High-dose-rate brachytherapy is typically used as a dose-escalating boost for external beam radiotherapy (EBRT) for prostate cancer. But some researchers believe it might be close to graduating to a viable monotherapy—an alternative, rather than an adjuvant, to prostate EBRT.
Brachytherapy involves the precisely positioned implantation of short-range radioisotope-containing beads at the sites of tumors, with little exposure of adjacent nontarget tissue to irradiation, thanks to the rapid irradiation dose fall-off. This is why prostate tumor brachytherapy is believed to allow the most conformal high dose radiotherapy possible to the prostate, sparing healthy, nontarget tissues more effectively than can be achieved with external beam radiotherapy (EBRT).1
Brachytherapy can be delivered via two different dose-rate techniques. Low-dose rates (LDR) emit 2 Gy/hour or less from permanently implanted radioisotope seeds. In contrast, high-dose-rate (HDR) brachytherapy delivers 10 Gy/hour or more, via temporarily implanted catheters.1 Both LDR and HDR brachytherapy beads are placed under guidance of transrectal ultrasound (TRUS).1 HDR delivers a high dose over a few minutes; LDR delivers its radiation dose over weeks or months.1
LDR brachytherapy is a standard monotherapy for patients with low- to intermediate-risk prostate cancer.1 HDR brachytherapy, in contrast, has been used as a radiation dose-escalating boost to EBRT in the treatment of prostate cancer.1,2 It is the most frequently used local EBRT dose-escalation technique for patients with intermediate and high-risk prostate cancer—an approach that appears to be less toxic than dose escalations involving increased EBRT dose.1
The authors of a phase 3 randomized clinical trial found that this increasingly popular radiation dose-escalation strategy is both safe and clinically efficacious in patients with localized prostate cancer, significantly reducing the risk of tumor recurrence (by 31%; P=.01) and improving relapse-free survival times (albeit not overall survival times), compared with EBRT alone—and doing so with rates of late urinary and rectal toxicity comparable to those associated with EBRT alone.3 Authors of a 2014 systematic review of data from 14 prospective and 24 retrospective single-institution clinical studies of HDR brachytherapy boost for prostate cancer similarly found encouraging rates of grade 3-4 late radiation toxicities, affecting fewer than 6% of patients overall.4
Recently, HDR brachytherapy has gained attention as a promising monotherapy alternative to EBRT, rather than an EBRT adjuvant or boost.1,5,6 High disease control rates have been reported for patients receiving HDR monotherapy, “particularly in patients with low- and intermediate-risk disease,” note Gerard Morton, MD, of the University of Toronto in Ontario, Canada, and P.J. Hoskin, MD, of the Mount Vernon Cancer Centre, Northwood, in Middlesex, England.1
Several studies have reported biochemical relapse-free survival rates exceeding 90% for such men, they note.1 “Selected high-risk patients have also been treated with HDR monotherapy, with reported recurrence-free survival of 79% to 93%,” they note.