Hypofractionated Preoperative Radiation Therapy to a Partial Tumor Volume in Retroperitoneal Sarcomas (RPS): A Dosimetric Comparison Between Intensity Modulated Radiation Therapy (IMRT) and Uniform Scanning Proton Therapy (PT)

S766

International Journal of Radiation Oncology  Biology  Physics

24 months with rates of OS at 12, 24, and 48 months of 74.4%, 57.9%, and 12.5%, respectively. Acute and late toxicities were rare with none higher than grade II. The most frequent toxicities included acute pain flare (n Z 2), acute nausea (n Z 3) and late cough (n Z 2). Conclusions: SBRT provides excellent local control for metastatic STS. Treatments were well tolerated with no side effects greater than grade II. Most patients received SBRT after failing multiple lines of chemotherapy and thus survival was poor. This study demonstrates that SBRT is an excellent option for local therapy in metastatic STS and warrants further investigation. Author Disclosure: K.W. Merrell: None. S.R. Francis: None. B. Mou: None. C.L. Hallemeier: None. K.R. Olivier: None.

NY, 2Procure Proton Therapy Center, Somerset, NJ, 3Memorial SloanKettering Cancer Center, New York, NY

3409 Does a Postoperative Boost in Patients With Low Necrosis Scores Improve Local Control in High-Grade Extremity Sarcomas? J.D. Pennington, O.C. Bismarck, M.T. Selch, F.C. Eilber, F. Eilber, and M. Kamrava; UCLA, Los Angeles, CA Purpose/Objective(s): For patients with high grade extremity sarcoma our institution utilizes an abbreviated pre-operative (pre-op) radiation regimen of 3.5 Gy x 8 fractions given with neoadjuvant ifosfamide-based chemotherapy. Patients with necrosis scores of < 50% are often treated with a post-operative (post-op) boost based on previous data suggesting higher local recurrence rates. We retrospectively reviewed patients treated with this pre-op regimen to determine whether local failure rates vary between those who did and did not receive a post-op boost. Materials/Methods: This was an IRB-approved study of patients treated in our Department with a pre-op regimen of 3.5 Gy x 8 fractions between 2002-2013. A total of 28 patients were identified. When post-operative RT was given, the median dose was 28 Gy. Charts were reviewed to assess for local failure, distant failure, and overall survival. Results: Median follow-up for the entire cohort was 36 months. Median age of the entire cohort was 47 years old. The preoperative tumor size was: < 5 cm (4), 5-10 cm (13), and 10 cm (11). Tumor location was upper extremity (8) and lower extremity (20). The distribution of histologies treated was: high grade NOS (9), synovial (7), liposarcoma (3), myxofibrosarcoma (3), Chondrosarcoma (2), and other (4). Clinical outcomes are presented in Table 1. Conclusions: The local recurrence risk in the entire cohort of patients was 4%. These data do not demonstrate an increased risk of local recurrence in patients with 50% necrosis score. Whether a boost is necessary in this subset of patients is not obvious. Author Disclosure: J.D. Pennington: None. O.C. Bismarck: None. M.T. Selch: None. F.C. Eilber: None. F. Eilber: None. M. Kamrava: None.

Scientific Abstract 3409; Table Entire Cohort Number of 28 patients Median follow36 up (months) Local Failure 1 (4%) Distant Failure 12 (43%) 10.8 Median time to any failure (months) Overall Survival 20 (71%)

50% necrosis 50% necrosis >50% necrosis score + pre-op score + pre and score + pre-op RT post-op RT RT 8

7

13

25

47

38

0 4 (50%) 10.2

1 (14%) 1 (14%) 13.6

0 7 (54%) 11.4

4 (50%)

6 (86%)

10 (77%)

3410 Hypofractionated Preoperative Radiation Therapy to a Partial Tumor Volume in Retroperitoneal Sarcomas (RPS): A Dosimetric Comparison Between Intensity Modulated Radiation Therapy (IMRT) and Uniform Scanning Proton Therapy (PT) O. Cahlon,1 S. Berry,1 T. Leven,2 J. Davis,2 P. Fox,2 K. Sine,2 and K.M. Alektiar3; 1Memorial Sloan-Kettering Cancer Center, New York,

Purpose/Objective(s): Several reports have explored limiting the radiation field only to the portion of the RPS at the interface between the tumor and posterior abdominal structures. A previous treatment planning study showed the feasibility of using IMRT for this approach. We undertook a dosimetric treatment planning comparison to determine if proton therapy can further reduce the dose to the surrounding normal tissues. Materials/Methods: Comparative treatment planning was performed using CT scans of 10 patients with RPS. The gross disease was delineated as GTV. The PTV was defined as the interface of the GTV with the posterior abdominal wall with a margin of 1.5 - 3 cm posteriorly and 1 cm elsewhere. The normal structures in the abdomen were contoured as organs at risk. The prescribed dose to the PTV was 30 Gy in five fractions. IMRT plans were generated with an inhouse treatment planning system with a 5 - 7 beam approach. PT plans were generated with CMS and typically utilized 2 - 3 beams. The majority of the dose was delivered through posteriorly angled beams. Institutional hypofractionated dose constraints were used for the cord (Dmax 25 Gy), cauda (Dmax 30 Gy), total kidney (V18 < 33%), and bowel (D 5cc < 30 Gy). Results: All 10 plans met all normal tissue dose constraints with both techniques. See the table below for detailed dose comparison. The mean PTV D95 for IMRT and PT was equivalent. There was no difference between treatment techniques in the D1cc to the cord or the D1cc to the cauda. There was no difference in the mean dose to the ipsilateral kidney with IMRT vs PT (9.2 Gy v 9.0; p Z 0.97). The mean contralateral kidney dose was significantly higher with IMRT than PT (2.2 Gy vs 0.94 Gy, p Z 0.03). The mean liver dose was significantly higher with IMRT than PT (3.2 Gy vs 1.7 Gy, p Z 0.01). The PT plan resulted in significantly less bowel exposure than the IMRT plan in both high and low dose regions. The bowel D5cc was higher with IMRT than PT (24.9 Gy vs 19.2 Gy, p Z 0.06). The bowel V15 was significantly higher with IMRT than PT (14.4% vs 1.4%, p Z <0.01) as well as the V5 (46.9% vs 13%, p < 0.01). Conclusions: Proton therapy and IMRT are both feasible techniques for hypofractionated preoperative radiation therapy to a partial tumor volume in RPS. Both offer excellent target coverage while meeting normal tissue dose constraints. While PT was able to reduce the dose to several of the abdominal structures, it is the reduction in bowel exposure with PT which would be expected to have the greatest clinical significance, and makes this treatment approach worth exploring clinically. Author Disclosure: O. Cahlon: None. S. Berry: None. T. Leven: None. J. Davis: None. P. Fox: None. K. Sine: None. K.M. Alektiar: None. Scientific Abstract 3410; Table

Dose comparison of IMRT vs Protons

Structure

IMRT Mean

PT Mean

P-value

PTV D95 Cord D 1 cc Cauda D 1 cc Ipsi kidney mean dose Contra kidney mean dose Mean liver dose Bowel D 5 cc Bowel V 15 Bowel V 5

29.7 Gy 19.4 Gy 17 Gy 9.2 Gy 2.2 Gy 3.2 Gy 24.9 Gy 14.4 % 46.9 %

29.9 Gy 19.2 Gy 15.7 Gy 9.0 Gy 0.94 Gy 1.7 Gy 19.2 Gy 1.4 % 13%

.27 .88 .10 .97 .03 .01 .06 <.01 <.01

3411 High-Dose Proton Beam Boosted Radiation Therapy in the Management of NoneSkull Base Chondrosarcomas A.D. Bhatt, A. Jacobson, R.Y. Lee, C. Giraud, J. Schwab, F. Hornicek, P. Nielsen, E. Choy, D. Harmon, T.F. DeLaney, and Y.E. Chen; Massachusetts General Hospital, Boston, MA Purpose/Objective(s): Surgery combined with radiation therapy (RT) has been shown to improve control rates in management of Chondrosarcomas (CS). It is often difficult to safely deliver the high doses required to control these tumors, in the range of 70.2-77.4 Gy, especially in the spinal location using photons. This can however be achieved with proton beam therapy