RESUMO
Background: CyberKnife SBRT is capable of producing dosimetry comparable to that created by HDR brachytherapy. Our original CyberKnife prostate SBRT schedule of 3,800 cGy/4 fractions ("high dose") was based upon favorable published prostate HDR brachytherapy experience. Subsequently, our trial was modified to allow a lower dose of 3,400 cGy/5 fractions ("moderate dose") in selected cases. Methods: Two hundred eighty-nine low and intermediate-risk patients were treated to either high dose (178 pts) or moderate dose (111 pts). The dose selection was individualized; high dose more commonly used in younger, intermediate-risk patients, and moderate dose more commonly used in older, low-risk patients. Results: Median PSA reached 5-year nadir levels of 0.034 ng/mL in the high dose, vs. 0.1 ng/mL in the moderate dose groups, respectively (p = 0.044 by year 4), with 62 vs. 44% reaching an ablation PSA nadir (<0.1 ng/mL) by year 5, respectively. Five year biochemical relapse free survival rates measured 98.3% for moderate dose and 94.3% for high dose groups, respectively (p = 0.1946). Five-year actuarial grade 2 genitourinary (GU) toxicity rates measured 11.6 vs. 8.7% for high dose vs. moderate dose groups, respectively, with a far lower incidence of grade ≥3 GU and grade ≥2 GI toxicity rates in both groups. Conclusions: Both regimens are efficacious in their respective, selected groups. Both arms have low grade ≥3 GU toxicity and ≥grade 2 GI toxicity. In favor of the original high dose regimen, it has longer follow-up, produces a lower PSA nadir value and is more likely to eventually produce an ablation PSA nadir (<0.1 ng/mL). In favor of the lower dose regimen, it also produces a low PSA nadir, and does so with a slightly lower grade 2 GU toxicity rate. As a lower PSA nadir could be the initial predictor a lower clinical relapse rate far beyond 5 years, even if no difference is apparent within that time frame, a practical strategy could be to more strongly consider the high dose regimen in those with the greatest potential longevity, while for those with a more limited longevity, particularly if they have minimal negative prognostic factors, the moderate dose regimen could be more attractive.
RESUMO
PURPOSE: Prostate stereotactic body radiotherapy (SBRT) may substantially recapitulate the dose distribution of high-dose-rate (HDR) brachytherapy, representing an externally delivered "Virtual HDR" treatment method. Herein, we present 5-year outcomes from a cohort of consecutively treated virtual HDR SBRT prostate cancer patients. METHODS: Seventy-nine patients were treated from 2006 to 2009, 40 low-risk, and 39 intermediate-risk, under IRB-approved clinical trial, to 38 Gy in four fractions. The planning target volume (PTV) included prostate plus a 2-mm volume expansion in all directions, with selective use of a 5-mm prostate-to-PTV expansion and proximal seminal vesicle coverage in intermediate-risk patients, to better cover potential extraprostatic disease; rectal PTV margin reduced to zero in all cases. The prescription dose covered >95% of the PTV (V100 ≥95%), with a minimum 150% PTV dose escalation to create "HDR-like" PTV dose distribution. RESULTS: Median pre-SBRT PSA level of 5.6 ng/mL decreased to 0.05 ng/mL 5 years out and 0.02 ng/mL 6 years out. At least one PSA bounce was seen in 55 patients (70%) but only 3 of them subsequently relapsed, biochemical-relapse-free survival was 100 and 92% for low-risk and intermediate-risk patients, respectively, by ASTRO definition (98 and 92% by Phoenix definition). Local relapse did not occur, distant metastasis-free survival was 100 and 95% by risk-group, and disease-specific survival was 100%. Acute and late grade 2 GU toxicity incidence was 10 and 9%, respectively; with 6% late grade 3 GU toxicity. Acute urinary retention did not occur. Acute and late grade 2 GI toxicity was 0 and 1%, respectively, with no grade 3 or higher toxicity. Of patient's potent pre-SBRT, 65% remained so at 5 years. CONCLUSION: Virtual HDR prostate SBRT creates a very low PSA nadir, a high rate of 5-year disease-free survival and an acceptable toxicity incidence, with results closely resembling those reported post-HDR brachytherapy.
