BOSTON – For men with newly diagnosed low- or intermediate-risk prostate cancer, stereotactic body radiotherapy (SBRT) in the right hands can be safe, with low radiation-associated toxicities and with cancer control rates that compare favorably with those produced with external-beam radiotherapy (EBRT), investigators in a multicenter study report.
At 5-year follow-up, there were no grade 4 toxicities, no treatment-related deaths, and just five grade 3 adverse events that occurred in 4 out of 309 patients treated with SBRT, said Robert Meier, MD, at the annual meeting of the American Society for Radiation Oncology.
Dr. Robert S. Meier
Using a standard radiology definition of recurrence as a more than 2 ng/mL increase in prostate-specific antigen (PSA) levels over posttreatment nadir, 97.1% of all patients were recurrence free at 5 years.
Among 172 patients with low-risk disease (T1b-T2, Gleason 6 or less and PSA 10 ng/mL or less), 97.3% were recurrence-free at 5 years, which compares favorably with the 93% seen in combined data from three large clinical trials of dose-escalated EBRT, Dr. Meier said.
For the 137 patients with intermediate risk disease (T1B-T2b, Gleason = 7, and PSA of 10 or less, or Gleason 6 or lower with a PSA between 10 and 20), 97% were recurrence-free at 5 years, a result “that matches the best results in radiotherapy for intermediate-risk patients, and matches the best results for, for example, dose-escalated IMRT [intensity-modulated radiation therapy],” he commented.
“The data is very encouraging,” commented Colleen Lawton, MD, professor and vice chair of radiation oncology at the Medical College of Wisconsin in Milwaukee.
Dr. Colleen Lawton
To determine the safety and efficacy of SBRT in men with newly diagnosed prostate cancer, Dr. Meier and coinvestigators at six centers in the United States designed a prospective study.
A total of 309 patients were enrolled, and all were treated with SBRT delivered in 5 fractions of 8 Gy each over 5 days with a robotic linear accelerator that tracks the prostate, and corrections for motion in three spatial dimension, as well as yaw, pitch, and roll.
The treatment-delivery pattern is shaped to constrain doses to the bladder, rectum, testes, and penile bulb.
Using standard dosimetry calculation, the total actual radiation dose delivered to the prostate is equivalent to approximately 100 Gy, Dr. Meier said.
The safety analysis was powered to consider a greater than 10% rate of grade 3-5 urinary or bowel side effects as excessive. The efficacy analysis was designed to ask whether 5-year recurrence-free rates in low-risk patients could equal or be superior to a historical control rate of 93%.
As noted before, there were no grade 4 toxicities and no treatment-related deaths, and the rate of grade 3 side effects was 2.7%. with two events occurring in low-risk patients, and three in intermediate-risk patients. The events, all genitourinary toxicities, occurred from 11 to 51 months after treatment. Grade 1 or 2 genitourinary toxicities at any time were seen in 53% and 35% of patients, respectively. Grade 1 or 2 GI toxicities were seen in 59% and 10%.
Five patients developed urinary retention which required temporary catheter placement.
The ideal candidate for the therapy is the unfavorable intermediate-risk patient, Dr. Meier said in the interview.
“These are the patients who, if they are going to get external-beam radiation, have to combine it with androgen ablation, and that has its own toxicities. SBRT did very well even in the unfavorable intermediate-risk patients, so I think that group, and for that matter any intermediate-risk patient, is ideally suited,” he said.
The study was supported by Accuray. Dr. Meier disclosed research grants from the company. Dr. Lawton reported no relevant financial disclosures.