Indications for Plan Adaptation in Patients with Adrenal Metastases Treated With Stereotactic MR-Guided Adaptive Radiation Therapy (SMART)

Purpose/objective(s) Stereotactic body radiation therapy (SBRT) is increasingly utilized in patients with oligometastatic and oligoprogressive disease; however, concerns about interfractional changes in gastrointestinal (GI) anatomy limit broad adoption and dose-escalation in the context of adrenal metastases. The objective of this study is to evaluate the need and indications for daily adaptive replanning in this patient population. Materials/methods Consecutive patients with adrenal metastases treated using respiratory-gated stereotactic magnetic resonance (MR)-guided adaptive radiation therapy (SMART) at a single institution were evaluated. Breath-hold 3D-MRI was acquired prior to each fraction to visualize the anatomy-of-the-day. To quantify the indication for adaptation, dose prediction (i.e., calculation of initial plan on the anatomy of the day) was performed. If adaptation was indicated (i.e., critical organ at risk [OAR] dose constraints were exceeded and/or desired target coverage was not achieved), the plan was reoptimized using the original beam parameters with adjustment of individual beam fluence. Comparisons between the predictive and adaptive dose were performed to evaluate adaptive replanning decision-making based on pre-defined institutional criteria. Dose differences between the predictive and adaptive doses were compared by paired sample t test. Results 16 patients with adrenal metastases underwent a total of 80 SMART fractions to a median dose of 50 Gy in 5 fractions. The median PTV was 58.9 cc (Range: 16.1 - 377.4 cc). Plan adaptation was performed in 15/16 patients at least once during the treatment course due to predefined GI OAR dose constraints being exceeded, after accounting for the anatomy of the day. The indications for adaptation were to improve target coverage in 19/80 fractions (24%), or reduce dose to at least one critical GI OAR in 33 fractions (41%), and for both conditions in 9 fractions (11%). The most commonly affected GI OAR for plan adaptation was the stomach for left-sided metastases (6/7 patients, 86 %) and duodenum for right-sided metastases (3/9 patients, 33 %). Plan adaptation allowed for dose-escalation to the GTV with statistically significant increase in the minimum dose (42 Gy vs. 34 Gy, P Conclusion SMART enabled dose-escalation with simultaneous OAR sparing in response to changes in the anatomy-of-the-day for patients with adrenal metastases. SMART using an ablative prescription dose should be explored in future prospective studies for targets in proximity to historically dose-limiting GI OARs.