(P 340) CENTRAL BOOST RADIATION THERAPY CAN ACHIEVE DURABLE LOCAL RESPONSE IN UNRESECTABLE RETROPERITONEAL SARCOMA

Objective: Optimal treatment of retroperitoneal sarcoma (RPS) remains to be defined. Surgical resection is utilized when the patient is fit for surgery and neoadjuvant and/or adjuvant therapy may be helpful for certain histologies in select patients and is being investigated in ongoing clinical trials. For patients with unresectable RPS, treatment may include palliative radiotherapy with standard fraction sizes (1.8 Gy to 2 Gy per daily fraction) and/or systemic chemotherapy. Here, we report the feasibility of using high-dose boost radiation (3 to 4 Gy) to the central part of the tumor in patients with unresectable RPS.

Methods: Five patients with unresectable RPS were treated with radiation therapy using a central boost technique with intensity modulated radiation therapy so that the edge of the tumor received 45 Gy to outer part of the tumor (planning target volume 1 or PTV1) in 1.8 to 2 Gy fractions and the central part of the tumor (PTV2) in 3 Gy to 4 Gy fractions in 5 to 25 fractions. Initial presentation, relevant work-up including imaging and laboratory findings, and delivered radiation treatment is documented and are reported. Locoregional response, the presence of distant disease, overall survival and general adverse events are reported.

Results: Key findings and dosing schedule is summarized in Tables 1 and 2. On average, IMRT doses of 45 Gy – 59 Gy was delivered to the outer part of the tumor (PTV1), while the central part of the tumor (PTV2) received 56 Gy – 75 Gy physical dose, which translates to 61 Gy – 80.2 Gy equivalent dose in 2 Gy fractions (EQD2) in an attempt to improve the response of the RPS to radiotherapy and achieve local control while minimizing the toxicity of treatment. Using this treatment technique, patients experienced mild radiation toxicities overall (CTCAE Grade 1 or 2) such as fatigue, nausea, and diarrhea alternating with constipation. Four of the five patients had clinically stable local disease at the time of their last documented follow up. Unfortunately, despite excellent local response to local radiation therapy, most patients developed metastatic disease (4/5), with the most common sites being lung followed by liver, which ultimately led to early death. In this case series of variable histology, the median survival post radiation therapy was 30 months. At this time, one patient (Case 5) is still alive and on imaging surveillance 36 months after radiotherapy.

Conclusion: Patients with unresectable RPS tolerate IMRT with central tumor boost irradiation well. This approach can delay tumor growth in the majority of patients and allows patients with responsive tumors to live with stable, localized disease for at least 2 to 3 years. To minimize radiation toxicity to the adjacent bowel and other organs while delivering up to 75 Gy to the central part of the tumor, we used sequential, interdigitated, or simultaneous integrated boost techniques. In this small case series of 5 patients, treatment was tolerated well, and excellent local responses were observed regardless of the timing of central boost. Given the high rates of metastatic disease that developed in responding patients, adjuvant systemic therapy may be considered for patients with unresectable RPS treated with aggressive radiation therapy to the central part of the tumor.