition and the dosimetric requirement of radiosurgery. Any newer, post-Gamma knife, radiosurgery system has to demonstrate comparable targeting precision and sharp dose gradient characteristics. Statistics shows that the total of number of beams used in Cyberknife treatment is often less than 200. Herein, we present a study to evaluate the dose distribution characteristics comparing with that of the traditional photon-based radiosurgery. Material and Method: An experimental beam path with beam positions (notes) distributed in 2Π solid angle was created to compare the dose distribution of a. Single isocenter plan with different collimator sizes and with the total number of beams ranging from 30 to 205. b. Multiple isocenter conformal plan and non-isocentric inverse conformal plan. c. Multiple non-coplanar beam arrangement and Multiple coplanar beam arrangement Results and Discussion: For single isocenter plans, no significant differences were observed in isodose levels ranging from 90% to 10%, with number of beams ranging from 55 to 205, distributed over 2Π solid angle. With beam number reduced to 50, the noticeable differences fall in the dose levels under 10%. The conformal planning using multiple isocenters increases the number of beams geometrically, while no-isocentric inverse planning can achieve excellent conformity with much less number of beams. However, close attention is needed to assure the dose fall-off is well controlled in all directions. Although the reduction of beam number may not affect significantly the dose gradient in the median-high dose region, to deliver all beams in a single plane does result in a much inferior dose distribution. The attempt to use a single co-planar delivery for radiosurgery is thus to be avoided.