Intermediate-risk prostate cancer is heterogenous. The absolute percentage of biopsied tissue positive for Gleason pattern 4 disease (APP4) is a possible prognostic measure. Here we sought to determine the impact of APP4 in a prospective multi-institutional pooled analysis of men with intermediate-risk prostate cancer treated with stereotactic body radiotherapy (SBRT).

Patients with intermediate-risk prostate cancer treated with SBRT (40 Gy in five fractions or 26 Gy in two fractions) with or without androgen deprivation therapy treated on prospective clinical trials were included. Pathology reports were queried to obtain APP4, calculated as the percentage of Gleason pattern 4 disease within the tumour(s) multiplied by the percentage of total biopsied tissue positive for disease divided by 100. The optimal APP4 cut-off points for biochemical failure and distant metastasis were calculated and used as a stratification in the cumulative incidence of biochemical failure and distant metastasis. Multivariable competing risk models were developed.

In tota, 227 patients were included. The median follow-up was 56.5 months. The optimal APP4 cut-off points were 5% for biochemical failure and 20% for distant metastasis. At 4 years, the cumulative incidence of biochemical failure was 23.6% and 2.3% for APP4 >5% and ≤ 5%, respectively (P < 0.0001). The cumulative incidence of distant metastasis was 12.5% for APP4 >20% and 1% for APP4 ≤ 20% (P = 0.02). APP4 sub-stratified favourable intermediate-risk prostate cancer and unfavourable intermediate-risk prostate cancer into groups at similarly low and similarly high risk of biochemical failure and distant metastasis. On multivariable competing risk analysis, APP4 >5% (P = 0.0004) was significantly associated with biochemical failure, but APP4 (log) was not for distant metastasis (P = 0.08).

APP4 may be an easily accessible promising prognostic measure for patients with intermediate-risk prostate cancer treated with SBRT. Incorporation of APP4 into prospective trials will help to determine its value.