2128 - Simultaneous Integrated Boost for Soft Tissue Sarcomas

Purpose/Objective(s): Intensity modulated radiation therapy (IMRT) is standard for soft tissue sarcomas (STS). However, sequential planning remains the most utilized technique for a boost, which is adapted from the 3D conformal RT era. Simultaneous integrated boost (SIB) is frequently used in IMRT and can reduce treatment duration and low dose expansion that results from sequential boost. We have adopted SIB planning, even in the preoperative setting when we anticipate a close or positive margin on final pathology. Herein, we describe our experience with SIB technique for STS.

Materials/Methods: An IRB approved retrospective review of STS patients treated with IMRT and SIB, with the boost receiving >2 Gy per fraction (fx), was performed. Demographic, dosimetric, toxicity, and oncologic data were assessed. The total planning target volume (PTV) and boost PTV were evaluated, with EQD2 calculated based on a/ß = 4. Toxicity was graded using CTCAE v5.0 with acute toxicity within 90 days of completion of treatment (surgery or RT).

Results: From 2017-2024, 22 patients with median follow up of 19 months were included. RT was preoperative in 13 and post-operative in 9. Tumor site was extremity (14), mediastinal (2), retroperitoneal [RP] (4), pelvic (1), or HN (1). T stage was T1-2 in 6 and T3-4 in 8 extremity; T2 in HN; T1-T2 in 2 and T4 in 5 mediastinal/RP/pelvic STS, with 2 being recurrent. Extremity myxoid liposarcoma and RP liposarcoma were common histologies. RT and SIB prescriptions are summarized in table below. Overall, median boost was 2.52 Gy/fx (2.1 – 7 Gy/fx). Median PTV of entire target volume was 930 cc (136 – 7004 cc), while SIB PTV was 171 cc (4 – 1629 cc), making up 24% of the entire volume on average. Of 14 extremity STS, 7 received preop RT, and 1 had a + final margin. Median percent viable tumor was 10% in extremity and 30% in mediastinal/RP/pelvic STS. In post-op group, all but one had a + margin. Overall, local control was 100% for extremity, HN, and mediastinal STS, versus 2 RP and 1 pelvic local recurrences. There were no grade 3+ acute toxicities. Wound healing complications occurred in 1 patient (7.7%) treated with preop 54 Gy in 18 fx. Late grade 2+ RT-related toxicity was limited to 1 case of grade 2 fibrosis.

Conclusion: SIB in STS is an effective technique for boosting postoperative positive margins or anticipated close or positive margins in the preoperative setting. This can help achieve highest chance of local control and eliminate consideration for post-operative boost in extremity patients treated pre-operatively. Although the follow up is limited, mild hypofractionation is unlikely to result in significant toxicity given small SIB volumes, but it may provide dosimetric advantages.