Siraj Husain

Sci-Thur PM – Brachytherapy 02: Positional accuracy in Pd-103 permanent breast seed implant (PBSI) brachytherapy at the Tom Baker Cancer Centre (TBCC)

Purpose: This retrospective study aims to quantify the positional accuracy of seed delivery in permanent breast seed implant (PBSI) brachytherapy at the Tom Baker Cancer Centre (TBCC). Methods: Treatment planning and post-implant CT scans for 5 patients were rigidly registered using the MIM Symphony™ software (MIM Software, Cleveland, OH) and used to evaluate differences between planned and implanted seed positions. Total and directional seed displacements were calculated for each patient in a clinically relevant ‘needle coordinate system’, defined relative to the angle of fiducial needle insertion. Results: The overall average total seed displacement was 10±8 mm. Systematic seed displacements were observed in individual patients and the magnitude and direction of these offsets varied among patients. One patient showed a significant directional seed displacement in the shallow-deep direction compared with the other four patients. With the exception of this one patient outlier, no significant systematic directional displacements in the needle coordinate system were observed for this cohort; the average directional displacements were −1±5 mm, 2±3 mm, and −2±4 mm in the shallow-deep, up-down, and right-left directions respectively. Conclusion: With the exception of one patient outlier, the magnitude of seed displacements were relatively consistent among patients. The results indicate that the shallow-deep direction possesses the largest uncertainty for the seed delivery method used at the TBCC. The relatively large uncertainty in seed placement in this direction is expected, as this is the direction of needle insertion. Further work will involve evaluating deflections of delivered needle tracks from their planned positions.

Poster — Thur Eve — 77: Implanted Brachythearpy Seed Movement due to Transrectal Ultrasound Probe‐Induced Prostate Deformation

The study investigated the movement of implanted brachytherapy seeds upon transrectal US probe removal, providing insight into the underlying prostate deformation and an estimate of the impact on prostate dosimetry. Implanted seed distributions, one obtained with the prostate under probe compression and another with the probe removed, were reconstructed using C-arm fluoroscopy imaging. The prostate, delineated on ultrasound images, was registered to the fluoroscopy images using seeds and needle tracks identified on ultrasound. A deformation tensor and shearing model was developed to correlate probe-induced seed movement with position. Changes in prostate TG-43 dosimetry were calculated. The model was used to infer the underlying prostate deformation and to estimate the location of the prostate surface in the absence of probe compression. Seed movement patterns upon probe removal reflected elastic decompression, lateral shearing, and rectal bending. Elastic decompression was characterized by expansion in the anterior-posterior direction and contraction in the superior-inferior and lateral directions. Lateral shearing resulted in large anterior movement for extra-prostatic seeds in the lateral peripheral region. Whole prostate D90 increased up to 8 Gy, mainly due to the small but systematic seed movement associated with elastic decompression. For selected patients, lateral shearing movement increased prostate D90 by 4 Gy, due to increased dose coverage in the anterior-lateral region at the expense of the posterior-lateral region. The effect of shearing movement on whole prostate D90 was small compared to elastic decompression due to the subset of peripheral seeds involved, but is expected to have greater consequences for local dose coverage.

MO‐D‐AUD B‐06: Source Motion in Permanent Implant Prostate Brachytherapy Due to Ultrasound Probe Deformation

Purpose: To determine the amount of seed motion in the prostate due to ultrasound probe deformation during permanent implant prostate brachytherapy.Method and Materials: A C‐arm was used to take variable angle images of clinical implants immediately after the last needle was delivered with the patient and ultrasound remaining in the treatment position, after the ultrasound probe was lowered, and after it had been removed with the patient remaining in the treatment position. Three dimensional seed coordinates were calculated and corresponding seed coordinates were compared to determine the motion induced by the ultrasound probe. A rigid body registration was performed and deformational effects were evaluated using the residual seed motion. Results: Seed positions over all patients moved, on average, 6.6 mm posterior, 1.6 mm caudal, 1.5 mm patient right and the mean total motion was 7.1 mm (range 2.1 mm – 12.3 mm). The mean for a single patient ranged from 5.3 mm (2.4 mm – 8.3 mm) to 9.7 mm (8.1 mm – 12.3 mm). The rigid body registrations showed rotation about an axis perpendicular to a sagittal plane in each patient (mean 4.2°, range 2.9° – 5.9°). The mean residual seed motion was 1.1 mm (0.2 mm – 4.4 mm) and showed non‐random deformational patterns. Conclusion: Final seed positions are significantly different from those delivered due to the ultrasound probe. Non‐random residual motion within the implant can be associated with deformation and may have dosimetric consequences.