MR-Guided Stereotactic Ablative Body Radiotherapy for Pancreatic Oligometastases from Renal Cell Carcinoma

PURPOSE/OBJECTIVE(S)

Patients with pancreatic metastases from renal cell carcinoma (RCC) have been reported to have an unusually favorable prognosis. Traditionally, treatment for these pancreatic oligometastases comprises either surgical resection or systemic therapy. However, stereotactic ablative body radiotherapy (SAbR) may be an effective, non-invasive alternative strategy, provided that it shows acceptable toxicity and high local control rates among treated patients.

MATERIALS/METHODS

We reviewed all patients treated with MR-guided SABR for pancreatic oligometastases from renal cell carcinoma at a single institute. We explored adverse events (AE), freedom from local progression (FFLP), progression-free survival (PFS), and freedom from start of systemic therapy (FFST) among treated patients. The Kaplan-Meier method was used to describe the time-to-event endpoints.

RESULTS

From June 2019 to September 2022, we identified 11 patients treated with MR-guided SAbR to 31 pancreatic oligometastases from RCC. Patients had a mean age of 65.6 years (±6.8) and had a median of 3 irradiated pancreatic metastases (range 1-7). Metastases were treated with 5 fractions of 7 Gy (n = 1 metastasis) or 8 Gy (n = 30 metastases) per fraction. One grade 3 adverse event (bleeding) was observed. At a median follow-up of 10.5 months (range 5-46 months), estimated FFLP at 1 year was 100%. 1y-PFS was 83% (95% CI 58-100%), and 1y-FFST was 91% (95% CI 75-100%).

CONCLUSION

First exploration of MR-guided SAbR to pancreatic oligometastases from RCC indicates that it can be an effective and safe treatment option. Moreover, MR-guided SAbR may facilitate deferral of systemic therapy initiation in this select group of patients with favorable prognosis.

Toward Global Consensus for MR-Guided Treatment Planning for Pancreatic Tumors on a 1.5 T MR-Linac

PURPOSE/OBJECTIVE(S)

MR-guided SBRT with a 1.5 T MR-Linac is a relatively new therapy for pancreatic tumors with varying expertise levels. Moreover, treatment planning in the upper abdomen can be challenging as target coverage is often compromised by dosimetric constraints of abutting bowel structures. This may lead to large differences between centers in protocols, practices. To increase harmonization a worldwide consortium was founded among 1.5 T MR-Linac users. In this work we report on the outcome of the first phase within this collaboration, which is the assessment of the baseline variation between the treatment planning protocols and subsequent dose distributions.

MATERIALS/METHODS

Twelve centers across three continents (North America, Europe, and Australia) participated in this consortium. Each center was sent the same two anonymized data sets reflecting two cases of locally advanced pancreatic cancer of different complexity levels. The data sets included a CT scan, a predefined structure set containing the gross target volume (GTV) and the OARs, a brief medical history, tumor motion characteristics, and auxiliary CT and MR imaging. Centers were asked to create an MRgRT treatment plan according to their clinical five-fraction SBRT protocol, using their institutional margin structures, beam setup, target prescriptions, and OAR constraints. Key DVH parameters that were evaluated are D99%, D90%, D50%, D1% for the GTV and D0.5cc for the duodenum, small bowel, and stomach.

RESULTS

In general, large variations were observed in planning objectives and machine settings yielding widely varying inhomogeneous dose distributions to both the tumor and organs at risk (Table 1). This was especially manifest for case 2 where the tumor abutted with both the duodenum and small bowel over a trajectory of multiple centimeters. Not only were different trade-offs between target coverage and OAR sparing observed, but also different strategies for optimizing the integral dose to the tumor.

CONCLUSION

These results indicate a large variety in the treatment planning strategies that could well translate to differences in outcome. Based on this first evaluation, the consortium will work towards a collective consensus protocol with a second evaluation round after internal discussions.