Quality indicators for permanent I-125 prostate seed implants : seven years post implant dosimetry evaluation

Full text: The aim of the current work is to assess the progress of prostate implant quality via post implant dosimetry over a 7-year time period. The roles of post implant dosimetry (PID) after permanent 1-125 implant are to: identify suboptimal implants; assess the dosimetric quality indicators and evaluate dose to organs at risk. The 7-years PID learning-curve shows clear changes in dosimetric trend. Beside the expected improvement in technique the following factors were investigated: the replacement of the computed tomography (CT)-delineation based PID with ultrasound (US)-CT fusion based, and the evolution and influence of parameters such as 090 and V I 00. The correlation between dosimetric parameters and clinical outcome was also evaluated. Results A study on 30 patients showed manual target contouring on CT tends to overestimate the prostate volume when compared to ultrasound data (mean difference 38.3%), translating to CT based D90 values being lower than US-CT D90 by an average of 6%. There was 4.7% patient relapse and urinary retention was reported in 2.7% of the patients. CT-based PID was found less reliable than US-CT fusion due to target overestimation. This result shows the biased interpretation of low D90 based on CT targeting and may not relate to patient relapse data. The low urinary retention statistics are in accordance with the PID data for the organ, as only 5.2% of patients had their PID D I 0 >218 Gy, i.e. above recommended GEC-ESTRO guidelines. Besides the 'learning' component, the 7-years PID D90 curve is influenced by PID technique.