Stereotactic Magnetic Resonance Guided Adaptive Radiation Therapy for Abdominopelvic Metastases.

PURPOSE/OBJECTIVE(S): Stereotactic body radiation therapy (SBRT) can be an effective method for treating metastases. However, the proximity of organs at risk (OARs) to the target can limit the safe delivery of ablative doses, particularly in the abdomen and pelvis where there can be significant interfraction and intrafraction motion. Stereotactic magnetic resonance guided adaptive radiation therapy (SMART) may allow for safer delivery of ablative radiation to abdominopelvic metastases. We report the feasibility and early outcomes of patients with abdominopelvic metastases treated with SMART using a 0.35T MR Linac in a (1) retrospective cohort and (2) a prospective phase 1 trial.

MATERIALS/METHODS: We identified 44 patients in the retrospective cohort and enrolled 13 patients onto the phase 1 protocol, all with abdominopelvic metastases. Patients with metastases of the liver, kidneys, or adrenal glands were excluded and were enrolled on companion phase 1 trials. Patients were treated between 10/29/19-2/18/21. For the retrospective cohort, dose prescription and coverage were at the discretion of the treating physician, with 67% of sites receiving 40 Gy/5 fractions. For the phase 1 cohort, patients were treated to 40 Gy/5 fractions daily with goal PTV coverage of V40 ≥95% (ideal) and V30 ≥95% (acceptable).

RESULTS: Overall, the 44 patients in the retrospective cohort were treated to 49 sites of abdominopelvic disease, and the 13 patients in the phase 1 cohort received treatment to one site of abdominopelvic disease each on protocol. The most common histology was prostate adenocarcinoma (57%). In total, 308 fractions were delivered, with 138 (45%) using a breath hold and 170 (55%) using free breathing. Online adaptation was necessary for 269/308 fractions (87%) to either satisfy critical OAR metrics (62%) or improve target coverage (38%). Additionally, there were two instances when on-board MRI allowed for the detection of bowel which had displaced the target to such a degree as to interfere with safe treatment delivery, even when utilizing adaptive planning. These two treatments were postponed to either later the same day or a subsequent day. Patients spent an average of 61 minutes in the treatment room. With median follow-up of 150 days (interquartile range 90-224 days), there were no deaths and one local failure (a patient with urothelial carcinoma who received 35 Gy/5 fractions at 102 days after start of SBRT). There was one grade ≥3 treatment-related toxicity (colonic hemorrhage in a patient with serous ovarian cancer who received 40 Gy/5 fractions at 86 days after start of SBRT in the setting of thrombocytopenia).

CONCLUSION: SMART is a feasible method for delivering ablative doses of radiation. Adaptive planning was necessary in a large portion of cases. Early data suggest a high rate of local control and low risk of significant toxicity. Longer-term results of the phase 1 protocol will be reported in the future.