- Email: [email protected]
Dosimetric Comparison of Gamma Knife and VMAT Radiosurgery for Vestibular Schwannoma
Volume 90 Number 1S Supplement 2014
Poster Viewing Abstracts S901
of SBRT, disappearance of symptoms, response of SBRT, and patient’s status and toxicities were evaluated. Results: There were 13 cases with metastatic thyroid carcinoma including 6 men and 7 women with a mean age of 46.85 years (range, 23-73 years) treated with SBRT. The papillary was the most common histologic type in 11 patients. The metastases of 5 patients were located in the cervical vertebra, 3 patients in lymph node, 3 patients in the skull and brain, 1 patient in the lung, as well as 1 patient in the ventricle, respectively. The most common symptom at metastases was pain, which was present in 5 patients with cervical vertebra metastases. The tracking models contained vertebral spine tracking system (7 patients), skull tracking system (4 patients), fiducial tracking system (1 patient), and synchrony tracking system (1 patient), respectively. The median PTVof SBRTwas 8.05 cc (range, 2.50-72.66 cc). The patients received a median five fractions (range, one to five fractions) with a median dose of 8 Gy per fraction (range, 6-23Gy). The median biologically equivalent dose (BED) was 59.5 Gy (range, 26.4-150 Gy). The dose was prescribed to the median 80% isodose line (range, 70-81%), which encompassed 95% of the PTV. To the end of the following-up, the metastatic sites of all patients were controlled, and the symptoms related to the metastases had relieved about one week after SBRT. There were 10 patients (76.9%) were alive without evidence of recurrence, and 3 patients (23.1%) died due to other reasons. In addition, no adverse events were observed. Conclusions: With excellent in-field local control and symptoms relieved without severe toxicities, SBRT for metastatic thyroid carcinoma seems to be feasible and safe for appropriately selected patients. Further research is warranted for more accurate selection of patients suitable for SBRT. Author Disclosure: H. Wang: None. M. Meng: None. C. Jiang: None. D. Qian: None. L. Zhao: None. Z. Yuan: None. P. Wang: None.
3754 A Planning Comparison of 3D Conformal Multiple Static Field, Conformal Arc, and Volumetric Modulated Arc Therapy for the Delivery of Stereotactic Body Radiation Therapy for Lung Cancer Z. Gabos,1 M.J. Dickey,2 S. Drodge,1 B. Murray,2 S. Ghosh,2 R. Scrimger,2 and W. Roa1; 1University of Alberta, Edmonton, AB, Canada, 2Cross Cancer Institute, Edmonton, AB, Canada Purpose/Objective(s): Utilization of Stereotactic Body Radiation Therapy (SBRT) for early stage non-small cell lung cancer (NSCLC) has increased. A number of different treatment delivery techniques are clinically utilized. The primary objective of this study was to compare treatment time, monitor units and dosimetric non-inferiority of 3D conformal multiple static field (MSF) to conformal arc (CA) and volumetric modulated arc therapy (VMAT) plans. Materials/Methods: This retrospective study identified 23 NSCLC patients previously treated with SBRT using a MSF technique. Plans consisted of 9-11 static coplanar and non-coplanar fields. Each patient was replanned using CA and VMAT techniques. The ratio of the prescription isodose volume to PTV volume (R100%), the maximum dose 2cm away
Scientific Abstract 3755; Table
Shots/MU Beam-on (min) 1400 (P < .0001) Target max (Gy) (P Z .0004) Brainstem max (Gy) (P Z .4255) Cochlea mean (Gy) (P < .0001) Brain V6 (Gy) (P Z .8101) Brain V12 (Gy) (P Z .9604) Conformity Index (< 2 cc) (P Z .5403) Conformity Index ( 2 cc) (P Z .01)
from the PTV (D2cm), percentage of lung receiving 20Gy or more (V20Gy), van’t Riet conformity number (CN), total monitor units (MU) and treatment time were evaluated. Non-inferiority comparisons to detect significance were performed using a two-tailed paired t-test. A p-value of 0.017 (0.05/3) was used to adjust for multiple hypothesis testing. Results: The VMAT plans had significantly lower R100% than either the MSF or CA, and the CA plans were significantly better than MSF plans [mean difference: MSF-v-CA Z 0.07 (p<0.0001); MSF-v-VMAT Z 0.15 (p<0.0001); CA-vVMAT Z 0.08 (p<0.0001)]. Looking at D2cm, the VMAT plans had significantly lower values than either the MSF or CA, and the CA plans were significantly better than MSF plans [mean difference (percentage of total dose): MSFv-CA Z 2.73 (p Z 0.005); MSF-v-VMAT Z 4.80 (p<0.0001); CA-v-VMAT Z 2.08 (p Z 0.006)]. Comparing the van’t Riet conformity numbers, the higher VMAT values were significantly better than either CA or MSF plans, while again the CA plans were better than MSF plans [mean difference: MSF-v-CA Z -0.5 (p<0.0001); MSF-v-VMAT Z -0.11 (p<0.0001); CA-v-VMAT Z -0.06 (p<0.0001)]. Total monitor units for the plans revealed that VMAT had the highest MUs, and CA had the lowest, with significant differences between all three techniques [mean MUs: 2687, 2164 and 2072 (p<0.0001)]. Evaluation of treatment times (not including imaging) demonstrated that conformal arcs were the quickest to deliver, significantly faster than VMAT or MSF [mean treatment time: 3.91 min, 4.73 min and 8.56 min respectively (p<0.0001)]. Conclusions: This study supports the use of CA for lung SBRT in eligible patients with significant dosimetric advantages over multiple static fields. VMAT demonstrated additional gains except increased monitor units and treatment time. Considering the additional optimization time during planning, treatment delivery verification, dedicated machine requirements and conflicting reports on the safe use of hypofractionated VMAT in the thorax, conformal arcs provide an adequate alternative to MSF for delivering SBRT. Author Disclosure: Z. Gabos: None. M.J. Dickey: None. S. Drodge: None. B. Murray: None. S. Ghosh: None. R. Scrimger: None. W. Roa: None.
3755 Dosimetric Comparison of Gamma Knife and VMAT Radiosurgery for Vestibular Schwannoma H. Kim, P. Potrebko, H. Liu, H.B. Eldredge-Hindy, V. Gunn, M. WernerWasik, D.W. Andrews, J.J. Evans, C.J. Farrell, K. Judy, and W. Shi; Thomas Jefferson University Hospital, Philadelphia, PA Purpose/Objective(s): Stereotactic radiosurgery (SRS) is a common treatment option for vestibular schwannoma (acoustic neuroma). Gamma Knife (GK) SRS is traditionally the technique of choice. With the increasing availability of volumetric modulated arc therapy (VMAT), LINAC SRS is an appealing alternative with the potential to avoid invasive frame based setup and shorten treatment time. We hypothesized that VMAT SRS for vestibular schwannoma can deliver comparable dosimetric outcomes to GK plans with potentially reduced treatment times.
Dosimetric comparison of VMAT and GK SRS plans. *Statistically significant difference to GK. Gamma Knife
VMAT 1 arc
VMAT 3 non-coplanar arcs
VMAT 5 non-coplanar arcs
10 3 20.5 8.0
4986 848 3.6 0.7*
5138 930 3.7 0.7*
5627 1061 4.0 0.9*
20.4 2.5*
21.5 2.8*
21.0 2.4*
10.3 4.7
9.6 4.8
9.1 4.7
6.2 1.4*
6.0 1.6*
24.0 10.6 5.7 6.8 1.7
7.7 1.4*
9.8 8.0
12.0 9.0
9.4 6.7
8.6 6.0
2.9 2.4
2.8 2.2
2.7 2.1
2.5 1.9
1.61 0.12
1.97 0.61
1.89 0.45
1.86 0.47
1.48 0.08
1.37 0.07
1.31 0.05*
1.24 0.11*
S902
International Journal of Radiation Oncology Biology Physics
Materials/Methods: Ten patients with vestibular schwannoma were included in this study. Each case was planned using GK forward planning and VMAT inverse planning software. VMAT plans were generated with 1 coplanar and 3/ 5 non-coplanar arcs. A custom VMAT planning structure defined a steep dose gradient surrounding the tumor. Dose to normal brain tissue volumes receiving 6, 8 and 12 Gy, conformity index (CI), and dose to brainstem and cochlea were recorded. Beam-on times were calculated with dose rates of 600 and 1400 MU/ min. Dosimetric outcomes for GK and VMAT plans were compared using a Kruskal-Wallis test and Dunn’s post-hoc test. Results: Median tumor volume was 2 cc (range 0.1-4.7 cc). A peripheral tumor dose of 12 Gy was prescribed. Excellent tumor coverage (>99.8%) was achieved with both GK and VMAT plans. Each of the VMAT arc groups had plans with lower target maximum and mean dose, as well as decreased beam-on time, compared to the GK plans (Table). CI was superior in VMAT (3 and 5 arc plans) to GK for targets 2 cc (p Z 0.01). Mean dose to cochlea was higher in VMAT 1 arc and lower in 3 and 5 arc plans, respectively, when compared to GK (p < 0.0001, p < 0.05). Each of the VMAT arc sets had plans with decreased beam-on time at 1400 MU/ min (Table) and 600 MU/min (data not shown) compared to the GK plans. Similar normal brain tissue V6, V8 and V12, and dose to brainstem were achieved with GK and VMAT (Table). Conclusions: In the present study, VMAT inverse planning for the treatment of vestibular schwannoma demonstrated similar dosimetric outcomes for sparing surrounding organs-at-risk as GK plans. There appears to be a benefit to VMAT compared to GK, in regard to treatment delivery time and mitigating hot spots. CI was superior in VMAT (3 and 5 arc plans) to GK for targets 2 cc. VMAT should be considered as a safe equivalent to GK for vestibular schwannoma SRS treatment. Author Disclosure: H. Kim: None. P. Potrebko: None. H. Liu: None. H.B. Eldredge-Hindy: None. V. Gunn: None. M. Werner-Wasik: None. D.W. Andrews: None. J.J. Evans: None. C.J. Farrell: None. K. Judy: None. W. Shi: None.
21.3 cc and the mean tumor volume on final fraction CBCT was 20.6 cc. The interfraction volume change for the two local failures was 0.02 and -0.1 cc, respectively. For tumors with local control, the median rate of change in volume (volume change divided by treatment duration) was nil. Wilcox rank sum test revealed that there was no association between dose fractionation scheme (3 vs 4-5 fractions) and volume change (p Z 0.93). The Kruskal-Wallis test revealed there was no significant relationship between volume change and whether the treatment was delivered over 3, 4 or 5 fractions (p Z 0.48). Larger tumor volume was not associated with larger change in volume on CBCT (p Z 0.91). Conclusions: In our small cohort, changes in lesion volume during treatment (as measured on first and last fraction CBCT) did not predict for local control. Visibly enlarging tumor volume during treatment does not predict for local recurrence and should not be used for prognostication. Early size changes during treatment with CBCT may be more consistent with imaging limitations such as CBCT image quality and patient-related factors, such as interstitial lung disease or local inflammatory products, such as TGF-b, than with local control. Author Disclosure: J. Zhung: None. T.A. DiPetrillo: None. D. Wazer: None. J.T. Hepel: None. L. Price: E. Research Grant; NIH CTSA UL1 TR000073. K.L. Leonard: None.
3756 Evaluation of Interfraction Changes in Tumor Volume to Predict Long-Term Outcomes of SBRT in Early-Stage NSCLC J. Zhung,1,2 T.A. DiPetrillo,1,2 D. Wazer,1,2 J.T. Hepel,1,2 L. Price,3 and K.L. Leonard1,2; 1Tufts Medical Center, Boston, MA, 2Rhode Island Hospital-Warren Alpert Brown Medical School, Providence, RI, 3Tufts Clinical and Translational Science Institute, Boston, MA Purpose/Objective(s): During the course of SBRT for NSCLC, changes may be noted in the volume of the primary lesion on cone beam CTs scans (CBCT) used for image guidance. We hypothesized that decrease in lesion size during SBRT treatment predicts for local control. This study aims to evaluate changes in lung lesion volume and to correlate this change with long-term local control outcomes. Materials/Methods: We evaluated patients with newly diagnosed T1-T2 N0 NSCLC treated with IMRT-based, image-guided SBRT at this institution from January 2008 to January 2013. For each patient, CBCT was obtained before each fraction. Diagnosis was confirmed by biopsy or by strict radiologic criteria. Nodal disease was excluded by CT/PET, mediastinoscopy or EBUS. The following parameters were evaluated: age, tumor location, stage, number of treatment beams, tumor dose and fractionation and failure dates. CBCT-derived target tumor volumes were contoured from both the initial and final fractions. Interfraction volume change was recorded as the difference in volume on initial and final fraction CBCT. The Wilcox rank sum test was used to compare interfractional change in volume with outcomes. Results: 39 patients met criteria for evaluation, with a median follow-up of 17 months. The median age was 73 (47-88 years). 82.5% were biopsy proven and 17.5% were confirmed on PET scan. 82% were T1N0 and 18% were T2N0. The median treatment was delivered using 50 Gy, with 12 treatment beams in 10 days. For the entire cohort, mean volume was 9.6 cc on first fraction CBCT and 10.0 cc on final fraction CBCT. There were two local failures, with a mean time to failure of 11 months (range 8-14 m). For these two lesions, the mean tumor volume on initial fraction CBCT was
3757 Use of FDG-PET to Guide Dose Prescription Heterogeneity in Stereotactic Body Radiation Therapy for Lung Cancers With Volumetric Modulated Arc Therapy: A Study of Feasibility B. Henriques,1 M. Antoine,1 R. Trouette,2 P. Lagarde,1 J. Benech,2 C. Zacharatou,1 M. Hatt,3 and P. Fernandez4; 1Department of Radiotherapy, Institut Bergonie´, Comprehensive Cancer Centre, F-33000, Bordeaux, France, 2Department of Radiotherapy, CHU Haut-Leveque, Bordeaux, France, 3LaTIM INSERM U1101, Brest, France, 4Department of Nuclear Medicine, CHU Pellegrin, Bordeaux, France Purpose/Objective(s): Dose distribution heterogeneity with uncontrolled hotspots within the tumor is frequent with stereotactic body radiation therapy (SBRT). The aim of this study is to assess for lung SBRT whether a positron emission tomography (PET) with 18F-Fluorodeoxyglucose (FDG) can be used to guide this heterogeneity with volumetric modulated arc therapy (VMAT) and simultaneous integrated boost (SIB) technique. Materials/Methods: For three patients with stage I lung cancer, a Computed Tomography (CT) simulation and a FDG-PET were performed and registered. The planning target volume was manually defined on CT simulation (PTVCT) whereas the biological target volume (BTV) was defined automatically with the fuzzy locally adaptive Bayesian (FLAB) method on PET data. Two VMAT plans were calculated, the first plan delivered 4 x 12Gy within the PTVCT and the second plan delivered with a SIB 4 x 12Gy and 13.8Gy (115% of the prescribed dose) within the PTVCT and the BTV respectively. The maximum dose within the PTVCT (Dmax PTVCT) had to be inferior to 60Gy (125% of the prescribed dose). Plans were evaluated through: the dose received by 95% and 99% of the PTVCT (D95% and D99%), the Dmax PTVCT, the D2cm (maximal dose at 2cm of the PTVCT), the R50% and R100% (ratios between PTVCT and the 50% and 100% isodoses respectively) and the dice similarity coefficient (DSC) between the isodose 115% and the BTV. DSC Z (Iso115% BTV) / (Iso115% BTV). Ideal value of DSC is 1 and allows verifying the location of the 115% isodose. Results: The tumor was located in the left inferior lobe with contact to the chest wall for patient 1, in the middle lobe for patient 2 and in the left upper lobe for patient 3. The mean PTVCT and BTV were 36.7 (12.5) and 6.5 (2.2) cm3 respectively. Both plans led to similar target coverage (similar D95% and D99% of the PTVCT) with same Dmax PTVCT (around 124% of the prescription dose) and similar doses to the OARs. The rapid fall-off of the dose outside the PTVCT, evaluated through the D2cm, R50% and R100%, was equivalent with or without SIB technique. On the other hand, the location of hotspots, evaluated through the DSC, was improved for the SIB plans with a mean DSC of 0.31 and 0.45 for the first and the second plans respectively. Conclusions: Use of PET to guide dose prescription heterogeneity and decrease arbitrary location of hotspots is feasible with VMAT and SIB for
Poster Viewing Abstracts S901
of SBRT, disappearance of symptoms, response of SBRT, and patient’s status and toxicities were evaluated. Results: There were 13 cases with metastatic thyroid carcinoma including 6 men and 7 women with a mean age of 46.85 years (range, 23-73 years) treated with SBRT. The papillary was the most common histologic type in 11 patients. The metastases of 5 patients were located in the cervical vertebra, 3 patients in lymph node, 3 patients in the skull and brain, 1 patient in the lung, as well as 1 patient in the ventricle, respectively. The most common symptom at metastases was pain, which was present in 5 patients with cervical vertebra metastases. The tracking models contained vertebral spine tracking system (7 patients), skull tracking system (4 patients), fiducial tracking system (1 patient), and synchrony tracking system (1 patient), respectively. The median PTVof SBRTwas 8.05 cc (range, 2.50-72.66 cc). The patients received a median five fractions (range, one to five fractions) with a median dose of 8 Gy per fraction (range, 6-23Gy). The median biologically equivalent dose (BED) was 59.5 Gy (range, 26.4-150 Gy). The dose was prescribed to the median 80% isodose line (range, 70-81%), which encompassed 95% of the PTV. To the end of the following-up, the metastatic sites of all patients were controlled, and the symptoms related to the metastases had relieved about one week after SBRT. There were 10 patients (76.9%) were alive without evidence of recurrence, and 3 patients (23.1%) died due to other reasons. In addition, no adverse events were observed. Conclusions: With excellent in-field local control and symptoms relieved without severe toxicities, SBRT for metastatic thyroid carcinoma seems to be feasible and safe for appropriately selected patients. Further research is warranted for more accurate selection of patients suitable for SBRT. Author Disclosure: H. Wang: None. M. Meng: None. C. Jiang: None. D. Qian: None. L. Zhao: None. Z. Yuan: None. P. Wang: None.
3754 A Planning Comparison of 3D Conformal Multiple Static Field, Conformal Arc, and Volumetric Modulated Arc Therapy for the Delivery of Stereotactic Body Radiation Therapy for Lung Cancer Z. Gabos,1 M.J. Dickey,2 S. Drodge,1 B. Murray,2 S. Ghosh,2 R. Scrimger,2 and W. Roa1; 1University of Alberta, Edmonton, AB, Canada, 2Cross Cancer Institute, Edmonton, AB, Canada Purpose/Objective(s): Utilization of Stereotactic Body Radiation Therapy (SBRT) for early stage non-small cell lung cancer (NSCLC) has increased. A number of different treatment delivery techniques are clinically utilized. The primary objective of this study was to compare treatment time, monitor units and dosimetric non-inferiority of 3D conformal multiple static field (MSF) to conformal arc (CA) and volumetric modulated arc therapy (VMAT) plans. Materials/Methods: This retrospective study identified 23 NSCLC patients previously treated with SBRT using a MSF technique. Plans consisted of 9-11 static coplanar and non-coplanar fields. Each patient was replanned using CA and VMAT techniques. The ratio of the prescription isodose volume to PTV volume (R100%), the maximum dose 2cm away
Scientific Abstract 3755; Table
Shots/MU Beam-on (min) 1400 (P < .0001) Target max (Gy) (P Z .0004) Brainstem max (Gy) (P Z .4255) Cochlea mean (Gy) (P < .0001) Brain V6 (Gy) (P Z .8101) Brain V12 (Gy) (P Z .9604) Conformity Index (< 2 cc) (P Z .5403) Conformity Index ( 2 cc) (P Z .01)
from the PTV (D2cm), percentage of lung receiving 20Gy or more (V20Gy), van’t Riet conformity number (CN), total monitor units (MU) and treatment time were evaluated. Non-inferiority comparisons to detect significance were performed using a two-tailed paired t-test. A p-value of 0.017 (0.05/3) was used to adjust for multiple hypothesis testing. Results: The VMAT plans had significantly lower R100% than either the MSF or CA, and the CA plans were significantly better than MSF plans [mean difference: MSF-v-CA Z 0.07 (p<0.0001); MSF-v-VMAT Z 0.15 (p<0.0001); CA-vVMAT Z 0.08 (p<0.0001)]. Looking at D2cm, the VMAT plans had significantly lower values than either the MSF or CA, and the CA plans were significantly better than MSF plans [mean difference (percentage of total dose): MSFv-CA Z 2.73 (p Z 0.005); MSF-v-VMAT Z 4.80 (p<0.0001); CA-v-VMAT Z 2.08 (p Z 0.006)]. Comparing the van’t Riet conformity numbers, the higher VMAT values were significantly better than either CA or MSF plans, while again the CA plans were better than MSF plans [mean difference: MSF-v-CA Z -0.5 (p<0.0001); MSF-v-VMAT Z -0.11 (p<0.0001); CA-v-VMAT Z -0.06 (p<0.0001)]. Total monitor units for the plans revealed that VMAT had the highest MUs, and CA had the lowest, with significant differences between all three techniques [mean MUs: 2687, 2164 and 2072 (p<0.0001)]. Evaluation of treatment times (not including imaging) demonstrated that conformal arcs were the quickest to deliver, significantly faster than VMAT or MSF [mean treatment time: 3.91 min, 4.73 min and 8.56 min respectively (p<0.0001)]. Conclusions: This study supports the use of CA for lung SBRT in eligible patients with significant dosimetric advantages over multiple static fields. VMAT demonstrated additional gains except increased monitor units and treatment time. Considering the additional optimization time during planning, treatment delivery verification, dedicated machine requirements and conflicting reports on the safe use of hypofractionated VMAT in the thorax, conformal arcs provide an adequate alternative to MSF for delivering SBRT. Author Disclosure: Z. Gabos: None. M.J. Dickey: None. S. Drodge: None. B. Murray: None. S. Ghosh: None. R. Scrimger: None. W. Roa: None.
3755 Dosimetric Comparison of Gamma Knife and VMAT Radiosurgery for Vestibular Schwannoma H. Kim, P. Potrebko, H. Liu, H.B. Eldredge-Hindy, V. Gunn, M. WernerWasik, D.W. Andrews, J.J. Evans, C.J. Farrell, K. Judy, and W. Shi; Thomas Jefferson University Hospital, Philadelphia, PA Purpose/Objective(s): Stereotactic radiosurgery (SRS) is a common treatment option for vestibular schwannoma (acoustic neuroma). Gamma Knife (GK) SRS is traditionally the technique of choice. With the increasing availability of volumetric modulated arc therapy (VMAT), LINAC SRS is an appealing alternative with the potential to avoid invasive frame based setup and shorten treatment time. We hypothesized that VMAT SRS for vestibular schwannoma can deliver comparable dosimetric outcomes to GK plans with potentially reduced treatment times.
Dosimetric comparison of VMAT and GK SRS plans. *Statistically significant difference to GK. Gamma Knife
VMAT 1 arc
VMAT 3 non-coplanar arcs
VMAT 5 non-coplanar arcs
10 3 20.5 8.0
4986 848 3.6 0.7*
5138 930 3.7 0.7*
5627 1061 4.0 0.9*
20.4 2.5*
21.5 2.8*
21.0 2.4*
10.3 4.7
9.6 4.8
9.1 4.7
6.2 1.4*
6.0 1.6*
24.0 10.6 5.7 6.8 1.7
7.7 1.4*
9.8 8.0
12.0 9.0
9.4 6.7
8.6 6.0
2.9 2.4
2.8 2.2
2.7 2.1
2.5 1.9
1.61 0.12
1.97 0.61
1.89 0.45
1.86 0.47
1.48 0.08
1.37 0.07
1.31 0.05*
1.24 0.11*
S902
International Journal of Radiation Oncology Biology Physics
Materials/Methods: Ten patients with vestibular schwannoma were included in this study. Each case was planned using GK forward planning and VMAT inverse planning software. VMAT plans were generated with 1 coplanar and 3/ 5 non-coplanar arcs. A custom VMAT planning structure defined a steep dose gradient surrounding the tumor. Dose to normal brain tissue volumes receiving 6, 8 and 12 Gy, conformity index (CI), and dose to brainstem and cochlea were recorded. Beam-on times were calculated with dose rates of 600 and 1400 MU/ min. Dosimetric outcomes for GK and VMAT plans were compared using a Kruskal-Wallis test and Dunn’s post-hoc test. Results: Median tumor volume was 2 cc (range 0.1-4.7 cc). A peripheral tumor dose of 12 Gy was prescribed. Excellent tumor coverage (>99.8%) was achieved with both GK and VMAT plans. Each of the VMAT arc groups had plans with lower target maximum and mean dose, as well as decreased beam-on time, compared to the GK plans (Table). CI was superior in VMAT (3 and 5 arc plans) to GK for targets 2 cc (p Z 0.01). Mean dose to cochlea was higher in VMAT 1 arc and lower in 3 and 5 arc plans, respectively, when compared to GK (p < 0.0001, p < 0.05). Each of the VMAT arc sets had plans with decreased beam-on time at 1400 MU/ min (Table) and 600 MU/min (data not shown) compared to the GK plans. Similar normal brain tissue V6, V8 and V12, and dose to brainstem were achieved with GK and VMAT (Table). Conclusions: In the present study, VMAT inverse planning for the treatment of vestibular schwannoma demonstrated similar dosimetric outcomes for sparing surrounding organs-at-risk as GK plans. There appears to be a benefit to VMAT compared to GK, in regard to treatment delivery time and mitigating hot spots. CI was superior in VMAT (3 and 5 arc plans) to GK for targets 2 cc. VMAT should be considered as a safe equivalent to GK for vestibular schwannoma SRS treatment. Author Disclosure: H. Kim: None. P. Potrebko: None. H. Liu: None. H.B. Eldredge-Hindy: None. V. Gunn: None. M. Werner-Wasik: None. D.W. Andrews: None. J.J. Evans: None. C.J. Farrell: None. K. Judy: None. W. Shi: None.
21.3 cc and the mean tumor volume on final fraction CBCT was 20.6 cc. The interfraction volume change for the two local failures was 0.02 and -0.1 cc, respectively. For tumors with local control, the median rate of change in volume (volume change divided by treatment duration) was nil. Wilcox rank sum test revealed that there was no association between dose fractionation scheme (3 vs 4-5 fractions) and volume change (p Z 0.93). The Kruskal-Wallis test revealed there was no significant relationship between volume change and whether the treatment was delivered over 3, 4 or 5 fractions (p Z 0.48). Larger tumor volume was not associated with larger change in volume on CBCT (p Z 0.91). Conclusions: In our small cohort, changes in lesion volume during treatment (as measured on first and last fraction CBCT) did not predict for local control. Visibly enlarging tumor volume during treatment does not predict for local recurrence and should not be used for prognostication. Early size changes during treatment with CBCT may be more consistent with imaging limitations such as CBCT image quality and patient-related factors, such as interstitial lung disease or local inflammatory products, such as TGF-b, than with local control. Author Disclosure: J. Zhung: None. T.A. DiPetrillo: None. D. Wazer: None. J.T. Hepel: None. L. Price: E. Research Grant; NIH CTSA UL1 TR000073. K.L. Leonard: None.
3756 Evaluation of Interfraction Changes in Tumor Volume to Predict Long-Term Outcomes of SBRT in Early-Stage NSCLC J. Zhung,1,2 T.A. DiPetrillo,1,2 D. Wazer,1,2 J.T. Hepel,1,2 L. Price,3 and K.L. Leonard1,2; 1Tufts Medical Center, Boston, MA, 2Rhode Island Hospital-Warren Alpert Brown Medical School, Providence, RI, 3Tufts Clinical and Translational Science Institute, Boston, MA Purpose/Objective(s): During the course of SBRT for NSCLC, changes may be noted in the volume of the primary lesion on cone beam CTs scans (CBCT) used for image guidance. We hypothesized that decrease in lesion size during SBRT treatment predicts for local control. This study aims to evaluate changes in lung lesion volume and to correlate this change with long-term local control outcomes. Materials/Methods: We evaluated patients with newly diagnosed T1-T2 N0 NSCLC treated with IMRT-based, image-guided SBRT at this institution from January 2008 to January 2013. For each patient, CBCT was obtained before each fraction. Diagnosis was confirmed by biopsy or by strict radiologic criteria. Nodal disease was excluded by CT/PET, mediastinoscopy or EBUS. The following parameters were evaluated: age, tumor location, stage, number of treatment beams, tumor dose and fractionation and failure dates. CBCT-derived target tumor volumes were contoured from both the initial and final fractions. Interfraction volume change was recorded as the difference in volume on initial and final fraction CBCT. The Wilcox rank sum test was used to compare interfractional change in volume with outcomes. Results: 39 patients met criteria for evaluation, with a median follow-up of 17 months. The median age was 73 (47-88 years). 82.5% were biopsy proven and 17.5% were confirmed on PET scan. 82% were T1N0 and 18% were T2N0. The median treatment was delivered using 50 Gy, with 12 treatment beams in 10 days. For the entire cohort, mean volume was 9.6 cc on first fraction CBCT and 10.0 cc on final fraction CBCT. There were two local failures, with a mean time to failure of 11 months (range 8-14 m). For these two lesions, the mean tumor volume on initial fraction CBCT was
3757 Use of FDG-PET to Guide Dose Prescription Heterogeneity in Stereotactic Body Radiation Therapy for Lung Cancers With Volumetric Modulated Arc Therapy: A Study of Feasibility B. Henriques,1 M. Antoine,1 R. Trouette,2 P. Lagarde,1 J. Benech,2 C. Zacharatou,1 M. Hatt,3 and P. Fernandez4; 1Department of Radiotherapy, Institut Bergonie´, Comprehensive Cancer Centre, F-33000, Bordeaux, France, 2Department of Radiotherapy, CHU Haut-Leveque, Bordeaux, France, 3LaTIM INSERM U1101, Brest, France, 4Department of Nuclear Medicine, CHU Pellegrin, Bordeaux, France Purpose/Objective(s): Dose distribution heterogeneity with uncontrolled hotspots within the tumor is frequent with stereotactic body radiation therapy (SBRT). The aim of this study is to assess for lung SBRT whether a positron emission tomography (PET) with 18F-Fluorodeoxyglucose (FDG) can be used to guide this heterogeneity with volumetric modulated arc therapy (VMAT) and simultaneous integrated boost (SIB) technique. Materials/Methods: For three patients with stage I lung cancer, a Computed Tomography (CT) simulation and a FDG-PET were performed and registered. The planning target volume was manually defined on CT simulation (PTVCT) whereas the biological target volume (BTV) was defined automatically with the fuzzy locally adaptive Bayesian (FLAB) method on PET data. Two VMAT plans were calculated, the first plan delivered 4 x 12Gy within the PTVCT and the second plan delivered with a SIB 4 x 12Gy and 13.8Gy (115% of the prescribed dose) within the PTVCT and the BTV respectively. The maximum dose within the PTVCT (Dmax PTVCT) had to be inferior to 60Gy (125% of the prescribed dose). Plans were evaluated through: the dose received by 95% and 99% of the PTVCT (D95% and D99%), the Dmax PTVCT, the D2cm (maximal dose at 2cm of the PTVCT), the R50% and R100% (ratios between PTVCT and the 50% and 100% isodoses respectively) and the dice similarity coefficient (DSC) between the isodose 115% and the BTV. DSC Z (Iso115% BTV) / (Iso115% BTV). Ideal value of DSC is 1 and allows verifying the location of the 115% isodose. Results: The tumor was located in the left inferior lobe with contact to the chest wall for patient 1, in the middle lobe for patient 2 and in the left upper lobe for patient 3. The mean PTVCT and BTV were 36.7 (12.5) and 6.5 (2.2) cm3 respectively. Both plans led to similar target coverage (similar D95% and D99% of the PTVCT) with same Dmax PTVCT (around 124% of the prescription dose) and similar doses to the OARs. The rapid fall-off of the dose outside the PTVCT, evaluated through the D2cm, R50% and R100%, was equivalent with or without SIB technique. On the other hand, the location of hotspots, evaluated through the DSC, was improved for the SIB plans with a mean DSC of 0.31 and 0.45 for the first and the second plans respectively. Conclusions: Use of PET to guide dose prescription heterogeneity and decrease arbitrary location of hotspots is feasible with VMAT and SIB for