Characterizing Rectal Dosimetry In Patients Who Have Received Definitive SBRT for Prostate Cancer: The 7-year Freedom From Proctitis In A Large Patient Cohort

Purpose:

Stereotactic Body Radiation Therapy (SBRT) is an increasingly utilized treatment option for men with localized prostate cancer Initial biochemical control rates appear favorable and long-term rectal toxicity rates continue to be investigated.  There is considerable variability in dosimetric objectives between large volume institutions and collaborative groups.  This study reports the 7-year freedom from grade 2+ Proctitis as it relates to rectal dosimetry at a high volume institution.

Material and Methods:

Between April 2, 2010 and August 25, 2017, 963 patients with prostate cancer were treated with definitive robotic-based SBRT.  The median pre-treatment PSA was 6.2ng/ml (0.3-82.49ng/ml).  Gleason scores were 6 (3+3) in 36.7%, 7 (3+4) in 32.5%, and Gleason 7 (4+3) in 18.7% and Gleason 8 (4+4) in 9.1%.  Based on NCCN risk categories, 29.9%, 57.1% and 13.0% had low, intermediate, and high risk disease, respectively.  792 patients received SBRT monotherapy and 171 were treated with concurrent Androgen Deprivation Therapy (ADT).  The median prescription dose was 3500cGy (3500-3625) delivered in 5 fractions.  Intrarectal amifostine was administered to patients prior to each fraction.  The median age was 66 years (41-92) and the mean CTV and PTV were 82.4cc’s (20.5-238.8) and 134.6 cc’s (45.1-331.2).  Long-term rectal toxicity was characterized as occurring ≥6 months post radiation and scored by RTOG late toxicity scale.  Freedom from grade 2+ Proctitis was assessed using the Kaplan-Meier method and analyzed using Cox regression methods.

Results:

7 patients developed grade 2 proctitis and 3 developed grade 3 proctitis.  Toxicity manifested:  50% within the first year, 30% between the first and second year, and 20% during the third year after treatment.  0.3% of all patients required hyperbaric or procedural intervention to manage toxicity.  With a mean follow-up of 24 months (6-137), the 7-year freedom from ≥ grade 2 and ≥ grade 3 proctitis was 93.6% and 99.4%, respectively.  The mean rectal V3600cGy, V3500cGy, and V3350cGy were 0.7cc’s, 1.7cc’s, and 3.3cc’s, respectively; the average rectal maximum dose was 3770.5cGy.  There was no statistical difference in rectal dosimetric paramaters between those with ≥ grade 2 proctitis and those without.  Age, race, anticoagulation use, histologic grade, and ADT did not predict for ≥ grade 2 proctitis in this cohort.

Conclusions: 

SBRT for prostate cancer is well tolerated with a low actuarial risk of grade 2+ proctitis.  With long-term follow-up, most cases occur within the first two years.  The rectal dosimetry for these patients can be assiduously investigated and compared to those without toxicity.  In patients treated with robotic intrafraction translation/rotation correction, concurrent amifostine, and with rigid dose constraints employed, a review of rectal dosimetry did not predict for proctitis occurrence.