Flattening Filter-free Beams for SBRT: Advantages and Risks

Flattening Filter-free Beams for SBRT: Advantages and Risks

S826 International Journal of Radiation Oncology  Biology  Physics 3616 Materials/Methods: Patients with brain metastases in RPA classes I and II...

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S826

International Journal of Radiation Oncology  Biology  Physics

3616

Materials/Methods: Patients with brain metastases in RPA classes I and II were randomized to single dose of SRS (Arm A) and 2 fractions of SRS (Arm B). Eligible patients were required to have 3 or less metastasis and largest tumor size should be less than 4 cm. Patients were excluded if RPA score is 3 or if neurological functional status was 3 or more. The primary tumor location was lung in 50 patients, breast in 10 patients, and other sites in 12 patients. SRS dose was decided according to tumor size. If tumor size was 2 cm or less, SRS dose was 22 Gy single dose or 2x14 Gy. If tumor size was between 2.1-3 cm, SRS dose was 18 Gy single dose or 2x12 Gy. If tumor size was between 3.1-4 cm, SRS dose was 15 Gy single dose or 2x10 Gy. All patients received whole brain irradiation of 25 Gy in 10 fractions within 2 weeks after completion of SRS. Results: A total of 72 patients were enrolled in this trial from 3 centers of Turkey. The median follow-up is 6.7 months (range between 0.5-23.6). The clinical features of two arms were similar with regard to RPA score, ECOG performance status, tumor size and number of metastases. Twenty-two patients died of disease. One-year actuarial local control rate was 72.4% for all patients. There was no significant difference between 1-year actuarial local control rate of two arms (69.1% for arm A vs. 58.4% for Arm B, p Z 0.5). We could not find any prognostic factors that may affect local control. One year overall survival for all patients was 46.8%. There was no significant difference between two arms (40.3% for Arm A vs. 58.4% for Arm B, p Z 0.35). The neurological functional status, performance status and tumor size were found to be significant prognostic factors determining overall survival, whereas RPA scores did not have any significant impact on survival. We observed only one brain necrosis around lesion in Arm A. Conclusions: Our randomized study did not show any difference of single fraction SRS and two fractions SRS with regard to local tumor control, overall survival and toxicity in the management of brain metastases. Author Disclosure: M. Cengiz: None. G. Ozyigit: None. Y. Guney: None. A. Mayadagli: None. G. Guler: None. M. Kocak: None. N. Ozseker: None. B. Kucukpilakci: None. M. Gultekin: None.

Dosimetric Effect of Implanted Hardware in Spine SBRT: Comparison of the Actual Planned Dose Using Pencil Beam Calculation to Monte Carlo T. Djemil,1 M. Ouzidane,1 S. Unal,1 C.A. Reddy,1 L. Angelov,2,3 J.H. Suh,1,3 and S.T. Chao1,3; 1Department of Radiation Oncology, Taussig Cancer Institute, Cleveland Clinic, Cleveland, OH, 2Department of Neurosurgery, Neurological Institute, Cleveland Clinic, Cleveland, OH, 3 Rose Ella Burkhardt Brain Tumor and Neuro-Oncology Center, Neurological Institute, Cleveland Clinic, Cleveland, OH Purpose/Objective(s): Spinal instrumentation is increasingly common in the treatment of degenerative disk disease, spondylolisthesis, trauma, infection, and tumors. A wide range of fixation devices may be implanted in the spine during these procedures. The purpose of this study is to evaluate the dosimetric effects of this hardware in spine stereotactic body radiation therapy (sSBRT) given the strict spinal cord dose restrictions. Materials/Methods: Previously treated sSBRT cases with implanted hardware surrounding the target were randomly selected from an IRBapproved database. Twenty-eight patients with 32 spinal lesions were evaluated. The 32 treatment plans were generated using the pencil beam algorithm with heterogeneity correction (PBHC). The average prescription dose (PD) was 15.4Gy [12Gy-16Gy]. The criteria for a plan to be acceptable were D90  PD for the CTV, V10  10% for the cord (V12  10% for the cauda equina), and the cord max point dose CMD  14Gy (CMD  16Gy for the cauda equina). In order to study the dosimetric effects of hardware, each of these plans was recalculated using Monte Carlo algorithm with heterogeneity correction (MCHC), as it is considered the gold standard in dose computation, especially in heterogeneous media. Dosimetric endpoints were extracted for the CTV (min, mean, max, and D90 doses) and the cord (D0.1cc, D1cc, V10 or V12, and CMD). These endpoints were compared to those from the clinically approved plans. Paired t-tests were used to compare the dosimetric endpoints obtained from the two calculation algorithms, PBHC versus MCHC. Results: For each of these endpoints, a significant difference was noted between the original PB and the MC plans (p<0.001). Overall, the dose endpoints from MCHC were found to be hotter than predicted by PBHC. The ratios of min, mean, max, and D90 doses from PBHC to MCHC were 0.941 [0.466-1.107], 0.966 [0.727-1.003], 0.949 [0.904-0.987], and 0.979 [0.890-1.061], respectively. A similar trend was observed for the cord: the ratios of D0.1cc, D1cc, and V10 or V12 were 0.933 [0.163-1.050], 0.945 [0.818-1.000], and 0.606 [0.033-0.878], respectively. V10 or V12 cord constraints were violated in 53.1% of the plans. The CMD constraints were also violated in 21.9% of the cases. Conclusions: Based on the results of this study, significant discrepancies in sSBRT dose to CTV and cord were observed when implanted hardware surrounds or is in the target. This typically resulted in the PBHC underestimating the dose and possibly resulting in a higher dose delivered to the spinal cord. A PBHC algorithm may not be adequate for dose calculation in this situation. A more robust algorithm such as Monte Carlo is strongly recommended. Author Disclosure: T. Djemil: None. M. Ouzidane: None. S. Unal: None. C.A. Reddy: None. L. Angelov: F. Honoraria; Brainlab. J.H. Suh: G. Consultant; Abbott Oncology. S.T. Chao: None.

3617 Comparison of Single Versus 2 Fractions of Radiosurgery for Brain Metastases: A Prospective Randomized Study M. Cengiz,1 G. Ozyigit,1 Y. Guney,2 A. Mayadagli,3 G. Guler,2 M. Kocak,3 N. Ozseker,3 B. Kucukpilakci,2 and M. Gultekin1; 1Hacettepe University, Faculty of Medicine, Department of Radiation Oncology, Ankara, Turkey, 2 Ankara Oncology Hospital, Ankara, Turkey, 3Kartal Education and _ Research Hospital, Istanbul, Turkey Purpose/Objective(s): To evaluate response rate and toxicity of single versus two fractions of stereotactic radiosurgery (SRS) for brain metastases in a prospective randomized study.

3618 Flattening Filter-free Beams for SBRT: Advantages and Risks W. Verbakel, C. Ong, S. Senan, J.P. Cuijpers, B.J. Slotman, and M. Dahele; VU University Medical Center, Amsterdam, Netherlands Purpose/Objective(s): Stereotactic body radiation therapy (SBRT) for lung and spine can require long treatment times. Recently, flattening filter free (FFF) with dose rates up to 2400 MU/min became available. This abstract describes the potential reduction in delivery times using FFF beams and presents data on the dosimetric effects of intrafraction patient motion and interplay, for spine and lung SBRT, respectively. Materials/Methods: Our earlier work evaluated plan quality and delivery time between FFF and FF plans for 10 lung and 10 spine SBRT patients. This demonstrated a reduction in average beam-on time from 3.6 (1.000 MU/min; FF) to 2.5 min (2.400 MU/min; FFF) for lung SBRT and from 6.7 (FF, range 3.7-10.7) to 2.8 min (FFF, 2.5-3.4) for spine SBRT. For investigated PTV lengths up to 11cm, FFF beams did not result in significant differences in plan quality. Three of the spine SBRT plans were selected to study in the TPS the effect of intrafraction patient shifts during delivery of 2 and 5 mm, lasting for 5-30 seconds. For each patient, the arc was first divided in 20 segments for which doses to spinal cord (sc) and sc+2mm were calculated. Shifts were performed at segments delivering the steepest dose gradients, and the dosimetric effects were calculated. These were summed with the remainder of the arc dose. For one patient, the FFF arc was divided into 7 segments and for each the effect of a 5mm shift lasting 10s was calculated. Maximum differences in GTV coverage and spinal cord (sc) maximum doses were assessed for FFF and FF plans. For two lung patients, each with 2 plans of 1 or 2 arcs, interplay between the moving tumor and moving MLC was investigated for tumor amplitudes of 20-25 mm for FF and FFF. Gamma evaluations (g, 3%, 1mm) were performed for each arc between measurements in a moving phantom and measurements in a static phantom convoluted with the motion pattern. Results: Intrafraction shifts of 2 mm for 10 s maximally increased sc doses on average by 15% for FFF and 4% for FF plans. Shifts of 5 mm for 5 s

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(simulating a large, but low probability shift e.g. due to coughing or pain surge) increased sc doses by on average 20% for FFF (max 26%) and 8% for FF plans (max 12%). The effect of 5mm shift for 10 s at different gantry angles showed that the increment of spinal cord dose was on average 22% (8-34%). No interplay effects were found for each arc of FF plans: g>1 was <1% but FFF plans exhibited more interplay: g>1 for 416% per arc and max dose deviation of 4-11%. Conclusion: Substantial reductions in delivery time are possible with FFF plans, especially for spine SBRT. However, fast FFF delivery can increase the dosimetric effect of intrafraction shifts and requires careful monitoring of patient stability. In addition, preliminary results indicate that interplay effects for moving tumors may be larger for FFF than for FF beams. Acknowledgment: This research was partly funded by Varian Medical System. Author Disclosure: W. Verbakel: H. Travel Expenses; Varian Medical Systems. C. Ong: None. S. Senan: E. Research Grant; Varian Medical Systems. F. Honoraria; Varian Medical Systems. H. Travel Expenses; Varian Medical Systems. J.P. Cuijpers: None. B.J. Slotman: E. Research Grant; Varian Medical Systems. F. Honoraria; Varian Medical Systems. H. Travel Expenses; Varian Medical Systems. M. Dahele: F. Honoraria; Varian Medical Systems. H. Travel Expenses; Varian Medical Systems.

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Hypofractionated Stereotactic Radiation Therapy for Symptomatic or Recurrent Meningioma: Two French Anticancer Center Experience M. Sunyach,1 C. Carrie,1 P. Bondiau,2 and C. Ginestet1; 1Centre Leon Berard, Lyon, France, 2Centre Antoine Lacassagne, Nice, France

A Dosimetric and Clinical Evaluation of Monotherapy With Hypofractionated Stereotactic Body Radiation Therapy in the Treatment of Intermediate-Risk Prostate Cancer A.W. Ju, S. Suy, S. Lei, E.K. Oermann, B.A. Sherer, H.N. Hanscom, J.S. Kim, J.H. Lynch, A. Dritschilo, and S.P. Collins; Georgetown University Hospital, Washington, DC Purpose/Objective(s): SBRT offers to improve the toxicity profile of radiation therapy for patients with clinically localized prostate cancers by limiting the volume of normal tissues exposed to high radiation doses and by taking advantage of the potential radiobiological benefits of hypofractionation. Prior prospective studies have demonstrated the efficacy of hypofractionated SBRT in the treatment of low-risk prostate cancer. This study examines the dose distribution of this modality in the treatment of intermediate-risk prostate cancer, where the dose distribution outside the prostatic capsule requires greater attention to ensure adequate coverage of potential extracapsular extension (ECE). Materials/Methods: Clinical records of 41 hormone naı¨ve intermediaterisk (T2b-T2c, Gleason 7 and/or PSA 10-20 ng/mL) prostate cancer patients who received fiducial-based image-guided robotic SBRT were reviewed. The prescribed dose was 35 or 36.25 Gy in 5 fractions to the planning target volume, defined as a 5 mm expansion from the prostate and proximal seminal vesicles except a 3 mm posterior expansion into the rectum. The distance from the prostatic capsule to the 33 Gy isodose line (corresponding to a BED of 66 Gy in conventionally fractionated radiation therapy) was measured in 45 degree intervals around the prostate at the mid-prostate, 1 cm from the base, and 0.5 cm from the apex. The

Poster Viewing Abstract 3619; Table

Axial level of the prostate 1 cm caudal to base

Mid-prostate

0.5 cm cranial to apex

equivalent dose to 66 Gy was chosen as this is the dose shown in postprostatectomy studies to be effective for treating microscopic ECE. PSA levels and clinical toxicities (CTCAEv4) were assessed at baseline and on follow-up. Results: The median 33 Gy isodose line extends 5 mm beyond the prostate capsule in all directions except posteriorly into the rectum. Shown in the table is the margin distribution for the 33 Gy isodose line. At a median follow-up of 22 months the progression-free survival rate was 97.6%. No Grade 3 or higher acute or late toxicities were observed. Conclusions: Pathologic studies estimate that microscopic disease extends 4 mm beyond the prostatic capsule in less than 5-10% of unfavorable-risk patients. We show that hypofractionated SBRT can deliver a clinically effective dose for microscopic ECE in most patients. Our results suggest that hypofractionated SBRT is a potentially safe and effective treatment for intermediate-risk prostate cancer. Prospective studies are needed to confirm our findings. Author Disclosure: A.W. Ju: None. S. Suy: None. S. Lei: None. E.K. Oermann: None. B.A. Sherer: None. H.N. Hanscom: None. J.S. Kim: None. J.H. Lynch: None. A. Dritschilo: None. S.P. Collins: H. Travel Expenses; Travel expenses for clinical talks covered by Accuray.

Background: Hypofractionated stereotactic radiation therapy for meningioma is under-investigated while radiosurgery (single fraction) as well as standard fractionated radiation therapy up to 50-54 Gy are more published. The aim was to investigate retrospectively the treatment results of intracranial meningiomas treated with hypofractionated stereotactic radiation therapy in two French anticancer centers (20072011). Methods: Forty-nine patients with 51 intracranial non-malignant meningiomas were treated by hypo fractionated stereotactic radiation therapy in five fractions using radiosurgical resection was performed in 22 patients. Among them, 14 had Grade I lesion, 1 had Grade II tumors and 7 lesions were diagnosed as Grade III according to the World Health Organization classification. Twenty-seven (27) lesions were diagnosed as meningioma based on radiological findings. Forty-one (41) patients (83%) presented with symptomatic meningioma at the time of the radiation therapy. The median largest diameter of planning target volume was 25mm (range, 1460). The total prescribed dose ranged from 25 to 40 Gy (median 35 Gy) administrated in five fractions, delivered every day or every two days on isodose 80%. Results: The median follow-up was 12 months (range 6 to 50) Neurologic improvement occurred in 17/41 (41.4%) symptomatic patients and 8 (8/49, 16%) presented worsening symptoms at last follow up. On these 8 patients, three were very young patients with aggressive meningioma, including two patients with progressive disease just after radiation therapy. Two other

The distance from the prostatic capsule to the 33 Gy isodose line

Direction of the distance measurement

Median distance from the capsule to the 33 isodose line

Anterior Left/right Left posterior-lateral/right posterior-lateral Anterior Left/right Left posterior-lateral/right posterior-lateral Anterior Left/right Left posterior-lateral/right posterior-lateral

7.7 mm 7.8 mm 10.4 mm 8.1 mm 7.5 mm 7.1 mm 9.1 mm 12.8 mm 6.8 mm

5th percentile of the distance 3.4 5.3 4.2 5.1 4.9 3.5 6.1 7.6 3.0

mm mm mm mm mm mm mm mm mm

25th percentile of the distance

75th percentile of the distance

5.9 mm 6.3 mm 6.9 mm 6.0 mm 6.2 mm 5.4 mm 7.7 mm 10.5 mm 5.1 mm

9.2 mm 10.2 mm 14.4 mm 11.2 mm 9.3 mm 9.6 mm 11.7 mm 14.2 mm 8.8 mm

95th percentile of the distance 13.5 12.7 23.9 14.2 13.8 12.7 14.1 19.9 13.3

mm mm mm mm mm mm mm mm mm