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Linac With Flattening Filter Free (FFF) Mode Based Virtual HDR Treatment for Localized Prostate Cancer: A Dosimetric Feasibility Study
S912
International Journal of Radiation Oncology Biology Physics
-0.7); the remaining patient presented with 32% GTV increase and 0.6 T/S ratio increase. In patients with no LR at last follow up, 21% presented a GTV increase (median 4%, range Z 1; 57%) associated in all cases with an increased T/S ratio (median 0.7, range: 0.5; 1.34). Volume increase was generally more important in patients that ultimately did recur after surgical pathology confirmation (median 19% vs 4%, range Z 2; 205% vs. 1; 57%). Decreased GTV was observed in 79% of lesions with no LR (median -13%, range Z -2; -55%) associated with an increased T/S ratio in 63% of cases (median 0.5, range Z 0.3; 0.8) and decreased T/S ratio in 37% of cases (median -1.3 range Z -0.1; -2.7). Conclusions: Early (3-6 month) changes in T1 and T2 volume seem to correlate with subsequent local failure. Patients with what may be small increases in tumor volume on the first post-SBRT MRI should be followed more closely for LR. Early T2 signal changes do not appear to correlate with LC. Author Disclosure: H. Bahig: None. D. Simard: None. L. LetourneauGuillon: None. D. Roberge: None. D. Donath: None. P. Wong: None. E. Filion: None. D. Beliveau-Nadeau: None. R. Doucet: None. P. Nicholson: None. L. Masucci: None.
Author Disclosure: T. Telivala: None. W. Matthew: None. D. Schwartz: None. D. Schreiber: None.
3781 Linac With Flattening Filter Free (FFF) Mode Based Virtual HDR Treatment for Localized Prostate Cancer: A Dosimetric Feasibility Study T. Telivala, W. Matthew, D. Schwartz, and D. Schreiber; Department of Veterans Affairs, Brooklyn, NY Purpose/Objective(s): Both stereotactic body radiation therapy (SBRT) as well as high dose rate brachytherapy (HDR) are being used with increasing frequency as a definitive treatment for early stage prostate cancer. Prior studies have reported that SBRT based plans were unable to achieve comparable intraprostatic doses while sparing the rectum and bladder. In this study we tested our ability to approximate the dosimetric parameters for targets and critical organ constraints using HDR for use by linear accelerator (Linac with FFF mode) based radiosurgery planning using the Flattening Filter Free mode. Materials/Methods: 10 randomly selected patients with common contour sets were planned using both simulated HDR catheters for the HDR brachytherapy plan and LINAC based virtual SBRT plans. LINAC based plans were performed using commercially available external beam TPS and HDR plans were performed using Brachytherapy TPS. The LINAC based plans were developed to resemble HDR dose distributions. The treatment delivery method comparisons were made utilizing PTV V100, V125, V150 and V200 coverage as well as normal tissue exposure (rectum, urethra, and bladder). Comparisons were made between the two plans using the two sided t-test. Statistical significance was defined as p<0.05. Results: Planning Target Volume (PTV) coverage for the prescription dose of EBRT with FFF based SBRT was slightly better than HDR based planning regarding V100 (97% for LINAC vs. 95% for HDR, p Z 0.003). There was a trend towards slightly decreased V125 for LINAC compared to HDR (62.94% vs. 65.6%, p Z 0.06). There was no difference between the two plans regarding theV150 (47.9% vs. 46.3%, p Z 0.72) but the V200 was significantly higher in the HDR plans (14.85% vs. 19.1%, p Z 0.003). Urethra Dmax (135% vs. 130%, p Z 0.05) was slightly higher for the LINAC plan. Rectal Dmax dose was lower for the LINAC plan (100% vs. 103%, p Z 0.02) but there were no differences between the two regarding rectal D2cc (93.6% vs. 94%, p Z 0.87). The bladder D2cc (150.4% vs. 159%, p Z 0.52), was overall similar for most plans but fluctuated significantly if catheters were placed in close proximity of the bladder. In these cases the bladder D2cc increased for the HDR plans. The bladder D15cc (98% vs. 78%, p Z 0.05), and the bladder dose fall off were lower for the HDR plan. Conclusions: It is possible while using the linac with the Flattening Free Filter mode to achieve radiation plans very similar to HDR, with excellent V100 coverage, high PTV V125, V150 doses and high V200 (albeit lower than HDR) while keeping similar organ constraint dose limits.
3782 Impact of Monte Carlo Treatment Planning on Local Control in Lung SBRT F. Alite, S. Jain, A. Sethi, E. Melian, and B. Emami; Loyola University Medical Center, Maywood, IL Purpose/Objective(s): Treatment planning uncertainties in lung SBRT can lead to significant differences between calculated and delivered doses. Using a Monte Carlo (MC) based algorithm, we evaluated the impact of target dose parameters on local control as a function of PTV size, location, and lung density. Materials/Methods: Twenty-five patients with non-small cell lung cancer treated consecutively with SBRT from 2009 to 2011 were reviewed (median follow up: 38 months). 21 patients had a prescription dose of 50 Gy in 5 fractions, and 4 patients were prescribed 60 Gy in 5 fractions. Treatment plans were normalized to deliver > 95% of prescription dose using 8-12 non-coplanar, conformal fields with heterogeneity-corrected pencil beam (PB) algorithm. To determine actual doses delivered, all plans were recalculated with Monte Carlo without changing beam angles, PTV to MLC margin, or monitor units. Tumors were classified according to PTV size (cc): small (< 25), medium (25-60), or large (> 60); location: adjacent to chest-wall/mediastinum vs. island; and lung density: low ( 600 HU) vs. high ( 600 HU). Target dose parameters, including Dmean, D90, and D99 were recorded. RTOG 0915 criteria were used to determine local control. Results: Average Dmean, D90 and D99 delivered to PTV were lower by 10.85%, 16.79%, and 187% respectively compared to PB calculated doses (p < 0.0001). This corresponded to a median BED of 76.5 Gy vs. prescribed BED of 100.6 Gy. Reduction from planned to delivered PTV D99 correlated significantly with tumor size, location and surrounding lung density. Small tumors showed an average reduction of 214% compared to 146% for larger tumors (p Z 0.01). Island tumors had an average decrease of 217% vs. 146% for adjacent tumors (p Z 0.0006). Tumors surrounded by low density lung had an average reduction of 214% vs. 136% for those in high density lung (p<0.0005). Based on RTOG 0915 criteria, 5 patients failed locally. 4 of 5 local failures were in island tumors and 3 of 5 were surrounded by low density lung, which are the characteristics that correlated with larger reductions from planned to delivered PTV dose. All patients with local failures had medium to large PTVs with more advanced clinical stage. None of the patients with local failure received the planned BED (median delivered BED was 83.3 Gy). Of patients who failed, those with island tumors surrounded by low density lung experienced the highest reductions from planned to delivered BED, ranging from 28-30%. Conclusions: Small, island tumors surrounded by low density lung experience significant under-dosing when planned with Pencil Beam algorithm, putting them at risk for local failure. Therefore, MC-based treatment planning algorithms should be considered in these cases. Choice of treatment planning system in lung SBRT plays a critical role in determining treatment outcomes. Author Disclosure: F. Alite: None. S. Jain: None. A. Sethi: None. E. Melian: None. B. Emami: None.
3783 Lowering Dose Prescription From the 85% to 60% Isodose Lines May Reduce Radiation Necrosis in Intracranial Tumors After LinacBased Radiosurgery B. Zhao,1 J. Jin,2 N. Wen,1 H. Yimei,1 M.U. Siddiqui,1 I.J. Chetty,1 and S. Ryu1; 1Henry Ford Health System, Detroit, MI, 2Georgia Regents University Cancer Center, Augusta, GA Purpose/Objective(s): The 12Gy radiosurgical volume (V12Gy) is a predictor of radiation necrosis for patients with intracranial tumors after radiosurgery. It has also been shown that the dose fall-off varies with
International Journal of Radiation Oncology Biology Physics
-0.7); the remaining patient presented with 32% GTV increase and 0.6 T/S ratio increase. In patients with no LR at last follow up, 21% presented a GTV increase (median 4%, range Z 1; 57%) associated in all cases with an increased T/S ratio (median 0.7, range: 0.5; 1.34). Volume increase was generally more important in patients that ultimately did recur after surgical pathology confirmation (median 19% vs 4%, range Z 2; 205% vs. 1; 57%). Decreased GTV was observed in 79% of lesions with no LR (median -13%, range Z -2; -55%) associated with an increased T/S ratio in 63% of cases (median 0.5, range Z 0.3; 0.8) and decreased T/S ratio in 37% of cases (median -1.3 range Z -0.1; -2.7). Conclusions: Early (3-6 month) changes in T1 and T2 volume seem to correlate with subsequent local failure. Patients with what may be small increases in tumor volume on the first post-SBRT MRI should be followed more closely for LR. Early T2 signal changes do not appear to correlate with LC. Author Disclosure: H. Bahig: None. D. Simard: None. L. LetourneauGuillon: None. D. Roberge: None. D. Donath: None. P. Wong: None. E. Filion: None. D. Beliveau-Nadeau: None. R. Doucet: None. P. Nicholson: None. L. Masucci: None.
Author Disclosure: T. Telivala: None. W. Matthew: None. D. Schwartz: None. D. Schreiber: None.
3781 Linac With Flattening Filter Free (FFF) Mode Based Virtual HDR Treatment for Localized Prostate Cancer: A Dosimetric Feasibility Study T. Telivala, W. Matthew, D. Schwartz, and D. Schreiber; Department of Veterans Affairs, Brooklyn, NY Purpose/Objective(s): Both stereotactic body radiation therapy (SBRT) as well as high dose rate brachytherapy (HDR) are being used with increasing frequency as a definitive treatment for early stage prostate cancer. Prior studies have reported that SBRT based plans were unable to achieve comparable intraprostatic doses while sparing the rectum and bladder. In this study we tested our ability to approximate the dosimetric parameters for targets and critical organ constraints using HDR for use by linear accelerator (Linac with FFF mode) based radiosurgery planning using the Flattening Filter Free mode. Materials/Methods: 10 randomly selected patients with common contour sets were planned using both simulated HDR catheters for the HDR brachytherapy plan and LINAC based virtual SBRT plans. LINAC based plans were performed using commercially available external beam TPS and HDR plans were performed using Brachytherapy TPS. The LINAC based plans were developed to resemble HDR dose distributions. The treatment delivery method comparisons were made utilizing PTV V100, V125, V150 and V200 coverage as well as normal tissue exposure (rectum, urethra, and bladder). Comparisons were made between the two plans using the two sided t-test. Statistical significance was defined as p<0.05. Results: Planning Target Volume (PTV) coverage for the prescription dose of EBRT with FFF based SBRT was slightly better than HDR based planning regarding V100 (97% for LINAC vs. 95% for HDR, p Z 0.003). There was a trend towards slightly decreased V125 for LINAC compared to HDR (62.94% vs. 65.6%, p Z 0.06). There was no difference between the two plans regarding theV150 (47.9% vs. 46.3%, p Z 0.72) but the V200 was significantly higher in the HDR plans (14.85% vs. 19.1%, p Z 0.003). Urethra Dmax (135% vs. 130%, p Z 0.05) was slightly higher for the LINAC plan. Rectal Dmax dose was lower for the LINAC plan (100% vs. 103%, p Z 0.02) but there were no differences between the two regarding rectal D2cc (93.6% vs. 94%, p Z 0.87). The bladder D2cc (150.4% vs. 159%, p Z 0.52), was overall similar for most plans but fluctuated significantly if catheters were placed in close proximity of the bladder. In these cases the bladder D2cc increased for the HDR plans. The bladder D15cc (98% vs. 78%, p Z 0.05), and the bladder dose fall off were lower for the HDR plan. Conclusions: It is possible while using the linac with the Flattening Free Filter mode to achieve radiation plans very similar to HDR, with excellent V100 coverage, high PTV V125, V150 doses and high V200 (albeit lower than HDR) while keeping similar organ constraint dose limits.
3782 Impact of Monte Carlo Treatment Planning on Local Control in Lung SBRT F. Alite, S. Jain, A. Sethi, E. Melian, and B. Emami; Loyola University Medical Center, Maywood, IL Purpose/Objective(s): Treatment planning uncertainties in lung SBRT can lead to significant differences between calculated and delivered doses. Using a Monte Carlo (MC) based algorithm, we evaluated the impact of target dose parameters on local control as a function of PTV size, location, and lung density. Materials/Methods: Twenty-five patients with non-small cell lung cancer treated consecutively with SBRT from 2009 to 2011 were reviewed (median follow up: 38 months). 21 patients had a prescription dose of 50 Gy in 5 fractions, and 4 patients were prescribed 60 Gy in 5 fractions. Treatment plans were normalized to deliver > 95% of prescription dose using 8-12 non-coplanar, conformal fields with heterogeneity-corrected pencil beam (PB) algorithm. To determine actual doses delivered, all plans were recalculated with Monte Carlo without changing beam angles, PTV to MLC margin, or monitor units. Tumors were classified according to PTV size (cc): small (< 25), medium (25-60), or large (> 60); location: adjacent to chest-wall/mediastinum vs. island; and lung density: low ( 600 HU) vs. high ( 600 HU). Target dose parameters, including Dmean, D90, and D99 were recorded. RTOG 0915 criteria were used to determine local control. Results: Average Dmean, D90 and D99 delivered to PTV were lower by 10.85%, 16.79%, and 187% respectively compared to PB calculated doses (p < 0.0001). This corresponded to a median BED of 76.5 Gy vs. prescribed BED of 100.6 Gy. Reduction from planned to delivered PTV D99 correlated significantly with tumor size, location and surrounding lung density. Small tumors showed an average reduction of 214% compared to 146% for larger tumors (p Z 0.01). Island tumors had an average decrease of 217% vs. 146% for adjacent tumors (p Z 0.0006). Tumors surrounded by low density lung had an average reduction of 214% vs. 136% for those in high density lung (p<0.0005). Based on RTOG 0915 criteria, 5 patients failed locally. 4 of 5 local failures were in island tumors and 3 of 5 were surrounded by low density lung, which are the characteristics that correlated with larger reductions from planned to delivered PTV dose. All patients with local failures had medium to large PTVs with more advanced clinical stage. None of the patients with local failure received the planned BED (median delivered BED was 83.3 Gy). Of patients who failed, those with island tumors surrounded by low density lung experienced the highest reductions from planned to delivered BED, ranging from 28-30%. Conclusions: Small, island tumors surrounded by low density lung experience significant under-dosing when planned with Pencil Beam algorithm, putting them at risk for local failure. Therefore, MC-based treatment planning algorithms should be considered in these cases. Choice of treatment planning system in lung SBRT plays a critical role in determining treatment outcomes. Author Disclosure: F. Alite: None. S. Jain: None. A. Sethi: None. E. Melian: None. B. Emami: None.
3783 Lowering Dose Prescription From the 85% to 60% Isodose Lines May Reduce Radiation Necrosis in Intracranial Tumors After LinacBased Radiosurgery B. Zhao,1 J. Jin,2 N. Wen,1 H. Yimei,1 M.U. Siddiqui,1 I.J. Chetty,1 and S. Ryu1; 1Henry Ford Health System, Detroit, MI, 2Georgia Regents University Cancer Center, Augusta, GA Purpose/Objective(s): The 12Gy radiosurgical volume (V12Gy) is a predictor of radiation necrosis for patients with intracranial tumors after radiosurgery. It has also been shown that the dose fall-off varies with