T Meyer

Verifying monitor unit calculations for tangential whole- breast fields in three-dimensional planning

Verification of the accuracy of monitor unit calculations is an essential component of quality assurance in radiation therapy. For tangential breast fields, monitor unit differences between primary calculations and second checks are usually larger than would be considered acceptable at other anatomic sites. Here, we present a simple model to reconcile the differences between sophisticated and simple algorithms, based on estimating the volume irradiated by the field, replacing the breast contour with a rectangular block having an equal volume, but using a new field width that provides almost equivalent scatter to the prescription point. This analysis can also assist the treatment planning physicist in selecting a tolerance window for verifying monitor unit calculations for tangential breast fields. PACS numbers: 87.53.Kn, 87.53.Tf, 87.53.Xd

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.

SU-E-T-123: Anomalous Altitude Effect in Permanent Implant Brachytherapy Seeds

Purpose: Permanent seed implant brachytherapy procedures require the measurement of the air kerma strength of seeds prior to implant. This is typically accomplished using a well-type ionization chamber. Previous measurements (Griffin et al., 2005; Bohm et al., 2005) of several low-energy seeds using the air-communicating HDR 1000 Plus chamber have demonstrated that the standard temperature-pressure correction factor, PTP, may overcompensate for air density changes induced by altitude variations by up to 18%. The purpose of this work is to present empirical correction factors for two clinically-used seeds (IsoAid ADVANTAGE™ 103Pd and Nucletron selectSeed 125I) for which empirical altitude correction factors do not yet exist in the literature when measured with the HDR 1000 Plus chamber. Methods: An in-house constructed pressure vessel containing the HDR 1000 Plus well chamber and a digital barometer/thermometer was pumped or evacuated, as appropriate, to a variety of pressures from 725 to 1075 mbar. Current measurements, corrected with PTP, were acquired for each seed at these pressures and normalized to the reading at ‘standard’ pressure (1013.25 mbar). Results: Measurements in this study have shown that utilization of PTP can overcompensate in the corrected current reading by up to 20% and 17% for the IsoAid Pd-103 and the Nucletron I-125 seed respectively. Compared to literature correction factors for other seed models, the correction factors in this study diverge by up to 2.6% and 3.0% for iodine (with silver) and palladium respectively, indicating the need for seed-specific factors. Conclusion: The use of seed specific altitude correction factors can reduce uncertainty in the determination of air kerma strength. The empirical correction factors determined in this work can be applied in clinical quality assurance measurements of air kerma strength for two previously unpublished seed designs (IsoAid ADVANTAGE™ 103Pd and Nucletron selectSeed 125I) with the HDR 1000 Plus well chamber.

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.

Poster — Thurs Eve‐35: Error reduction in variable angle implant reconstruction by optimization of imaging geometry

Movement towards intraoperative dosimetric analysis of permanent seed brachytherapy implants has driven seed localization techniques that may be applied in an operating suite. Variable angle reconstructions with a C-arm are a widely available, cost effective and easily integrated solution but are subject to mechanical limitations on imaging geometry accuracy. Phantom reconstructions of known seed locations were used to show optimization of the imaging geometry for consistency with the observed datasets in multiple variable angle images can be used to obtain more accurate source and detector positions and reduce uncertainty in reconstructed seed positions. Furthermore, the most difficult part of backprojection methods is the matching of corresponding seeds between the variable angle images. With improved accuracy in imaging geometry, tighter constraints on possible seed matches can be used. This reduces the number of potential matches between images. Application to typical clinical data has shown a reduction in the number of possible matches. The benefits are observed when optimizations are performed on very small subsets of matched seeds, allowing a small number of obvious matches due to orientation or peripheral positioning to greatly simplify matching over the remainder of the implant.