Physics Contributions
Potential role of various dosimetric quality indicators in prostate brachytherapy

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Abstract

Purpose: Postoperative CT-based dosimetric analysis provides detailed information regarding the coverage and uniformity of an implant, but the assessment of implant quality remains an unanswered and controversial issue. There is no disagreement that a good implant should cover the target volume with an adequate dose, but there is no consensus as to what represents an adequate dose.

Materials and Methods: The American Brachytherapy Society has recently proposed that prostate brachytherapy quality be measured in terms of the following parameters: D90, V100, and V150 where D90 is defined as the minimal dose covering 90% of the prostate volume and V100 and V150 are defined as the percent volume of the prostate receiving at least 100% or 150% of the prescribed minimal peripheral dose (mPD), respectively. We report detailed day 0 dosimetric evaluation for 60 consecutive prostate brachytherapy patients implanted via a standard transperineal ultrasound guided approach in terms of D90, D100, V90, V100, and V150 and also the maximal and average rectal and urethral dose.

Results: Dosimetric evaluation resulted in a V100 greater than 80% of the prostate volume and a D90 greater than 90% of the mPD in the entire patient population. There was a statistically significant difference between the quality scores of 125I implants and 103Pd implants with the 125I mean V100 and D90 at 95.3% volume and 109.9% mPD, respectively, vs. 103Pd at 91.8% volume and 103.7% mPD. Likewise, the rectal and urethral doses as a fraction of mPD were significantly lower in 103Pd than in 125I implants. This occurred despite the fact that palladium implants were typically preplanned with significantly better coverage and hotter V150 than iodine implants. We consider V150 to be an important parameter for determining dose homogeneity although the clinical utility of dose homogeneity remains unknown. The mean V150 was 45.6 ± 9.6% volume. There was no additional dosimetric utility from a determination of V90 while D100 was found to be overly sensitive to steep dose gradients at the periphery of the prostate.

Conclusions: This report represents the first detailed postimplant day 0 dosimetric evaluation comparing ABS recommended quality parameters used to evaluate prostate brachytherapy. At the present time, no long-term clinical outcomes are available because of short follow-up. As PSA based follow-up data becomes available, however, this report may help define what represents an adequate implant.

Introduction

Transperineal ultrasound guided prostate brachytherapy supported by sophisticated computerized treatment planning has emerged as a viable option in the management of early stage carcinoma of the prostate gland. Measured in terms of local control and freedom from biochemical failure, the results of conformal brachytherapy have been found to be as a favorable as the most positive radical prostatectomy series 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. The wide spread acceptability and reproducibility of the favorable results cited will be dependent on appropriate patient selection, assignment to the appropriate treatment regimen and the ability of brachytherapists to reproduce the technical aspects of the procedure. Postoperative CT-based dosimetric analysis provides detailed information regarding the coverage and uniformity of an implant. It also affords the ability to compare various intraoperative techniques and provides a sound basis for future improvement. Recent detailed reports have been published by Willins and Wallner 12, 13, Merrick et al. 14, 15, Prestidge et al. 16, 17, Moerland et al. (18), and Stock et al. 19, 20, 21. A dose response curve for 125I ultrasound guided prostate brachytherapy has also recently been reported (21).

The assessment of implant quality, however, remains an unanswered and controversial issue. Bice and Prestidge (22) evaluated 23 previously described parameters of implant quality with the conclusion that “no single best assessment tool has been demonstrated for prostate brachytherapy.” They also concluded that assessment of implant quality should be based on dose-volume histogram analysis. Quality is easy to conceptualize but more difficult to quantitate. There is no disagreement that a good implant should cover the target volume, but there is no consensus as to what represents an adequate dose. The American Brachytherapy Society (ABS) has recently proposed that brachytherapy quality be measured in terms of the following parameters: D90, V100, and V150 (23). D90 is defined as the minimal dose covering 90% of the prostate volume and V100 and V150 are defined as the percent volume of the prostate receiving at least 100% or 150% of the prescribed minimal peripheral dose (mPD), respectively. V100 and V150 are identical with the parameters proposed by Saw and Suntharalingam, which they coined the Coverage Index and the Dose Nonuniformity Ratio (24). To date, no study has compared the relationships between these parameters at the same point in time after the implant procedure. Multiple postoperative dosimetric publications have evaluated either a V100 12, 14, 15, 18, 25, 26, a D90 19, 20, 21 or a D80 and D100 16, 17. Herein, we report detailed dosimetric evaluation for 60 consecutive prostate brachytherapy patients implanted via a standard transperineal ultrasound guided approach in terms of D90, D100, V90, V100, and V150 and also the maximal and average rectal and urethral dose.

Section snippets

Materials and methods

Sixty consecutive patients who underwent transperineal ultrasound guided conformal prostatic brachytherapy from early December 1997 through mid-May 1998 using either 125I1 or 103Pd2 for clinical T1/T2 carcinoma of the prostate gland were retrospectively evaluated. The choice of isotope was dependent upon Gleason score. Because of significant inaccuracy of Gleason grading, all cases originating from outside institutions were reviewed prior to formulation of a treatment plan (27). Thirty patients

Results

A summary of the dosimetry data is listed in Table 2. The postimplant CT prostate volumes were nearly the same as the planning volumes of Table 1. This allowed evaluation of the preplan execution: the mean V100 for the study was 94.1% volume compared to 99.5% volume in the preplans. The mean D90 was 107.8% of the mPD. 81.7% of the cohort (49/60) had a D90 exceeding the prescribed dose and all patients had a D90 exceeding 0.9 × mPD. The mean and minimum of the quality parameters D90 and V100

Discussion

The management of clinically organ confined carcinoma of the prostate gland remains one of significant controversy within the uro-oncology community. Prostate brachytherapy has emerged however, as a viable option for the treatment of early stage carcinoma of the prostate gland. Recent reports have reported that in terms of local control and freedom from biochemical failure, the results of conformal brachytherapy have been found to be as favorable as the most positive radical prostatectomy

References (36)

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