Three-dimensional summation of rectal doses in brachytherapy combined with external beam radiotherapy for prostate cancer.

BACKGROUND AND PURPOSE: To determine the dose constraints for rectal bleeding in brachytherapy (BRT) combined with external beam radiotherapy (EBRT). MATERIALS AND METHODS: Post-BRT, pelvic computed tomography images were used for subsequent EBRT planning and BRT postplans in 37 patients. The physical doses for each plan were converted to biologically effective doses, and corresponding voxel doses were integrated to plot the summed dose-volume histogram (sum-DVH). Between 5 patients with (bled-pts) and 32 without (spared-pts) grade 2 or 3 rectal bleeding, the differences in the mean minimal dose (rDn) covering the rectal volume of 0.5-10.0 cc and the rectal volume (rVn) receiving the calculated dose of 20-150Gy were compared. RESULTS: The differences in the summed-rDn were determined by BRT exposure, while those of the summed-rVn were determined in the low-dose range and superimposed in the high-dose range by EBRT exposure. Of the 13 patients with rV150 of >1.2 cc, 4 were bled-pts (30.8%). Of the 24 patients with rV150 of ≤ 1.2cc, 1 was a bled-pts (4.2%) (p=0.024; odds ratio, 10.2; CI (95%), 1.0-104.3). CONCLUSIONS: The mono-scale DVH analysis is a promising method for exploring the threshold for rectal bleeding in combined radiotherapy.

Effects of ellipsoid prostate deformation on dose delivery during permanent interstitial brachytherapy.

PURPOSE: To elucidate the potential effects of prostate deformation on dose distribution during Iodine-125 ((125)I) seed implantation brachytherapy for prostate cancer. METHODS AND MATERIALS: A retrospective analysis of 245 patients who underwent only transperineal brachytherapy for low-risk prostate adenocarcinoma was performed. The maximum diameters of the prostate were measured before treatment by transrectal ultrasound volumetry along right to left (RL), anterior to posterior (AP), and apex to base (Length) directions. The seeds were inserted by the modified peripheral loading method using real-time ultrasound-guided seed placement. The ellipsoid deformation rates in the axial plane (E(ax)) and in the sagittal plane (E(sag)) were defined as [RL-AP]/RL and [Length-AP]/Length, respectively. The correlation between them and the dose-volume histogram parameters at 30 days after the operation was evaluated. A simulation test was additionally performed to ascertain the change in dose distribution among virtual volumes built in a radiotherapy planning device that corresponds to prostates with increased Eax or Esag. RESULTS: The mean Esag and Eax of patients were 0.313 (range, -0.28 to 0.844) and 0.261 (range, -0.02 to 0.54), respectively. Esag showed a positive correlation with dose (Gy) covering 90% of the prostate volume (pD(90)), prostate volume (%) covered by 100% of the prescribed dose (pV(100)), the rectal volume (cc) irradiated by 100% of the prescribed dose (rV(100)), and the rectal volume (cc) irradiated by 150% of the prescribed dose (rV(150)), whereas Eax showed a positive correlation with prostate volume (%) covered by 150% of the prescribed dose (pV(150)) and the urethral dose (Gy) delivered to 5% of its volume (uD(5)). The simulation test suggested that the prescribed dose resulted in the best coverage in patients with increased E(sag), and that patients with increased E(ax) exhibited poor urethral sparing from overdosage. CONCLUSION: In the seed implantation method, ellipsoid deformation of the prostate causes higher rectal dose exposure or dose delivery to the urethra.