Splenic Irradiation and Late Volume Changes Following Stereotactic Ablative Radiotherapy (SABR) for Adrenal Tumors

Abstract

Splenic irradiation may increase the risk of life-threatening infections. New guidelines recommend limiting mean spleen doses (MSD) to <10Gy, based on data derived after chemoradiotherapy and conventional radiotherapy. Splenic changes after abdominal SABR are not well characterized. We studied splenic doses during MR-guided SABR for adrenal tumors, and evaluated late changes in splenic volumes (SV). AnEthics approved institutional database was accessed for details of patients treated using MR-SABR on a 0.35T unit for adrenal metastases between 2016-2023. No splenic planning constraints were routinely applied during this period. The spleen was delineated on treatment planning breath-hold CT and MR-scans and MSD, relative V5-10-20-30 values derived. On follow-up CT scans, SV was measured at 6, 12 and 24 months. Intergroup comparisons were performed with Wilcoxon rank-sum and Fisher exact tests; linear and logistic regression analyses computed to assess variables associated with the MSD and SV reduction >20% respectively. Of 113 adrenal tumors treated, 56% were left-sided. MR-SABR was mostly delivered in 5 fractions to 50 Gy (53%), in 3 fractions to 45 Gy (13%) or 16-24 Gy single-fraction (14%). The median MSD and V10 were 9.7 Gy (range, 1.5-28.4) and 46.3% (range, 0-100%) for left-sided treatments, 1.5 Gy (range, 0.2-5.9) and 0% (range, 0-6.2%) for right-sided treatments. Systemic therapy was administered in 51% of patients, either during or after MR-SABR. Baseline SV, GTV and use of systemic treatment did not significantly differ between right and left-sided tumors. MSD>10Gy was only seen in 44% of left-sided tumors, accounting for 25% of all patients. Multivariable linear analysis revealed treatment laterality (p<0.001), PTV dose (p = 0.02) and larger GTV size (p<0.001) to be significantly associated with MSD. Repeat imaging was available at 6 months (n = 59, 56% left-sided), 12 months (n = 47, 57% left-sided) and 24 months (n = 31, 65% left-sided). In the overall population, MSD >10Gy correlated with greater decreases in SV at 6 months (-19.5% vs -1.7%, p = 0.003), 12 months (-17.0% vs -3.9%, p = 0.03) and 24 months (-24.6% vs -9.3%, p = 0.06). For patients with an MSD >10Gy, a >20% decrease in SV was observed in 46% at 6 months, and 40% at 12 months. SV reduction >20% was associated with the MSD, V5 and V10 in multivariable logistic regression analysis. An MSD>10Gy was seen in 44% of all left adrenal SABR plans, with late SV reductions observed in 40% of them. The functional implications are unclear, but suggest a need to give more priority to splenic doses during abdominal SABR.