Stereotactic body radiotherapy (SBRT) has shown good long-term outcomes and low rates of severe toxicity, making it a valid treatment option for men with low- and intermediate-risk prostate cancer, according to three studies recently presented at the 36th European Society for Radiotherapy and Oncology (ESTRO) Annual Meeting.
All three studies explored the long-term safety and efficacy of the advanced technique, designed to deliver high doses of radiation therapy to the tumor in a few fractioned radiation treatments (usually up to five), and with a high specificity to the tumor site.
Researchers at the Swedish Cancer Institute in Seattle recruited 309 patients across 21 cancer centers to gather data on long-term treatment outcomes. Earlier reports have only been composed of single-center studies, and the research team wanted data from a broader pool.
All patients were treated with SBRT, and researchers compared results to those obtained in earlier studies of people treated with external beam radiotherapy.
Researchers followed study participants — 172 with low-risk, and 137 with intermediate-risk cancer — for a median of 61 months. Only five patients experienced grade 3 toxic effects, which all affected the genitals or urinary system. More severe side effects were not seen during the study.
Analyses showed that five-year survival was 95.6 percent, with disease-free survival of 97.1 percent.
Looking separately at low- and intermediate-risk patients, researchers noted that the 97.3 percent disease-free survival seen in the low-risk group was better than the 93 percent reported for patients treated with external beam radiotherapy in earlier studies. Intermediate-risk patients had a similar disease-free survival as patients with low-risk cancer— 97.1 percent — but the difference was not statistically significant.
The study also measured patient-reported quality of life issues. Urinary irritative symptoms were more bothersome up to a year after treatment, and patients reported bowel problems during the first month. These issues improved over time, and no other negative effects on quality of life were reported.
Researchers, however, noted a steady decline in sexual quality of life over the five years of the study, but the differences were not significant.
The second study — by a research team at Hacettepe University in Turkey — explored robotic SBRT using the CyberKnife system, a fully robotic radiation delivery system that treats cancerous and non-cancerous tumors and other targets. The team compared different treatment approaches, in which radiation was given either sequentially or every other day.
The study enrolled 106 patients, with low- and intermediate-risk cancer. Patients were followed for a median of 56 months, that is, nearly five years.
The team also assessed the rate of the so-called prostate-specific antigen (PSA) bounce phenomenon, in which PSA levels transiently rise after radiotherapy. The phenomenon was observed in 16 percent of patients.
The five-year biochemical relapse-free survival (deemed as a rise in PSA levels) in the whole group was 93.4 percent. But a closer look at the data revealed that people who experienced a bounce had a slightly higher relapse-free survival than those who did not—94.2% compared to 89%.
Bounce was, however, equally likely in the 58 patients receiving sequential and the 48 receiving treatment every other day.
Toxicity was also similar, making the researchers conclude that the treatment schedule did not affect outcomes.
A third team with Erasmus MC Cancer Institute in the Netherlands compared SBRT to high-dose-rate brachytherapy, in which pieces of radioactive material are placed in or near a tumor. Patients received the same dose of radiation, given in four fractions.
Among the 329 enrolled patients with low- and intermediate-risk cancer, 206 received brachytherapy and 123 were treated with SBRT using the CyberKnife system.
Researchers followed up on patients seven times during the first year, and then every six months. The median follow-up was 36 months.
The team specifically screened for late side effects of the treatment. Only nighttime genitourinary toxicities were more common in the SBRT group. Rates of all other toxic effects were similar in the two groups.