Optimal source distribution for focal boosts using high dose rate (HDR) brachytherapy alone in prostate cancer

Purpose: To investigate the optimal distribution of sources using high dose rate brachytherapy to deliver a focal boost to a dominant lesion within the whole prostate gland based on multi-parametric magnetic resonance imaging (mpMRI). Methods: Sixteen patients with prostate cancer underwent mpMRI each of which demonstrated a dominant lesion. There were single lesions in 6 patients, two lesions in 4 and 3 lesions in 6 patients. Two dosimetric models and parameters were compared in each case. The first model used 10 mm intervals between needles, and the second model used additional needles at 5 mm intervals between each needle in the boost area. Results: Three of thirty-two plans did not achieve the plan objectives. These three plans were in the first model. A higher median urethral volume was seen in the 'unsuccessful' group (2.7 cc, and 1.9 cc, respectively, p-value = 0.12). Conformity indices of the second model were also better than the first model (COIN index; 0.716 and 0.643, respectively). Conclusions: Focal monotherapy based on mpMRI achieves optimal dosimetry by individualizing the needle positions using 5 mm spacing rather than 10 mm spacing within the boost volume. A larger urethral volume may have an adverse effect on this distribution. Formal clinical evaluation of this approach is currently underway.