# Changes in splenic volumes following stereotactic ablative radiotherapy (SABR) to adrenal tumors

## TL;DR

This study found that a significant portion of patients receiving SABR for adrenal tumors experience reduced spleen volume, especially when the spleen is exposed to higher radiation doses.

## Contribution

The study identifies associations between splenic dose metrics and volume reduction following adrenal SABR, and demonstrates feasibility of dose optimization.

## Key findings

- 40–50% of patients with mean spleen dose >10 Gy experienced a >20% spleen volume reduction at 6–24 months.
- Baseline splenic volume, mean dose, and V5-10Gy were associated with significant spleen volume reduction.
- Re-planning with spleen dose optimization was feasible while meeting other organ constraints.

## Abstract

•Reductions in splenic volume at 6/12/24 months after adrenal SABR were observed in 40–50 % with mean spleen dose >10 Gy.•Baseline splenic volume, mean spleen dose, spleen V5-10Gy were associated with a >20 % spleen volume reduction at 6 months.•Re-planning with spleen dose optimization was feasible and still meeting other organs at risks’ institutional constraints.

Reductions in splenic volume at 6/12/24 months after adrenal SABR were observed in 40–50 % with mean spleen dose >10 Gy.

Baseline splenic volume, mean spleen dose, spleen V5-10Gy were associated with a >20 % spleen volume reduction at 6 months.

Re-planning with spleen dose optimization was feasible and still meeting other organs at risks’ institutional constraints.

Splenic irradiation can result in life-threatening infections. Updated dose constraints have been recommended for patients undergoing chemoradiotherapy and conventional radiotherapy but splenic constraints were not specified in trials of stereotactic ablative radiotherapy (SABR). We studied splenic doses in patients undergoing SABR for adrenal metastases and late changes in splenic volume (SV).

Patients treated with breath-hold MR-guided SABR for adrenal metastases were identified from an Ethics-approved database. Splenic dose constraints were not routinely used. The spleen was delineated retrospectively on both breath-hold CT and MR-scans. Mean spleen dose (MSD) and relative V5-10-20-30Gy values were derived from the baseline plan. SV was measured on available follow-up CT scans at 6–12–24 months. Regression analyses were performed to assess SV changes in relation to splenic dose and other parameters.

SABR was delivered to 113 adrenal tumors mostly using 5 fractions (64 % of tumors), 3 fractions (19 %) or a single fraction (14 %). Systemic therapy was administered during or within 3 months preceding/after SABR in 51 % of patients. Left-sided tumors comprised 56 % of total, and baseline median MSD and V10Gy were 9.7 Gy (range 1.5–28.4 Gy) and 46.3 % (range 0–100 %), respectively. Corresponding values for right-sided adrenal plans were 1.5 Gy (0.2–5.9 Gy) and 0 % (0–6.2 %), respectively. In multivariable analysis, a higher MSD was significantly associated with left laterality (p < 0.001), higher prescription dose (p = 0.02), and larger GTV (p < 0.001). An MSD of > 10 Gy was observed in 28 patients (25 %). Among these, a greater than 20 % decrease in SV was found in 46 % of patients with available follow-up at 6 months (n = 59), 40 % at 12 months (n = 47) and 50 % at 24 months (n = 31).

Substantial reductions in spleen volume occur in 40–50 % of patients treated with adrenal SABR with an MSD of >10 Gy. The clinical relevance of splenic atrophy merits further study.