The dependence of prostate postimplant dosimetric quality on CT volume determination

Purpose: The postoperative evaluation of permanent prostate brachytherapy r equires a subjective determination of the implant volume. This work investi gates the magnitude of the effect that various methods of treatment volume delineation have on dosimetric quality parameters for a treatment planning philosophy that defines a target volume as the prostate with a periprostati c margin. Methods and Materials: Eight consecutive prostate brachytherapy patients wi th a prescribed dose of 145 Gy from I-125 as monotherapy comprised the stud y population. The prostate ultrasound volume was enlarged to a planning vol ume by an average factor of 1.8 to encompass probable extracapsular extensi on in the periprostatic region. For this cohort, the mean pretreatment para meters were 30.3 cm(3) ultrasound volume, 51.8 cm(3) planning volume, 131 s eeds per patient, and 42.9 mCi total activity. On CT study sets obtained le ss than 2 hours postoperatively, target volumes mere drawn using three meth ods: prostate plus a periprostatic margin, prostate only which excluded the puborectalis muscles, the periprostatic fat and the periprostatic venous p lexus, and the preplanning ultrasound magnified to conform to the magnifica tion factor of the postimplant CT scan. Three sets of 5 dosimetric quality parameters corresponding to the different volumetric approaches were calcul ated: V100, V150, and V200 which are the fractions of the target volume cov ered by 100, 150, and 200% of the prescribed dose, and D90 and D100, which are the minimal doses covering 90 and 100 % of the target volume. Results: The postoperative CT volume utilizing the prostate plus margin tec hnique was comparable to the initial planning volume (mean 55.5 cm3 vs. 51. 8 cm3, respectively) whereas those determined via superimposing the preplan ultrasound resulted in volumes nearly identical to the initial ultrasound evaluation (mean 32.4 cm(3) vs. 30.3 cm3). The prostate only approach resul ted in volumes approximately 25% larger than the ultrasound volume approach . Despite the volume determinations being markedly different, no significan t differences between the approaches were appreciated for V100, V150, V200, and D90. Large variations seen in D100 were uncorrelated to any of the oth er parameters and make D100 unsuitable as a quality indicator. Conclusions: In terms of a logarithmic measure, the variation between volum etric approach for V100, V150, V200, and D90 was less than one-fifth the va riation of the CT volumes. These results which indicate relative independen ce of postimplant CT volume determination and dosimetric quality are only v alid for a planning philosophy that includes the prostate with a periprosta tic margin as the target volume. (C) 1999 Elsevier Science Inc.