Purpose: High-Dose-Rate Brachytherapy (HDR-BT) is one treatment options for localized prostate cancer. Magnetic resonance imaging (MRI) allows for delineation of the dominant lesion (DL) and thus offers the potential of a focal boost. The question remains of how much dose can safely be prescribed to the dominant lesion without increasing toxicity during whole gland treatments.
Methods: The retrospective planning study includes 50 patients who previously underwent HDR-BT to the whole gland. All patients had the DL contoured. Plans were re-optimized with our in-house planning-system allowing for constrained optimization. Plans were constrained to our current clinical metrics with the objective to maximize the dose to the DL. The dose escalation for the focal boost was analyzed with respect to lesion size and location.
Results: Volumes for the gland and DL ranged from 6.0cc-47.2cc and 0.2cc-16.8cc, respectively. D90 of the whole gland was 107.8%±1.9%. Based on a Wilcoxon rank-sum test, key metrics for urethra an rectum did not differ significantly (p>0.05) when compared to clinical approved plans. D90 for the DL was (141.6±23.5)%. The maximum dose escalation was 220% for the smallest and 110% for the largest lesion. As per study design dose metrics for orangs at risk were always met. Dose escalation for the DL is anti-proportional to its size and plateauing to D90=135% of the prescribed dose for lesions greater than 4cc. Boosting the DL in the posterior portion of the gland is limited by urethra and rectum with a median D90=129.3%. This contrasts with the anterior portion with a D90=145.4%. Those findings are independent of the laterality.
Conclusion: Focal boosts are limited by the size of the dominant lesion and location within the gland. The results are used as a baseline for achievable dose escalation and can be incorporated in the treatment planning for MRI guided HDR-BT.