Adaptive Stereotactic Body Radiation Therapy in the Management of Oligometastatic Uterine Leiomyosarcoma: A Clinical Case Report

Summary: Safe delivery of stereotactic body radiation therapy (SBRT) to large (>5 cm) oligometastatic abdominopelvic tumors can be challenging, especially in tumors that require a higher biologically effective dose (BED) for tumor control such as sarcomas. Adaptive SBRT (A-SBRT) involves real-time replanning while the patient is on the treatment table, potentially allowing for improved dose coverage and greater sparing of organs at risk (OARs). Our case report illustrates the benefit of CT-based A-SBRT in the treatment and management of an oligometastatic uterine leiomyosarcoma patient with a rapidly enlarging pelvic recurrence. The patient consented to A-SBRT using an online adaptive CT-guided linear accelerator. She received 35 Gy in five fractions (BED3=117 cGy, α/β=3) to the planning target volume (PTV), delivered every other day. The patient was simulated using a free-breathing CT scan with and without intravenous contrast and after oral contrast administration. After the fusion of the two scans, the gross tumor volume (GTV) and all OARs were delineated; the PTV was defined as GTV+3-mm margin. To spare regions of the small bowel that overlap the PTV, a planning optimization PTV (PTV_Opt), defined as the PTV excluding small bowel, was derived for each treatment fraction. Treatment plans prioritized small bowel sparing followed by coverage of the PTV_Opt and the full PTV. The patient had a CT chest/abdomen/pelvis performed approximately 1 month after completion of treatment to evaluate disease response. The patient is now seen in follow-up every 8 to 12 weeks with repeat imaging; toxicity is also assessed by the provider during this visit. On average, adaptive plans showed an increase in PTV_Opt V100% of 1.9% and a decrease in small bowel Dmax of 12% as demonstrated in Table 1. Initial post-treatment reimaging demonstrated an increase in the size of the recently treated pelvic mass, interpreted to be treatment effect. Repeat imaging three months post-treatment showed a decrease in the size of the right pelvic mass from 8.7 cm x 6.0 cm to 6.5 cm x 4.7 cm. Overall, the patient tolerated the treatment well and acute side effects were limited to looser, more frequent stools that self-resolved. The patient experienced no acute Grade 2 or higher treatment toxicities. Approximately eight months post-treatment, the patient shows continued local radiographic response and now-resolved acute Grade 1 gastrointestinal toxicity. She has preserved bowel and bladder function. In this novel case study of a large oligometastatic abdominopelvic tumor, adaptive plans showed superior OAR sparing and target coverage compared to scheduled plans. The patient has had a durable, local radiographic response eight months post-treatment. Thus, CT-based A-SBRT may be a safe, ablative treatment option for patients with large pelvic tumors, especially those with radioresistant histology. Maryanne Jude. Lubas (she/her/hers), DO (Presenting Author) - Fox Chase Cancer Center; Joseph Panetta, PhD (Co-Author) - Fox Chase Cancer Center; Robert H. Freeman (he/him/his), MD (Co-Author) - Fox Chase Cancer Center; Joshua Meyer, MD (Co-Author) - Fox Chase Cancer Center