Purpose: The purpose of this study is to determine dose-response relations regarding the endpoint of hematocrit level drop after pelvic irradiation in patients undergoing external beam radiotherapy for prostate carcinomas.
Methods: A retrospective analysis was conducted on 221 patients treated between 2014 and 2016 at a single institution for prostate cancer. Patients received either 78Gy to primary prostate or 70Gy post-prostatectomy. All patients had complete blood counts collected at baseline (pre-RT) and 3 months post-RT, which were analyzed in conjunction with dose volume histograms corresponding to eight different pelvic structures.
Results: Pelvic structures that upon irradiation correlated with significant decreases in hematocrit were the following: the bone volume of os-coxae bilaterally superior to the acetabulum (OCUB), the bone volume of the entire os-coxae bilaterally (OCTB), and the bone volume of the whole pelvis (BVWP). The structure showing the highest correlation was OCUB with a maximum AUC of 0.74. For V20Gy < 30% the odds ratio was 9.8 with 95% confidence interval (CI) of 2.9-32.9. For mean dose to OCUB, an AUC of 0.73 was observed where the dose threshold was 23Gy and the odds ratio was 2.7 and 95%CI 1.3-5.6. For OCTB, AUC was 0.69 with a V20Gy < 50% showing an odds ratio of 9.3 (95%CI 2.7-31.8). For BVWP, AUC was 0.71 with a V20Gy < 40% demonstrating an odds ratio of 7.5 (95%CI 2.2-25.4). Further, mean dose < 24Gy had an odds ratio of 2.6 (95%CI 1.1-6.1) showing an AUC of 0.72.
Conclusion: These findings confirm the association of damage to pelvic bone marrow with blood counts drop. The doses to OCUB, OCTB and BVWP were found to correlate with a significant drop of hematocrit. A threshold of V20Gy < 30% or mean dose < 23Gy to OCUB may reduce almost ten-fold the risk for this endpoint.