SMART ablation of lymphatic oligometastases in the pelvis and abdomen: Clinical and dosimetry outcomes.

Purpose: To demonstrate dosimetry benefits and report clinical outcomes of stereotactic magnetic resonance (MR)-guided online adaptive radiotherapy (SMART) of abdominopelvic lymphatic oligometastases.

Patients & Methods: Prospective registry data of 26 patients with 31 oligoprogressive lymphatic metastases (1-2 lesions) who received SMART between April 2020 and April 2021 was analyzed. Prostate cancer was the most common histology (69%). Most patients (63%) had received previous abdominopelvic radiotherapy (RT). SMART was delivered in 3-7 fractions based on planning target volume (PTV) location and previous dose exposures. For SMART, the baseline plan was recalculated on daily 3D MR-imaging (predicted plan), and plan adaptation was mandatory in case of planning objective violations.

Results: Plan adaptation was mostly performed due to violation of planning objectives in the predicted plan (134/140 fractions, 96%) and significantly improved plan dosimetry: (1) PTV coverage was increased (predicted: median 89%, adapted: median 95%, p < 0.001), (2) organs-at-risk (OAR) overdoses were reduced (predicted: 27/140 (19%), adapted: 1/140 (1%), p < 0.001) and (3) PTV overdoses were reduced (predicted: 21/140 (15%), adapted: 1/140 (1%), p < 0.001). After a median follow-up of 9.8 months, one patient had in-field tumor progression and twelve patients had out-field tumor progression (at 6 months: progression-free survival: 63% [46-88%], local control rate: 97% [90-100%]). Treatment was tolerated well and no grade ≥3 toxicity was reported.

Conclusion: SMART improves target volume coverage and yields superior OAR protection compared to non-adaptive radiotherapy, thus representing an innovative approach to challenging cases, such as repeated radiotherapy.