Normal tissue dosimetric comparison between HDR prostate implant boost andconformal external beam radiotherapy boost: Potential for dose escalation

Purpose: To compare the dose and volume of bladder and rectum treated using high-dose-rate (HDR) prostate implant boost versus conformal external beam radiotherapy boost, and to use the dose-volume information to perform a cr itical volume tolerance (CVT) analysis and then estimate the potential for further dose escalation using HDR brachytherapy boost. Methods and Materials: Using CT scan data collected before and after patien ts underwent HDR prostate implant, a 7-field conformal prostate-only extern al beam treatment plan and HDR brachytherapy treatment plan were constructe d for each patient. Doses to the normal structures were calculated. Dose-vo lume histograms (DVH) were plotted for comparison of the two techniques. Wi lcoxon signed rank test was performed at four dose levels to compare the do se to normal structures between the two treatment techniques. The acute and late effects of HDR brachytherapy were calculated based on the linear-quad ratic (LQ) model. CVT analyses were performed to calculate the potential do se gain (PDG) using HDR brachytherapy boost. Results: The volume of bladder and rectum receiving high dose was significa ntly less from implant boost. On the average, 0.19 cc of the bladder receiv ed 100% of the brachytherapy prescription dose, compared with 5.1 cc of the bladder receiving 100% of the prescription dose in the 7-field conformal e xternal beam radiotherapy boost. Similarly, 0.25 cc of the rectum received 100% of the dose with the implant boost, as compared to 2.9 cc in the confo rmal external beam treatment. The implant also delivered higher doses insid e the prostate volume. On average, 47% of the prostate received greater tha n or equal to 150% of the prescription dose. The CVT analysis revealed a ra nge of PDG using the HDR brachytherapy boast which depended on the followin g variables: critical volume (CV), critical volume tolerance dose (CVTD), n umber of HDR fractions (N), and the dose of external beam radiotherapy (XRT ) delivered with brachytherapy boost. The PDG varied from -3.45% to 10.53% for tumor with an alpha-beta ratio of 10 and 7.14% to 64.6% for tumor with an alpha-beta ratio of 1.5 based on the parameters used for calculation in this study. Conclusions: HDR brachytherapy can provide better sparing of rectum and bla dder while delivering a higher dose to the prostate. Even with the increase d late effects of high dose per fraction, there is still a potential for do se escalation beyond external radiotherapy limits using HDR brachytherapy. (C) 2000 Elsevier Science Inc.