Case Report: MR-guided adaptive stereotactic body radiotherapy for splenic metastases: a case series of technical feasibility and early clinical outcomes

We present a case series of four patients with five splenic metastases treated with stereotactic body radiation therapy (SBRT) using a 1.5-T Unity (Elekta AB, Stockholm, Sweden) magnetic resonance linear accelerator (MR-Linac). While SBRT is a standard of care for the treatment of oligometastatic disease, splenic SBRT is not well established due to its rarity. MRI-guided adaptive radiotherapy (MRgART) offers superior soft tissue visualization and the unique ability to adapt treatment plans in real time. In our series, splenic oligometastases originated from primary malignancies of melanoma, colorectal adenocarcinoma, and non-small-cell lung cancer. Prescription doses ranged from 40 to 50 Gy in five fractions, with gross tumor volumes between 2.4 and 22.7 cm 3 . At 12 months, the local control was 80%. One patient developed grade 2 lymphopenia without infection following two courses SBRT with a total mean spleen dose of 24.4 Gy. Our experience suggests that MRgART is a technically feasible, effective, and non-invasive treatment for splenic metastases, offering excellent disease control with an acceptable toxicity profile. However, the benefits of splenic SBRT should be balanced against the potential risk of hematological toxicity, and prophylactic vaccinations for hyposplenism should be considered for this patient population.