Stereotactic Body Radiation Therapy in Prostate Cancer: Where Do We Stand?

 From: Journal of Current Oncology

Prostate cancer, the risk of which increases with increasing age, is common in men worldwide. As per Globocan (2018), its incidence and mortality in India were 4.4% and 2.9%, respectively; the incidence rose to 5.5% while mortality reduced to 2.7% in 2020. The cancer stage and aggressiveness as well as patient preference and their life expectancy decide the treatment choice. In prostate cancers of low and favorable intermediate risk, external beam radiotherapy (EBRT) is an integral therapeutic option either alone or in combination with other therapies. Research has revealed that the overall survival in prostate cancer can be enhanced by combining RT with androgen-deprivation therapy (ADT). In contrast to conventional RT, it has been shown that biochemical control after EBRT can be enhanced with dose escalation via stereotactic body radiotherapy (SBRT). SBRT refers to a stereotactic RT procedure for treating tumors with ultrahigh doses in a single fraction in a total of 5 or fewer hypofractionated fractions. It has been shown that SBRT can achieve dosimetric parameters similar to high-dose-rate brachytherapy. Hence, it can deliver high radiation doses with a sharp dose falloff and has been increasingly used in prostate cancer. Indeed, the outcome of SBRT in prostate cancer of low and intermediate risk is well established. The present review summarizes the therapeutic role of SBRT in prostate cancer.

Compared to other tumors that proliferate faster, the intrinsic radiosensitivity to repair capacity ratio (𝛼/𝛽 ratio) in prostate cancer is lower. Thus, prostate cancer appears radiobiologically similar to healthy tissue. The 𝛼/𝛽 ratio for tumors and early responding tissues (skin and mucosa) is estimated to be approximately 8 to 10 Gy, whereas that for late responding tissues (bladder or rectal mucosa, muscle, and connective tissue) is approximately 3 to 5 Gy. The considerably low prostate cancer 𝛼/𝛽 ratio of 1.5 Gy (95% confidence interval [CI], 0.8-2.2 Gy) was derived by Brenner and Hall. Fowler et al. obtained a ratio of 1.5 Gy (1.25-1.75 Gy), which was confirmed by Miralbell et al. This phenomenon of prostate cancer behaving similar to healthy tissue has been studied using DNA labeling, which revealed hypoxic cells. The hypoxia was attributed to poor tumoral blood supply, thereby slowing the proliferation and reducing the 𝛼/𝛽 ratio of the tumor. The low 𝛼/𝛽 ratio of prostate cancer suggests that hypofractionation is a better therapeutic option, but late rectal complications need to be accounted for implementing hypofractionation. Of note, the 𝛼/𝛽 ratio of late rectal complications has been found to be approximately 3 Gy.