SBRT Dose Research Articles

TU‐A‐304‐02: Treatment Simulation, Planning and Delivery for SBRT

Increased use of SBRT and hypo fractionation in radiation oncology practice has posted a number of challenges to medical physicist, ranging from planning, image‐guided patient setup and on‐treatment monitoring, to quality assurance (QA) and dose delivery. This symposium is designed to provide updated knowledge necessary for the safe and efficient implementation of SBRT in various linac platforms, including the emerging digital linacs equipped with high dose rate FFF beams. Issues related to 4D CT, PET and MRI simulations, 3D/4D CBCT guided patient setup, real‐time image guidance during SBRT dose delivery using gated/un‐gated VMAT or IMRT, and technical advancements in QA of SBRT (in particular, strategies dealing with high dose rate FFF beams) will be addressed. The symposium will help the attendees to gain a comprehensive understanding of the SBRT workflow and facilitate their clinical implementation of the state‐of‐art imaging and planning techniques.Learning Objectives: Present background knowledge of SBRT, describe essential requirements for safe implementation of SBRT, and discuss issues specific to SBRT treatment planning and QA. Update on the use of multi‐dimensional (3D and 4D) and multi‐modality (CT, beam‐level X‐ray imaging, pre‐ and on‐treatment 3D/4D MRI, PET, robotic ultrasound, etc.) for reliable guidance of SBRT. Provide a comprehensive overview of emerging digital linacs and summarize the key geometric and dosimetric features of the new generation of linacs for substantially improved SBRT. Discuss treatment planning and quality assurance issues specific to SBRT. Research grant from Varian Medical Systems

TU‐A‐304‐01: Introduction and Workflow of Image‐Guided SBRT

Increased use of SBRT and hypo fractionation in radiation oncology practice has posted a number of challenges to medical physicist, ranging from planning, image‐guided patient setup and on‐treatment monitoring, to quality assurance (QA) and dose delivery. This symposium is designed to provide updated knowledge necessary for the safe and efficient implementation of SBRT in various linac platforms, including the emerging digital linacs equipped with high dose rate FFF beams. Issues related to 4D CT, PET and MRI simulations, 3D/4D CBCT guided patient setup, real‐time image guidance during SBRT dose delivery using gated/un‐gated VMAT or IMRT, and technical advancements in QA of SBRT (in particular, strategies dealing with high dose rate FFF beams) will be addressed. The symposium will help the attendees to gain a comprehensive understanding of the SBRT workflow and facilitate their clinical implementation of the state‐of‐art imaging and planning techniques.Learning Objectives: Present background knowledge of SBRT, describe essential requirements for safe implementation of SBRT, and discuss issues specific to SBRT treatment planning and QA. Update on the use of multi‐dimensional (3D and 4D) and multi‐modality (CT, beam‐level X‐ray imaging, pre‐ and on‐treatment 3D/4D MRI, PET, robotic ultrasound, etc.) for reliable guidance of SBRT. Provide a comprehensive overview of emerging digital linacs and summarize the key geometric and dosimetric features of the new generation of linacs for substantially improved SBRT. Discuss treatment planning and quality assurance issues specific to SBRT. Research grant from Varian Medical Systems

TU‐A‐304‐03: Quality Assurance, Safety, and Other Practical Aspects of SBRT

Increased use of SBRT and hypo fractionation in radiation oncology practice has posted a number of challenges to medical physicist, ranging from planning, image‐guided patient setup and on‐treatment monitoring, to quality assurance (QA) and dose delivery. This symposium is designed to provide updated knowledge necessary for the safe and efficient implementation of SBRT in various linac platforms, including the emerging digital linacs equipped with high dose rate FFF beams. Issues related to 4D CT, PET and MRI simulations, 3D/4D CBCT guided patient setup, real‐time image guidance during SBRT dose delivery using gated/un‐gated VMAT or IMRT, and technical advancements in QA of SBRT (in particular, strategies dealing with high dose rate FFF beams) will be addressed. The symposium will help the attendees to gain a comprehensive understanding of the SBRT workflow and facilitate their clinical implementation of the state‐of‐art imaging and planning techniques.Learning Objectives: Present background knowledge of SBRT, describe essential requirements for safe implementation of SBRT, and discuss issues specific to SBRT treatment planning and QA. Update on the use of multi‐dimensional (3D and 4D) and multi‐modality (CT, beam‐level X‐ray imaging, pre‐ and on‐treatment 3D/4D MRI, PET, robotic ultrasound, etc.) for reliable guidance of SBRT. Provide a comprehensive overview of emerging digital linacs and summarize the key geometric and dosimetric features of the new generation of linacs for substantially improved SBRT. Discuss treatment planning and quality assurance issues specific to SBRT. Research grant from Varian Medical Systems

Is RCC truly radioresistant? Local control rates of metastatic renal cell carcinoma (RCC) to the lung using stereotactic radiotherapy (SBRT).

445 Background: The use of radiation therapy (RT) for renal cell carcinoma (RCC) is controversial because RCC has traditionally been considered to be “radioresistant”. Based on this concern, current studies have suggested the need for higher doses with conventional fractionation (CF) or the use of SBRT with RCC. The purpose of this study is to determine the efficacy of these modalities by comparing the radiographic and symptomatic local control (LC) rates in patients with metastatic RCC to the lung. Methods: We retrospectively analyzed the radiographic and symptomatic RT response in 27 consecutive RCC patients with 37 RCC lung lesions treated between 2005 to 2014. Twenty-six (70.2%) were treated with SBRT and eleven (29.8%) with CF. Only 9 patients (33.3%) had symptoms prior to RT. Median SBRT dose and fraction was 50 Gy (range 25-60) and 3 (range 1-6) versus CF 30 Gy (range 20-55) and 10 (range 5-22) respectively. Toxicity was evaluated by CTCAE 4.0. Results: Median follow up was 16 months (range 1-102). Rates of radiographic local control with SBRT and CF were 92.3% vs. 55.4% respectively (p=0.0016). Under univariate analysis, predictors for radiographic LR were gross tumor volume (GTV) > 20 cc (p < 0.0001), planned tumor volume (PTV) of > 100 cc (p < 0.0001), and biologic effective dose (BED) < 70 Gy (p=0.0001). Subanalysis of CF demonstrated that a BED < 54 Gy was also a strong predictor for LR (p=0.0012). Under multivariate analysis, PTV was a strong predictor for radiographic LR (odds ratio [OR] = 42.2, p=0.0492). For the 9 symptomatic patients, 3 of 4 (75%) patients in the CF arm and 4 of 5 (80%) in the SBRT arm experienced partial or complete symptomatic relief. The median time to relief of symptoms in the SBRT and CF arms were 0.5 and 2 months, respectively. All patients receiving CF had symptomatic LR compared to none in the SBRT arm (p=0.0124). One patient in the SBRT arm had grade 2 hiccups and one with grade 2 pneumonitis in the CF arm. There were no other grade 2 toxicities or higher. Conclusions: SBRT demonstrated an encouraging local control rate and was capable of safely providing symptomatic relief to patients. The traditional view of radioresistance in RCC may not apply in the era of SBRT.

Integrating stereotactic body radiation therapy in stage II/III non-small cell lung cancer: is local control important?

Local control for advanced non-small cell lung cancer (NSCLC) remains a significant problem with chemoradiation local failure rates in the chest of 30–50%. Despite attempts at dose escalation with conventional radiation therapy techniques, toxicities limit the amount of radiation that can be delivered. For stage I NSCLC, mounting evidence supports the use of hypofractionated radiation therapy (SBRT) to gain high local control rates with acceptable toxicity. For healthy patients with stage II/III NSCLC, the National Comprehensive Cancer Network guidelines suggest surgery is the preferred standard of care for patients with <N2 nodes or T3 tumors. In select patients who are surgical candidates or have more extensive disease, guidelines may include pre-operative chemoradiation followed by surgery, although this remains controversial and is the subject of a current national clinical trial (RTOG 0839). Dose escalation through conventional radiation therapy planning suggests that we can improve outcomes in stage III patients, but toxicity remains problematic. It follows that with improvements in imaging and delivery of radiotherapy, dose escalation with SBRT incorporation may improve local control in stage II/III NSCLC for medically inoperable patients. The rationale for dose escalation and some of the considerations for incorporation of SBRT dose escalation in stage III lung cancer are reviewed here.

Stereotactic Body Radiotherapy in Patients with Previous Pneumonectomy: Safety and Efficacy

There are limited treatment options for patients with prior pneumonectomy and a new lung malignancy. The safety and efficacy of stereotactic body radiotherapy in this subpopulation has not been well defined. Postpneumonectomy patients treated with lung SBRT were identified from a prospective single institution database. Treatment toxicity was recorded prospectively using the Common Terminology Criteria for Adverse Events version 3.0. Disease recurrences were categorized as local, regional, or distant metastatic disease. Overall survival was calculated using the Kaplan-Meier method. Of 406 patients, 13 postpneumonectomy patients were identified and 14 tumors were treated with SBRT. Median age was 69 years. Three lesions were biopsy confirmed. The SBRT doses were 60 Gy/3 (n = 1), 54 Gy/3 (n = 1), 48 Gy/4 (n = 7), 60 Gy/8 (n = 2), and 50 Gy/10 (n = 3). Median follow-up was 24 months. Two patients had grade 3 radiation pneumonitis 3 and 4 months post-SBRT; they died 3 and 1 months later, respectively, one of myocardial infarction and the other of progressive dyspnea thought to be related to congestive heart failure. There were no local failures, one regional failure, and three distant failures. Median survival was 29 months, 1 and 2 year overall survival were 69% (95% confidence interval: 48-100%) and 61% (95% confidence interval: 39-95%), respectively. SBRT in patients with prior pneumonectomy poses challenges because of limited lung reserve. However, local control and long-term survival can be achieved using SBRT in this inoperable population. Careful consideration must be given to radiation planning to minimize the risk of radiation pneumonitis.

Commissioning and verification of the collapsed cone convolution superposition algorithm for SBRT delivery using flattening filter‐free beams

Linacs equipped with flattening filter‐free (FFF) megavoltage photon beams are now commercially available. However, the commissioning of FFF beams poses challenges that are not shared with traditional flattened megavoltage X‐ray beams. The planning system must model a beam that is peaked in the center and has an energy spectrum that is softer than the flattened beam. Removing the flattening filter also increases the maximum possible dose rates from 600 MU/min up to 2400 MU/min in some cases; this increase in dose rate affects the recombination correction factor, Pion, used during absolute dose calibration with ionization chambers. We present the first‐reported experience of commissioning, verification, and clinical use of the collapsed cone convolution superposition (CCCS) dose calculation algorithm for commercially available flattening filter‐free beams. Our commissioning data are compared to previously reported measurements and Monte Carlo studies of FFF beams. Commissioning was verified by making point‐dose measurement of test plans, irradiating the RPC lung phantom, and performing patient‐specific QA. The average point‐dose difference between calculations and measurements of all test plans and all patient specific QA measurements is 0.80%, and the RPC phantom absolute dose differences for the two thermoluminescent dosimeters (TLDs) in the phantom planning target volume (PTV) were 1% and 2%, respectively. One hundred percent (100%) of points in the RPC phantom films passed the RPC gamma criteria of 5% and 5 mm. Our results show that the CCCS algorithm can accurately model FFF beams and calculate SBRT dose distributions using those beams.PACS number: 87.55.kh

SU‐E‐T‐476: 4‐Dimensional Dosimetric Comparison of Stereotactic Body Radiation Therapy for Lung Tumor With Versus Without Tumor Tracking Technique

Purpose: Recently a new treatment device capable of gimbal‐based tumor tracking (Vero, BrainLAB AG, Feldkirchen, Germany) was installed at our institution. The purpose of this study is to compare dose‐volume and radiobiological parameters associated with treatment efficacy and normal tissue toxicity for lung SBRT using a conventional linac with and without tumor tracking. Methods: A digital torso phantom having the capability to simulate respiratory and cardiac motion was used for this work. Spherical targets having a diameter of 20mm were centrally located within the right lung. 12 non‐coplanar, non‐opposing conformal beams were designed for linac planning with and without tumor tracking to deliver 60Gy in 3 fractions, prescribed to cover 95% of the target. 4D dose calculations were performed using the Monte Carlo algorithm in iPlan™ using a voxel tracking technique for the moving target and surrounding tissue. Diaphragm motion was varied from 10–30 mm in the superior‐inferior direction to simulate different motion amplitudes while limiting chest wall motion to 10mm in the anterior‐posterior direction. Dose‐volume analysis was performed using parameters previously shown to correlate with normal tissue toxicity for lung SBRT. Results: All indices used to assess normal tissue toxicity were reduced for the tumor tracking model compared with non‐tracking delivery. Mean lung dose, equivalent ipsilateral lung dose delivered in 2Gy fractions, and V13 were reduced by 17%, 21%, and 22% and 27%, 30%, and 36% for small and large diaphragm motion, respectively. The maximum heart dose was reduced by 7% for small diaphragm motion and by 9% for larger diaphragm motion. Conclusion: This work validates the ability of tumor tracking techniques to increase the dose to moving targets while reducing dose to surrounding normal tissues. It is expected that SBRT dose escalation using tumor tracking techniques will improve local control while reducing rates of normal tissue toxicity.

SU‐E‐T‐197: Is the COMPASS 3D Dosimetry System An Accurate and Safe Quality Assurance Tool for Verifying Stereotactic Body Radiotherapy Dose Delivery for Non‐Small‐Cell Lung Carcinoma?

Purpose: Quality assurance for small field fractionated stereotactic radiotherapy treatment is critical and challenging due to the resolution of detectors used for measurements. We evaluate whether COMPASS system can be used as a good QA tool for SBRT dose delivery. Methods: 40 SBRT NSCLC cases were utilized in this study. PTV size ranged from 3cmx3.4cm to 8.1cmx6.7cm. Prescription doses to PTVs were 30Gy to 60Gy delivered in 3 to 5 fractions. For each case, a representative plan designed in Eclipse AAA using either RapidArc or dynamic‐IMRT was sent to COMPASS for calculating predicted dose. The reconstructed dose distributions were updated by using the measurement corrections. Measurements were made using a MatriXX Evaluation system mounted on the gantry at a SDD of 76.2cm, with a resolution of 7.62mm. The target dose‐volume parameters of D99%(dose received by 99% of target volume), D95%, D85%, D80%, D50%, D30%, D20%, D10%, D1%, average dose, and average gamma were analyzed against that of Eclipse. The OAR dose differences of the average dose and average gamma were also compared. Results: Compared to Eclipse, PTV average dose in COMPASS varied by 0.8% from predicated dose, and by 1.9% from reconstructed dose. PTV dose‐volume parameters show good agreement on predicated dose(±3%) and relatively poorer agreement on reconstructed dose(±7%). PTV dose‐volume parameters in low dose region show a better agreement than that in high dose region. The average gamma differences were up to 0.6 in PTV and 0.15 in OARs. The average absolute dose difference shows a maximum of 0.17Gy in OARs. There is no significant difference between VMAT and IMRT plans. Conclusion: COMPASS can be used as an independent QA tool for small field SBRT. COMPASS predicated doses show a better agreement with Eclipse when compared to reconstructed dose distributions. The limitation of the MatriXX resolution should be considered.

Chasing Liver Tumors in Image-Guided SBRT: Impact on the Delivered Dose

Due to poor tumor contrast on cone-beam CT (CBCT) the liver is used as a surrogate during image-guided SBRT, potentially limiting the accuracy of treatment. The delivered liver SBRT doses for image-guidance strategies based on either whole-liver alignment or the tumor directly were investigated.

Computed 88% TCP Dose for SBRT of NSCLC

Computed 88% TCP Dose for SBRT of NSCLC

Reirradiation of recurrent salivary gland malignancies with fractionated stereotactic body radiation therapy

PurposeThe purpose of this study was to review a single-institution experience with the reirradiation of recurrent salivary gland tumors using fractionated stereotactic radiosurgery (SBRT).MethodsBetween 2003 and 2011, 18 patients diagnosed with recurrent, previously irradiated, salivary gland carcinomas were treated with SBRT reirradiation. Median age was 68 for all patients with most tumors being of major salivary gland origin. Most patients did not undergo surgical resection, and among those that did, all had positive margins. Only seven patients received chemotherapy, and the median SBRT dose was 30 Gy given in five fractions with a median cumulative dose of 91.1 Gy.ResultsThe median overall survival (OS), progression-free survival (PFS), and local control (LRC) were 11.5, 3.5, and 5.5 months, respectively. The 2-year OS, PFS, and LRC rates were 39%, 24%, and 53%, respectively. Statistical analysis identified presence of gross disease and interval to reirradiation as negative predictors of survival outcomes on both univariate and multivariate analyses (p < 0.05). On multivariate analysis, tumor volume was a negative predictor of survival outcomes (p < 0.05). Long-term toxicity analysis revealed four patients in the reirradiated group with soft tissue necrosis, which correlated with the cumulative dose (p = 0.01).ConclusionOur data suggest that SBRT is a reasonable treatment option for reirradiation of salivary gland tumors, but further studies are warranted.

Abstract IA16: A prospective trial of SBRT boost for stage III non-small cell lung cancer

Abstract Purpose/Objective: Escalation of radiation dose in stage III lung cancer has been problematic and no prospective studies have confirmed a survival benefit for patients receiving &amp;gt;60Gy concurrent with chemotherapy. Nonetheless, conventional treatment approaches may not deliver adequate doses to the primary tumor. We report on the feasibility of using SBRT as a means of dose escalation of the primary tumor mass for patients found to have resolution of mediastinal nodal disease following conventional chemoradiotherapy (CRT). The goal was to deliver a biological effective dose (BED) of more than 100 Gy to the primary tumor mass (CRT + SBRT boost doses combined). Methods: Patients with biopsy proven stage II/III NSCLC underwent CT or PET-CT scanning one month following completion of conventional CRT (59.4 Gy or greater). Eligible patients were required to have achieved either negative or only minor FDG residual uptake in nodal disease within the mediastinum. Patients with PET positive residual disease (≤7 cm) within the site of the primary tumor were included. Tumor volumes were defined using 3d planning based on the location of the initial tumor and PET uptake. The prescription dose was 10 Gy per fraction X 2 (20 Gy total dose, BED10 =112Gy combined CRT and SBRT boost dose) and later patients with tumors touching the mediastinum as per RTOG definition received SBRT to the residual tumor of 650cGy X 3 (1950 total dose). Patients underwent routine follow-up with repeat CT scans every 3 months or PET/CT as indicated. The primary endpoints were toxicity and tumor response. Target enrollment is 37. Results: 35 patients (33 reviewed here) have been enrolled with a median age 63. Stages included were IIB (n=4), IIIA (n=18), and IIIB (n=13). Additionally, 9 patients were screened by 1 month post-CRT PET/CT and found to be not eligible: 2 patients with a CR after initial therapy and 7 with significant residual uptake in the mediastinum or metastatic disease. Pathologies included squamous cell carcinoma (75%), adenocarcinoma (11%), large cell carcinoma (10%) and lung cancer NOS (4%). Median combined BED was 110 Gy (range 110–120). Mean PTV volume was 51.3 cm3 (range 46–2571cc) and average treatment coverage to 20 Gy prescription dose was 96.8%. Grade &amp;lt;3 toxicities have been very low. Only 4 local failures have occurred. Of these, two presented approximately 12 months after treatment with grade 5 toxicity, both of whom had large cavitary recurrences of their malignancies which involved the hilum. One patient had an SBRT boost volume encompassing the hilum, the other did not, but both anecdotally died of pulmonary hemorrhage. Of the 33 patients reported here, 16 patients had “medial” tumors of which 10 received 1000cGy X 2 boosts to the hilum. As a safety precaution, for patients with tumor volumes touching the hilum, a revised SBRT dose of 650cGy X 3 (1950cGy total boost, BED10 =102Gy) was delivered. Thus far, 6 patients received 650cGy X 3 boost that included the hilum in most cases. No toxicities have been noted in these patients. Conclusions: The use of linac-based SBRT boost as a means of dose escalation for limited residual NSCLC following definitive chemoradiotherapy is a feasible treatment approach with minimal toxicity.

A Dosimetric Comparison of Brachytherapy versus Tomotherapy versus Stereotactic Body Radiotherapy (SBRT) For The Delivery Of A Rectal Tumor Boost

trewith 26 Gy in 4 consecutive dailyfractions prescribedto the deepest radial margin of the tumor. These CT scans were acquired with the endorectal applicator in placewhich distends the rectal wall and facilitates rectal immobilization and tumor localization. From these scans, Tomotherapy and 7-field linac-based SBRT dose distributions were generated for the same dose schedule of 26 Gy in 4 fractions. SBRT plans were prescribed to an isodose envelope covering 95% of the target and ensuring 99% of the target receives a minimum of 90% of the prescribed dose. For this comparative dosimetric study, the PTV was considered equal to the GTV for all plans. One-way ANOVA testing was used to compare mean values observed for select dose-volume parameters. Results: All modalities achieved complete coverage of the target by the prescription dose. The target near maximum dose (D2%) was 175.9 Gy, 26.7 Gy and 29.1 Gy for brachytherapy, Tomotherapy and SBRT respectively ( p= 0.00). For the brachytherapy, Tomotherapy, and SBRT plans, respectively: the conformality index (prescription vol (cc)/target vol (cc)) was 4.1, 1.5 and 1.5 ( p= 0.01) and the homogeneity index ((D2%-D98%)/D50%) was 2.79, 0.03 and 0.13 ( p= 0.00). Conclusions: The Tomotherapy and SBRT plans provided better target conformality and homogeneity compared to endorectal brachytherapy. Brachytherapy was also associated with significantly higher doses to the uninvolved rectal wall and adjacent anal canal. Conformal external beam techniques may be preferable for boost delivery, particularly with patients for whom the irradiated tissue will not subsequently be resected.

MO-D-BRB-08: Optimization of the Collimator Design for a Novel Stereotactic Radiotherapy Device for Breast Cancer

Purpose: A dedicated stereotactic gamma irradiation device, GammaPod from Xcision Medical Systems, has been developed to treat breast cancer. An optimization of the collimator design is presented for the hemi-spherical source carrier housing the Co60 irradiators. Methods: The GammaPod unit contains 36 Co-60 sources modulated by 36 tungsten collimators positioned along 6 columns in a non-coplanar and isocentric arrangement. The rotating collimator structure accommodates two different sizes of circular holes in addition to a position where sources are fully blocked. 9 dual-collimator arrangements with diameter combinations of 5–15, 5–25, 5–30, 10–20, 10–25, 10–30, 15–25, 15–30 and 20–30 millimeters were investigated. Treatment plans were generated for five patients using MC dose distributions following a pre-operative SBRT strategy and dose fractionation of 3×20Gy. The collimator arrangements are evaluated in terms of treatment plan quality and treatment efficiency. Results: Of the nine different arrangements, four (5–15, 5–25, 5–30 and 10–20 milllimeters) resulted in treatment lengths in excess of 30 minutes (range 40.6–59.7). In the remainder, treatments required 16.4, 21.9, 22.6, 27.4, 28.7 minutes for 20–30, 15–25, 15–30, 10–25 and 10–30 millimeter collimators respectively. All arrangements achieved target dose coverage goals (D95=DRx). Low dose spillage to ipsilateral breast, defined as V50% and V25% were least for the 15–25mm collimator, 8.3% and 15.6% respectively, while 20–30mm was only slightly higher, 8.4% and 16.3% (V50%=[8.3–9.0%] and V25%=[15.6–19.3%]). Among all five patients, the differences in all dosimetric parameters were statistically insignificant for these two collimators (p=0.47, 0.50 and 0.47 for V25%, V50% and V100%). The only significant parameter was treatment duration with population averages of 14.6 minutes for 20–30mm versus 26.7 for 15–25mm (p < 0.01). Conclusions: Among all collimator arrangements, no significant dosimetric advantage was observed with any collimator size. However, the 20–30mm collimator set was deemed most practical due to shortest treatment time required. Some co-authors declare employment and shareholder interests with Xcision Medical Systems.

SU‐E‐T‐883: Clinical Application of Monte Carlo for SBRT: Mediastinal Lung

Purpose: Treatment planning systems have provided Monte Carlo dose calculations for several years but many physicians are still hesitant to use them clinically due to lack of data. Therefore we recalculated 200 Ray Tracing treatment plans using Monte Carlo with heterogeneity corrections and compared to SBRT dose tolerance limits. Methods: From among these 200 CyberKnife cases, 25 mediastinal lung cases are presented in this study. An extensive literature review obtained 105 published SBRT dose tolerance limits for the mediastinal critical structures aorta, bronchi, esophagus, heart, and trachea. These limits were partitioned into high‐risk and low‐risk categories. The DVH Evaluator software tool was used to generate DVH Risk Maps for these critical structures, which superimpose a) published dose tolerance limits b) unified high‐risk and low‐risk trends and c) published adverse event doses, onto Monte Carlo patient data to assess risk of adverse events. Results: Recalculated treatment plan data is within the expected range of published SBRT dose tolerance limits, providing optimism for clinical use. None of the patients experienced any Grade 3 or higher adverse events. The low‐risk dose tolerance limits were exceeded 22 times in these cases with no severe adverse event, thus helping to validate their safety. Conclusions: The range of doses calculated by Monte Carlo for our historical patient data is compatible with published SBRT dose tolerance limits. SBRT dose tolerance limits should be fine‐tuned by Monte Carlo dose calculations in long‐term statistical followup studies.Disclosure: The first author has developed the DVH Evaluator software.

MO-D-BRB-07: Phantom Validation and Clinical Application of Monte Carlo for Small Field SBRT

Purpose: Small field dosimetry is challenging in homogeneous medium and extremely difficult in an inhomogeneous medium. Monte Carlo dose calculation algorithms are considered as the most accurate for treatment planning. We present our validation of the Monte Carlo algorithm in the Accuray Multiplan system using measurements in a cork phantom. We also recalculated Ray Tracing treatment plans with the Monte Carlo algorithm and compared to SBRT dose tolerance limits. Methods: In our validation measurements with a cork phantom, an Exradin A16 ion chamber was used for collimators from 60mm to 20mm on a CyberKnife, and a PTW 60012 stereotactic diode for collimators from 60mm to 5mm. A literature review of more than 500 published SBRT dose tolerance limits was partitioned into high-risk and low-risk categories. Two hundred CyberKnife treatment plans were recalculated using Monte Carlo and compared to the dose limits. The DVH Evaluator software tool was used to generate DVH Risk Maps for 25 critical structures throughout the body, which superimpose a) published dose tolerance limits b) unified high-risk and low-risk trends and c) published adverse event doses, onto Monte Carlo patient doses to assess risk of adverse events. Results: The Monte Carlo calculations matched the Exradin A16 measurements to within 2.5% for field sizes down to 20mm, and matched the PTW 60012 measurements to within 2.5% for all field sizes down to 5mm. Recalculated treatment plan data is within the expected range of published SBRT dose tolerance limits, providing optimism for clinical use. Conclusions: The Accuray MultiPlan Monte Carlo algorithm is accurate even for small fields in heterogeneous media. The range of doses calculated by Monte Carlo for our patient data is compatible with published SBRT dose tolerance limits. SBRT dose tolerance limits should be fine-tuned by Monte Carlo dose calculations in long-term statistical followup studies. Disclosure: The first author has developed the DVH Evaluator software.

Effect of Breathing Motion on SBRT Dose Accumulation within the Human Thorax

Effect of Breathing Motion on SBRT Dose Accumulation within the Human Thorax

SU‐GG‐T‐547: New Measures for Evaluating SBRT Dose Distribution Quality

Purpose. To define a measure that indicates how well the dose distribution in SBRT treatment satisfies standards of dose symmetry and compactness required by SBRT RTOG protocols. Method. Any SBRT dose has to provide the adequate coverage of the PTV and away from PTV it has to decrease fast enough to minimize the volume of tissue in the organ in which the tumor is contained (e.g. lung, liver) (see RTOG 0236). To measure the tightness of dose around PTV one may create a sequence of regions that increase their detachment from PTV quantifying the dose in subsequent regions away from the target. The convenient approach is to build histograms based on a sequence of shells that wrap the target one after the other. This can be achieved by creating sequence of shell structures in the TPS system (script in case of Pinnacle) for which values of minimal, maximal and average dose will be automatically assigned and displayed in the form of appropriate histogram. Results. The script has been written for Pinnacle 8.0m and examples of dose distributions for sequence of SBRT plans have been analyzed by means of computed for these plans histograms. Histograms provide natural means for verification of SBRT RTOG parameters denoted as R50 and D2cm. These histograms provide, however, also additional information on symmetry and steepness of dose gradient in SBRT dose distributions. Conclusion. As in SBRT the quality of treatment depends on the location relative to the target of volumes of tissue exposed to given dose levels measures that go beyond DVH and quantify spatial properties of dose distribution in tissue are needed for the assessment of SBRT treatment quality.

SU‐GG‐T‐388: Assessing Dose Inaccuracies in the Superposition/Convolution Algorithm with Monte Carlo for SBRT Lung Tumors

Purpose: To assess dose inaccuracies in the superposition/convolution dose algorithm with Monte Carlo and how it affects SBRT lung treatments. Materials &amp; Methods: The treatment strategy for the SBRT tumors at MGH is to use 3DCRT for tumors with motion amplitude of less 5mm. The SBRT treatment planning uses either 2 or 3 non‐coplanar arcs, with a total of 8–10 beams. Following the dose planning guidelines provide by RTOG 0813 and 0618, the PTV is conformed with an isodose between 60% and 80% value. This will guarantee that the penumbra tail does not extend to deeply into the lung, while manipulating the hotspots of 120–140%, such as to situate them within the PTV region. SBRT dose calculation is performed with the commercial CMS superposition/convolution algorithm (SCA) and verified with in‐house developed Monte Carlo (MC) dose engine. SCA was also compared with MC in heterogeneous GAMMEX phantom to assess in a wide range of heterogeneous materials the accuracy of SCA. Results: A cohort of 10 SBRT patients was studied with both SCA and MC dose algorithms comparing PTV coverage after dose normalization. The SCA was shown to incorrectly estimate the dose in the PTV by 1–3% relative to the MC prediction, before normalization. The SCA differences occurred when the tumor was situated very close to a bone structure, such as rib cage. Similar results were observed in the irradiation of the heterogeneous phantom. The SCA dose algorithm inaccurately predicts doses if the region is shadowed by a high density material such as bone. Conclusions: Lung Cancer is the number one cause of cancer mortality in both men and women. Inaccuracies in the SCA dose computation can affect the treatment outcome of SBRT. This study suggests that accurate dosimetry in SBRT treatment of lung patients requires the use of Monte Carlo techniques.