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Stereotactic body radiation therapy can be an effective treatment for oligometastases. However, safe delivery of ablative radiation therapy is commonly limited by the proximity of mobile organs at risk (OARs), as interfraction and intrafraction motion can result in severe complications. The goal of this study is to determine the feasibility, safety, and disease control outcomes of stereotactic magnetic resonance guided adaptive radiation therapy (SMART) in patients with abdominopelvic oligometastases. We identified 101 patients with abdominopelvic oligometastases, including 20 patients enrolled on phase 1 protocols, who were consecutively treated with SMART on a 0.35 T MR Linac at a single institution from October 2019 to September 2021, excluding patients with metastases of the liver, kidneys, or adrenal glands. Local control and overall survival were analyzed using the Kaplan-Meier method. Adverse effects were assessed using the Common Terminology Criteria for Adverse Events (CTCAE) v5.0. Overall, 114 sites of disease were treated. The most common histology was prostate adenocarcinoma (53.5%, n=60), and 57.0% (n=65) of sites were centered in the pelvis. 79.8% of sites (n=91) were treated to 8 Gy × 5, and 43.0% (n=49) were treated with breath hold. Online adaptation was used in 86.6% of delivered fractions to either improve the sparing of OARs (57.2%) or target coverage (29.4%). In addition, there were two cases where the 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. These two fractions were postponed to either later the same day or a subsequent day. The median time required for adaptation was 24 minutes, and the median time in the treatment room was 58 minutes. With median follow-up of 11.4 months, the 12-month local control was 93% and higher for prostate adenocarcinoma vs other histologies (100% vs 84%, log-rank P =0.009). The 12-month overall survival was 96% and higher for prostate adenocarcinoma vs other histologies (100% vs 91%, log-rank P =0.046). Three patients developed grade 3 toxicities (colonic hemorrhage at 3.4 months and urinary tract obstructions secondary to ureteral stenoses at 10.1 and 18.4 months). SMART is a feasible and safe method for delivering ablative radiation therapy to abdominopelvic oligometastases. Early data indicate a high rate of local control and low risk of significant toxicity. Adaptive planning was necessary in the large majority of cases. The advantages of SMART warrant its further investigation as a standard option for the treatment of abdominopelvic oligometastases. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]