Control charts for evaluation of quality of low-dose-rate brachytherapy for prostate cancer

PURPOSE: Variations in dosimetric outcomes among patients treated with low-dose-rate brachytherapy for prostate cancer exist, even when implants are within dose constraints. Here, we used control charts to investigate reasons for intra-patient dosimetric variability. MATERIAL AND METHODS: Univariate and multivariate control charts for prostate V(100) (percentage of prostate volume that received 100% of prescribed radiation dose), D(90) (radiation dose to 90% of prostate volume), and RV(100) (rectal wall volume that received 100% of prescribed radiation dose) were generated for 212 consecutive prostate cancer patients implanted with iodine-125 ((125)I) radioactive seeds at the Princess Margaret Cancer Centre. Control limits were calculated based on the first fifty implants. Data points that were out of control were identified, and their pre-treatment and post-treatment dosimetric and clinical parameters were compared to data points that were in-control, using Student’s t-test. RESULTS: All implants were clinically acceptable. Twelve data points exceeded multivariate control limits. Ten of those points fell below the lower control limit of V(100) control chart. Average prostate edema in the 10 out-of-control patients on both multivariate and V(100) charts was 8.3%, as compared to 0.4% for in-control patients (p < 0.04). Two patients were observed to be out-of-control on multivariate control chart, but not on V(100) control chart, and were found to have a reduction in prostate volume of 19.1% and 20.1% at one month after seed implant, compared to prostate volumes of pre-implantation evaluations. CONCLUSIONS: Control charts helped in identifying cases with out-of-control variability in post-plan prostate dosimetry. Post-treatment prostatic edema and contraction are important factors predicting variability in patients treated with (125)I permanent seed brachytherapy.