PHSOR08 Presentation Time: 9:35 AM

Abstract

Purpose/Objectives

Interstitial HDR brachytherapy to the prostate involves precise, localized delivery to targets with high dose gradients, sparing adjacent organs at risk (OAR). Due to the proximity of the rectum and bladder to the target, deviations in the template position or catheter with respect to patient anatomy can change dose to those structures. While utilizing plastic interstitial catheters allows the patient to receive hyperthermia therapy during their course of interstitial treatment, the plastic catheters were observed to show significant deflection when the metal stylets are removed for treatment. The flexion of the source path for this particular afterloader has been previously demonstrated to show significant deviation within rigid intracavitary devices. The purpose of this study is to assess the magnitude and deflection of the plastic catheters for prostate interstitial plans and determine whether using flexible wires of similar thickness to the source cable for visualization on pre-treatment imaging is suitable for reconstructing the actual treatment received.

Materials/Methods

Pre-treatment CT images where the stylets were utilized for catheter visualization were registered to a subsequent CT scan with the metal stylets removed. The original treatment dwell positions and times were adjusted to the second CT to evaluate three-dimensional catheter displacement and dose distributions calculated for the bladder, rectum, urethra, and target. The plans were subsequently reconstructed on a pre-planning CT with flexible 20-gauge wires instead of metal stylets in order to determine their suitability as a surrogate for stylets.

Results

Absolute 3D displacement for all patients was 4.9 mm ± 3.0 mm (n = 1645 catheter dwell positions for all treatment plans and patients, mean ± SD, p<0.001) comparing plans with the stylet in versus out. Absolute catheter deflection magnitude for interstitial treatments increased with the removal of the metal stylets with a difference of 1.07° ± 0.83° (n = 153 catheters for all treatment plans and patients, mean ± SD, p<0.01). The plans utilizing flexible wire instead of stylets are observed to correlate with the plans with no stylets with absolute 3D displacement and angular deflection of 1.7 ± 1.1 mm and 0.36° ± 0.3° respectively (n = 67 catheters, mean ± SD, p>0.2). There was an overall increase in bladder D2cc (p<0.01) with the stylets utilized vs removed. Bladder D2cc, rectal D2cc, and urethra D0.1cc showed no appreciable difference between utilizing flexible wires for visualization and the plans without (p>0.10). There is no appreciable difference in target coverage comparing the plan types (p>0.5).

Conclusions

Catheter reconstruction in interstitial prostate treatments with CT imaging revealed significant changes in catheter positioning with respect to the OAR volumes once the stylets are removed for treatment. While the catheters maintain their geometry relative to each other, they move with the prostate as a whole. Using flexible wires of similar thickness to the source cable allow for more accurate tracking during planning without distorting the patient's anatomy, preserving the integrity of the treatment plan.