Purpose: The aim of this study was to model the correlation of the dose to different parts of the pelvis with a significant drop in hematocrit 3 months after prostate cancer radiotherapy.
Methods: 221 patients with complete treatment and follow-up data were included in this analysis. Patients received either 78Gy to primary prostate or 70Gy post-prostatectomy and 54Gy to the pelvic lymph nodes. All patients had complete blood counts (CBCs) collected at baseline (pre-RT) and 3 months post-RT. The net difference of each CBC metric post-RT vs. baseline was calculated and the level of the 15 percentiles defined the thresholds of response in each case. The doses to 8 different pelvic structures were derived and fitted 20 CBC metrics using the Relative Seriality (RS) NTCP model and the biologically equivalent uniform dose (BEUD).
Results: The structure os coxae upper (above acetabulum) bilateral (OCUB) showed the highest correlation for a significant level of hematocrit drop. The values for the D50, γ and s parameters of the RS model were 26.9 Gy (25.9-27.9), 1.3 (1.2-2.2) and 0.12 (0.10-0.83), respectively. The AUC of BEUD was 0.73 and patients with BEUD to UCUB ≥ 27Gy had 8.2 times higher rate of significant hematocrit drop vs. < 27Gy. The bone volume of the whole pelvis (BVWP) and the os coxae total bilateral (OCTB) also showed significant correlations for the same endpoint (AUC values of 0.72 and 0.69, respectively). In those cases, similar BEUD thresholds were found (27Gy and 28Gy, respectively) but with lower odds ratios (2.8 and 2.9, respectively).
Conclusion: The dose-response curve of os coxae upper bilateral for a significant hematocrit drop could be determined by fitting the clinical data with the RS NTCP model. A threshold of BEUD < 27Gy was found to significantly reduce the risk for this endpoint.