Superior Post-Treatment Biopsy Outcomes with High Dose SBRT Compared to High-Dose Conventionally Fractionated IMRT for Clinically Localized Prostate Cancer

Abstract

<h3>Purpose/Objective(s)</h3> To compare post-treatment biopsy outcomes for clinically localized prostate cancer patients treated with high-dose conventionally fractionated intensity modulated radiotherapy (CF- IMRT) versus high-dose stereotactic body radiotherapy (SBRT). <h3>Materials/Methods</h3> 451 patients with low and intermediate risk prostate cancer treated with high dose external beam radiotherapy underwent a post-treatment biopsy which was routinely performed at our institution to assess local control status. Biopsies were performed 2-3 years after completion of radiotherapy. 221 patients were treated with CF-IMRT to a median dose of 75.6 Gy (range: 64.8-86.4) and 230 patients were treated with SBRT to a median dose of 40 Gy in 5 fractions (range: 32.5-42.0). A 6-month course of androgen deprivation therapy (ADT) was used in conjunction CF-IMRT and SBRT in 38% and 17% of patients. The percentage comprising low, favorable intermediate and unfavorable intermediate risk group were similar (CF-IMRT: 23%, 36% and 41%; SBRT: 18%, 49%, 33%). The histological outcome of the prostate biopsy was classified as positive (prostatic adenocarcinoma without typical radiation–induced changes), negative (no evidence of carcinoma) or severe treatment effect. <h3>Results</h3> The incidence of a positive post treatment among the IMRT and SBRT cohorts were 49% and 17%, respectively (p< 0.001). The incidence of a positive biopsy for low risk, favorable- intermediate risk and unfavorable- intermediate risk patients treated with IMRT and SBRT were: IMRT: 40%, 52%, and 52%; SBRT: 7.1%, 17%, and 21%, respectively. Multivariable analysis evaluating co-variates associated with a positive post-treatment biopsy was IMRT (P< 0.001 CI:0.12, 0.33) with 5-fold higher odds of a positive biopsy compared to SBRT. Other variables including the use of ADT and the risk group did not significantly impact on the biopsy outcome. <h3>Conclusion</h3> The use of SBRT in the treatment of low and intermediate risk prostate was associated with improved post-treatment biopsy outcomes compared to conventionally fractionated IMRT. These findings were observed even in the setting of high-dose IMRT to prescribed doses of ≥75.6 Gy. We hypothesize that these findings may be related to the greater ablative potential of high dose SBRT compared to IMRT.