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

Abstract 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 (p D 90 ), prostate volume (%) covered by 100% of the prescribed dose (p V 100 ), the rectal volume (cc) irradiated by 100% of the prescribed dose (r V 100 ), and the rectal volume (cc) irradiated by 150% of the prescribed dose (r V 150 ), whereas Eax showed a positive correlation with prostate volume (%) covered by 150% of the prescribed dose (p V 150 ) and the urethral dose (Gy) delivered to 5% of its volume (u D 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.