Prostate cancer (PCa) is the second most common solid tumor in men.1 Although PCa carries a high cure rate, it also remains the second leading cause of male cancer death.2 Despite dose escalation regimes using external beam radiotherapy (EBRT), rates of biochemical recurrence (BCR) are between 15% and 57%.3 and the most common site of relapse is within the prostate gland.4 One-third of patients with BCR will go on to develop metastatic disease or die from PCa.5

It is well established that radiation dose escalation leads to improved disease control in PCa.6,7 Brachytherapy (BT) can deliver highly conformal treatment with doses in excess of 100 Gy to a target volume, whilst simultaneously sparing the nearby organs at risk (OARs). This dosimetric advantage over EBRT is due to the inverse square law, which defines dose around a radiation source resulting in very steep dose gradients. The use of BT in combination with EBRT to optimize the therapeutic ratio through dose escalation to the primary site is well established in gynaecological, particularly cervix, and head and neck cancers.

These principles have led to studies investigating the role of whole gland BT boost in combination with EBRT for PCa. The rationale for targeting the whole gland is due to the multifocal nature of PCa and high degree of genomic inter- and intra-foci heterogeneity.8