Robotic SBRT in Prostate Cancer Patients Younger Than 50 Years Old-Updated Results.

PURPOSE/OBJECTIVE(S): Stereotactic Body Radiation Therapy (SBRT) is a standard therapeutic option for men with prostate adenocarcinoma. The median age of prostate cancer in the US is 66 but patients as young as 35 have been reported. Many younger patients will have surgery rather than SBRT for localized prostate cancer but some will be treated with SBRT. There is a paucity of data on the outcomes of this younger subset. This study reports updated outcomes on patients younger than 50 treated with SBRT at a single institution and compares outcomes to older patients.

MATERIALS/METHODS: Between April 2006 and December 2022, 6,130 patients with prostate cancer were treated with inhomogeneous-dosed SBRT using a robotic linear accelerator. Information was available for 4,143 patients. 3568 (86.12%) of patients were treated with a median dose of 3500cGY (3500-3625) delivered over 5 consecutive fractions prescribed to the 83-85% isodose line, and the remaining 575 (13.88%) other patients receiving a median dose of 4500cGY (4500-5400) to the pelvis in conventional fractionation followed by a 3 fraction SBRT boost of 2100 cGY (1950-2100) over 3 consecutive fractions. Androgen deprivation Therapy (ADT) was prescribed in 1,035 (24.98%) of these cases. The mean age was 67.4 years old. 48 patients were younger than 50 years old (mean age 46.6). 4,095 patients were 50 or older. Patients were divided into prognostic D'Amico risk groups with 43.75%, 50.00%, 6.25% of patients falling in the low, intermediate, and high-risk stratifications in the younger cohort and 23.88%, 57.05%, 19.07% in the older cohort respectively. Pretreatment PSA was 1.72 - 43.2 (median: 5.4) in the younger group and 0.3 - 661 (median: 6.5) in the older group. In the younger group, Gleason scores were 6 in 47.92%, 7 in 47.92%, and 8-10 in 4.16%. 44 younger patients were treated with SBRT alone. 4 patients also received supplemental external beam radiation (median dose 4500cGY) and 5 patients (10.42%) received Androgen Deprivation Therapy (ADT) as part of their treatment regimen. In the older group, Gleason scores were 6 in 29.84%, 7 in 54.14%, and 8-10 in 16.02%. 3522 were treated with SBRT alone. 573 patients also received supplemental external beam radiation (median dose 4500cGY) and 1030 patients (25.15%) received Androgen Deprivation Therapy (ADT) as part of their treatment.

RESULTS: At 75 months the 6-year biochemical relapse free survival was 95.83% in younger patients compared to 98.41% in older patients using the Phoenix definition of biochemical failure. The 6-year median post treatment PSA was 0.3 in younger patients and 0.2 in the older patients. There were no significant differences in the risk stratification between the 2 groups.

CONCLUSION: This represents the largest series evaluating outcomes in very young patients treated with definitive SBRT for prostate cancer. With updated 6-year follow up, SBRT remains an effective treatment for this younger subset of patients. Continued follow up will be required to see if these results remain durable.