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Purpose:
Based on data characterizing prostate cancer as retaining a low α/β ratio, radiation therapy employing Stereotactic Body Radiation Therapy (SBRT) has the potential for improving biochemical and quality of life outcomes through higher biological and lower physical dosing. Prospective and retrospective analyses have demonstrated sustainable disease-free survival (DFS) and acceptable quality of life outcomes compared with historical controls. Despite its growing popularity, there is a paucity of data that characterizes the effect of SBRT delivery with different dose rates on freedom from rectal and bladder toxicity.
Material and Methods:
Between April 11, 2006 and August 13, 2018, 1673 patients with localized prostate cancer were treated with definitive SBRT at a large academic hospital. 1283 (76.7%) were treated on a LINAC with a nominal dose rate of 800 Monitor Units/minute while 390 (23.3%) were treated on a LINAC with a dose rate of 1000 Monitor Units/minute. Both units (800MU and 1000MU) delivered non-coplanar treatment on a robotic platform. The median prescription dose was 3500cGy (3500-3625) delivered in 5 fractions. Grade 2+ rectal or bladder side effects were characterized as occurring ≥6 months post radiation and scored by RTOG late toxicity scale. Statistical analyses were conducted using the Kaplan-Meier method and Pearson Chi-Square testing.
Results:
A larger percentage of patients treated on the 1000MU unit were younger than age 65 (40.9% vs. 33.5%, p=.026) and were treated for NCCN high risk disease (16.0% vs. 11.9%, p<.0001). Patients treated on the 1000MU unit were more likely to be prescribed 3625cGy than those treated on the 800MU LINAC (14.9% vs. 6.9%, p<.0001); they also were more likely to receive ADT (23.4% vs. 18.3%, p<.0001). At a median follow-up of 22 months (6, 148), 26 patients experienced grade 2+ bladder toxicity, with 17 of these patients experiencing a grade 3 event. During the same follow-up period, 20 patients experienced grade 2+ rectal toxicity, with 7 experiencing a grade 3 event. There were no grade 4 toxicities. The 5-year freedom from grade 2+ bladder toxicity did not differ by 800MU or 1000MU unit (97.1% vs. 98.3%, p=.409). The freedom from grade 2+ rectal toxicity also did not differ by 800MU or 1000MU unit (97.9% vs. 98.3%, p=.757).
Conclusions:
With 5 fraction dosing of 3500cGy- 3600cGy, SBRT for prostate cancer is well tolerated with a low risk of grade 2+ rectal or bladder toxicity. Despite statistical variation in patient composition and dosing between two LINACS employing different dose rates, a modest increase in dose rate delivery was not associated with an increase in late toxicity.
