Stereotactic Focal Radiotherapy as an Alternative Treatment to Active Surveillance for Low and Intermediate Risk Prostate Cancer

The stereotactic radiation therapy procedure involves at first placement of 3-4 fiducial markers, a planning computed tomography (CT) scan as well as a planning MRI, both fused thereafter based on fiducial markers. The delay between fiducial markers placement and planning CT Scan - MRI must be of at least 10 days. SBRT delivers 36.25 Gy in 5 fractions prescribed on the 80% isodose or higher. The fractions are delivered with a CyberKnife device every other day for 2 weeks. If constraints to organs at risk can be fulfilled with other radiation therapy devices, the latter can be used for treatment as well. Organs-at-risk (rectal wall, bladder wall, bladder neck, urethra, as well as contralateral neurovascular bundles, contralateral external sphincter and contralateral pudendal vessels) are delineated (see constraints used in the first citation). Gross target volume (GTV) is delineated on the MRI while considering the hypointense T2-weighted nodule, the hypointense apparent diffusion coefficient, and the hyperintense perfusion zone. At first, the GTV is expanded by 1cm in all directions to generate the clinical target volume (CTV). The CTV is then cropped to the limits of the prostate. If the volume ratio CTV/Prostate is under 30%, the expansion margin of the GTV is increased progressively within the prostate by 0.1 cm increments so that the CTV achieves at least 30% of the prostate volume. Hence, a maximum of 1.5 cm margin around the GTV is allowed and encouraged. The CTV is then expanded by 3 mm to generate the planning target volume (PTV).