Anatomic variation and dosimetric consequences of neoadjuvant hormone therapy before radiation therapy for prostate cancer

Purpose: To characterize anatomic variation during neoadjuvant androgen deprivation (NAD) and determine a treatment planning strategy to maintain acceptable normal tissue dose while treating potential microscopic disease in the original (pre-NAD) tumor bed. Methods and Materials: We retrospectively examined the effects of treating the post-NAD anatomy with plans derived before and after NAD in a group of 44 patients enrolled in an institutional review board-approved protocol. An 8-field intensity modulated radiation therapy (IMRT) treatment plan was generated on anatomy both before and after NAD for the first 35 patients. The pre-NAD treatment plan was applied to the post-NAD anatomy to evaluate the effect of complete pre-NAD tumor bed treatment on normal tissue sparing, and the post-NAD treatment plan was applied to the pre-NAD anatomy to investigate whether microscopic disease might go untreated in the location of the pre-NAD tumor bed. Results: The prostate decreased in volume by an average of about 14 cm³ (24.3%) and was correlated with NAD duration (P = .002). The prostate center of volume systematically shifted in the inferior direction (mean = 1.4 mm, P = .005) and inferior shift was correlated with absolute volume reduction of the prostate (P = .044) in a multivariate model containing rectal and bladder volume change and initial prostate volume. Pre-NAD treatment planning resulted in a significant increase in the bladder volume (P < .01) but little increase in the rectal volume treated to all dose levels. Post-NAD treatment planning resulted in decreased treatment of the prostate and seminal vesicles (on the pre-NAD anatomy) at the prescribed and 95% isodose levels (prostate: P = .033 and 0.025; seminal vesicles: P < .001). Conclusions: Anisotropic volume reduction of the prostate was found during NAD and correlated with NAD duration. Post-NAD based treatment planning can minimize excess bladder and rectal dose.