2442 - Dose-Volume Constraints for Adrenal Stereotactic Ablative Radiotherapy: A Single-Institution Analysis
Presenter(s)
S. J. Domal1, M. Miljanic1, A. Gao2, J. Ma1, O. Hamidi1, C. Ahn3, R. D. Timmerman1, and R. Hannan1; 1Department of Radiation Oncology, University of Texas Southwestern Medical Center, Dallas, TX, 2Department of Population and Data Sciences, University of Texas Southwestern Medical Center, Dallas, TX, 3Kidney Cancer Program, Simmons Comprehensive Cancer Center, University of Texas Southwestern Medical Center, Dallas, TX
Purpose/Objective(s): Dose-volume constraints for the adrenal glands in stereotactic ablative radiotherapy (SAbR) remain undefined, creating uncertainty when treating adrenal lesions while preserving endocrine function. We hypothesized that sparing sufficient adrenal volume below a threshold dose would correlate with preservation of function following SAbR. We sought to derive conservative planning constraints associated with prevention of primary adrenal insufficiency (PAI).
Materials/Methods: Among 66 patients treated with adrenal SAbR at one institution, seven had metastases involving their only remaining adrenal gland and were evaluable for post-treatment PAI. Treatment consisted of 3-5 fractions (680-1,400 cGy per fraction). Cumulative DVHs were biologically converted to a five-fraction reference using the Universal Survival Curve (USC) model with multiple a/ß values (1.5-10 Gy) as sensitivity analysis. PAI was defined per Endocrine Society guidelines: early morning serum cortisol <5 µg/dL with plasma ACTH >2-fold the upper reference limit. Spared-volume metrics were compared between PAI-positive (n=4) and PAI-negative (n=3) patients using the Mann-Whitney U test. A separation analysis identified the dose threshold maximizing inter-group non-overlap.
Results: Conventional dose metrics (Dmean, D5-D100) did not reliably distinguish PAI risk (p=0.0518). Volume-based metrics demonstrated significant separation (p=0.0319). Maximal separation occurred at 40 Gy in five fractions (36 Gy in three fractions): PAI-positive patients had 0% of adrenal volume spared below this threshold, whereas PAI-negative patients had =50% spared (=3 cc). Group separation was preserved across all three USC model variants (a/ß = 1.5, 3, and 10 Gy), confirming robustness to radiobiological assumptions. The three-fraction equivalent was independently verified against unconverted DVHs from patients originally treated in three fractions.
Conclusion: A planning constraint of V40Gy <45% in five fractions (V36Gy <45% in three fractions), or =3 cc of adrenal gland spared below these thresholds, was associated with preserved endocrine function. These findings provide the first dose-volume planning guidance for adrenal SAbR and may inform treatment planning for patients with solitary adrenal glands or bilateral adrenal metastases. Prospective validation is warranted.