Partial Volume Pre-Operative Irradiation of Retroperitoneal Sarcoma

Abstract

<h3>Purpose/Objective(s)</h3> There is no single standard of care for retroperitoneal sarcoma (RPS), but surgical resection is the mainstay of treatment while preoperative radiation (RT) may reduce local recurrence (LR). The STRASS trial evaluated preoperative RT and failed to show an improved abdominal recurrence free survival. These results must be considered in the context of tumor histology, endpoint, and protocol compliance (e.g., 65% RT plan compliance and 26% had major deviations). Our institutional approach for RPS is preoperative RT followed by resection and often intra-operative RT (IORT). But for large masses where RT might produce substantial morbidity, partial-volume RT (partial-RT) is utilized to target only the area(s) at greatest risk of LR while avoiding the majority of the tumor and adjacent normal tissues. This approach is based on the concept that the infiltrative portion of RPS (i.e., the site with the highest LR risk) is often a small volume, with the rest of the tumor having a pushing border, with a low risk of LR. We describe dosimetric comparisons and outcomes following partial-RT and total-volume RT (total-RT) in the pre-operative setting for adults with RPS. <h3>Materials/Methods</h3> RPS patients treated with RT from 2014-2021 were identified from an IRB-approved retrospective institutional database. Patients receiving partial-RT were retrospectively re-planned for treatment using a total-RT technique. The same planning target volume (PTV) and organ at risk (OAR) dosimetric goals (as-per STRASS) were applied to both partial-RT and conventional total-RT plans to allow direct comparison. IMRT was utilized for all plans with either two rotational arcs (n = 4) or tomotherapy (n = 1). Radiation dose was 45 Gy over 25 fractions (n = 4) or 25 Gy over 5 fractions (n = 1). <h3>Results</h3> Of the 5 patients treated with partial-RT, 4 patients also received IORT and 1 patient did not complete the intended course of preoperative RT. Median follow up was 14 months (range: 8 – 35). One patient died with distant metastatic disease, 1 patient is alive with an isolated local recurrence that is outside the previously irradiated field, 2 patients were lost to follow up, and 2 patients are alive with no evidence of disease. The average partial-RT PTV was 2,172 cc (range: 1,059 to 4,833) compared to 9,165 cc (3,535 to 16,376) for total-RT, yielding an average change in PTV volume of -75% (-87% to -64%). The average change in dose when using partial-RT was -73% (-99% to 0%) for bowel V45, -43% (-59 to -23%) for mean liver, -44% (-99% to 35%) for contralateral kidney V18, and -58% (-87% to -35%) for bladder V30. <h3>Conclusion</h3> Partial-RT is feasible and substantially reduces the dose and volume of irradiated normal tissue and in a limited number of patients, no in-field recurrences were observed. This technique should be evaluated in a future prospective study investigating patients with RPS who undergo preoperative RT to partial volumes to assess toxicity and disease control.