Purpose: Permanent prostate brachytherapy has inherent intraoperative organ deformation due to the inflatable trans-rectal ultrasound probe cover. Since the majority of the dose is delivered with reduced prostate deformation, the dosimetry approved at the time of implant may not accurately represent the dose delivered to the target and organs at risk. We aimed to evaluate the biological effect of the prostate deformation and its correlation with patient-reported outcome.
Methods: We prospectively acquired ultrasound images of the prostate pre- and post-probe cover inflation for 27 patients undergoing Iodine-125 seed implant. Coordinates of implanted seeds from the approved clinical plan were transferred to the undeformed prostate to simulate the actual dosimetry using a machine learning-based deformable image registration. DVHs of both plans were reduced to Tumor Control Probability (TCP) and Normal Tissue Complication Probability (NTCP) with assumed generalized equivalent uniform dose distribution. The change in patient-reported rectal and urinary symptoms six months post procedure were correlated with NTCP using area under the curve (AUC) and odds ratio (OR).
Results: From the deformed clinical plan to the undeformed plan, the average TCP decreased from 98.9% to 94.2% and the average NTCP changed from 0.4% and 26.7% to 18.0% and 17.3% for the urethra and rectum, respectively (p<0.05). For the diarrhea symptom NTCP>20%, the undeformed plan showed AUC=0.75, OR=8.9 (1.3-58.8) while the deformed plan showed AUC=0.56 and OR=1.4 (0.2-9.0). For the urinary control NTCP>15%, the undeformed plan showed AUC=0.70, OR=6.9 (0.6-78.0) while the deformed plan showed AUC=0.51 and no reported OR since all NTCP were below 6.6%.
Conclusion: With the consideration of organ deformation, TCP decreased while urethra NTCP showed worse sparing and rectal NTCP showed better sparing. The NTCP of the undeformed plan shows a strong correction with patient-reported diarrhea and urinary control, while the original clinical plan does not.