Tyler Meyer

Inter- and Intra-Observer Variability in Prostate Definition With Tissue Harmonic and Brightness Mode Imaging

PURPOSE The objective of this study was to compare the relative utility of tissue harmonic (H) and brightness (B) transrectal ultrasound (TRUS) images of the prostate by studying interobserver and intraobserver variation in prostate delineation. METHODS AND MATERIALS Ten patients with early-stage disease were randomly selected. TRUS images of prostates were acquired using B and H modes. The prostates on all images were contoured by an experienced radiation oncologist (RO) and five equally trained observers. The observers were blinded to information regarding patient and imaging mode. The volumes of prostate glands and areas of midgland slices were calculated. Volumes contoured were compared among the observers and between observer group and RO. Contours on one patient were repeated five times by four observers to evaluate the intraobserver variability. RESULTS A one-sample Student t-test showed the volumes outlined by five observers are in agreement (p > 0.05) with the RO. Paired Student t-test showed prostate volumes (p = 0.008) and midgland areas (p = 0.006) with H mode were significantly smaller than that with B mode. Two-factor analysis of variances showed significant interobserver variability (p < 0.001) in prostate volumes and areas. Inter- and intraobserver consistency was quantified as the standard deviation of mean volumes and areas, and concordance indices. It was found that for small glands (≤35 cc) H mode provided greater interobserver consistency; however, for large glands (≥35 cc), B mode provided more consistent estimates. CONCLUSIONS H mode provided superior inter- and intraobserver agreement in prostate volume definition for small to medium prostates. In large glands, H mode does not exhibit any additional advantage. Although harmonic imaging has not proven advantageous for all cases, its utilization seems to be judicious for small prostates.

Implanted brachytherapy seed movement reflecting transrectal ultrasound probe-induced prostate deformation

PURPOSE Compression of the prostate during transrectal ultrasound-guided permanent prostate brachytherapy is not accounted for during treatment planning. Dosimetry effects are expected to be small but have not been reported. The study aims to characterize the seed movement and prostate deformation due to probe pressure and to estimate the effects on dosimetry. METHODS AND MATERIALS C-arm fluoroscopy imaging was performed to reconstruct the implanted seed distributions (compressed and relaxed prostate) for 10 patients immediately after implantation. The compressed prostate was delineated on ultrasound and registered to the fluoroscopy-derived seed distribution via manual seed localization. Thin-plate spline mapping, generated with implanted seeds as control points, was used to characterize the deformation field and to infer the prostate contour in the absence of probe compression. Differences in TG-43 dosimetry for the compressed prostate and that on probe removal were calculated. RESULTS Systematic seed movement patterns were observed on probe removal. Elastic decompression was characterized by expansion in the anterior-posterior direction and contraction in the superior-inferior and lateral directions up to 4 mm. Bilateral shearing in the anterior direction was up to 6 mm, resulting in contraction of the 145 Gy prescription isodose line by 2 mm with potential consequences for the posterior-lateral margin. The average whole prostate D90 increased by 2% of prescription dose (6% max; p < 0.01). CONCLUSIONS The current investigation presents a novel study on ultrasound probe-induced deformation. Seed movements were characterized, and the associated dosimetry effects were nonnegligible, contrary to common expectation.

Dosimetric variations in permanent breast seed implant due to patient arm position.

PURPOSE Planning and delivery for permanent breast seed implant (PBSI) are performed with the ipsilateral arm raised; however, changes in implant geometry can be expected because of healing and anatomical motion as the patient resumes her daily activities. The purpose of this study is to quantify the effect of ipsilateral arm position on postplan dosimetry. METHODS AND MATERIALS Twelve patients treated at the Tom Baker Cancer Centre were included in this study. Patients underwent two postimplant CT scans on the day of implant (Day 0) and two scans approximately 8 weeks later (Day 60). One scan at each time was taken with the ipsilateral arm raised, recreating the planning scan position, and the other with both arms down in a relaxed position beside the body, recreating a more realistic postimplant arm position. Postplans were completed on all four scans using deformable image registration (MIM Maestro). RESULTS On the Day 0 scan, the V200 for the evaluation planning target volume was significantly increased in the arm-down position compared with the arm-up position. Lung, rib, and chest wall dose were significantly reduced at both time points. Left anterior descending coronary artery, heart, and skin dose showed no significant differences at either time point. CONCLUSIONS Although some dosimetric indices show significant differences between the arm-up and arm-down positions, the magnitude of these differences is small and the values remain indicative of implant quality. Despite the delivery of the majority of dose with the arm down, it is reasonable to use CT scans taken in the arm-up position for postplanning.

A framework for clinical commissioning of 3D‐printed patient support or immobilization devices in photon radiotherapy

Abstract Purpose The objective of this work is to outline a framework for dosimetric characterization that will comprehensively detail the clinical commissioning steps for 3D‐printed materials applied as patient support or immobilization devices in photon radiotherapy. The complex nature of 3D‐printed materials with application to patient‐specific configurations requires careful consideration. The framework presented is generalizable to any 3D‐printed object where the infill and shell combinations are unknown. Methods A representative cylinder and wedge were used as test objects to characterize devices that may be printed of unknown, patient‐specific dimensions. A case study of a 3D‐printed CSI immobilization board was presented as an example of an object of known, but adaptable dimensions and proprietary material composition. A series of measurements were performed to characterize the materials kV radiologic properties, MV attenuation measurements and calculations, energy spectrum water equivalency, and surface dose measurements. These measurements complement the recommendations of the AAPMs TG176 to characterize the additional complexity of 3D‐printed materials for use in a clinical radiotherapy environment. Results The dosimetric characterization of 3D‐printed test objects and a case study device informed the development of a step‐by‐step template that can easily be followed by clinicians to accurately and safely utilize 3D‐printed materials as patient‐specific support or immobilization devices. Conclusions A series of steps is outlined to provide a formulaic approach to clinically commission 3D‐printed materials that may possess varying material composition, infill patterns, and patient‐specific dimensions.

Technical Note: Empirical altitude correction factors for well chamber measurements of permanent prostate and breast seed implant sources

Purpose: Previous studies in the literature have measured an altitude effect for low‐energy brachytherapy seeds; a correction factor applied in addition to PTP to account for the breakdown of Bragg–Gray cavity theory at low energies in well‐type ionization chambers. In clinical practice, many centers use altitude correction factors that are not seed‐model‐specific. The purpose of this work is to present altitude correction factors for several seed models without documented factors in the literature. Methods: An in‐house constructed pressure vessel was used with a well‐type ionization chamber to measure the air‐kerma strength of the IsoAid Advantage (Pd‐103), Theragenics AgX100 (I‐125), and Nucletron selectSeed (I‐125) at a pressure range representative of those encountered worldwide. The TheraSeed 200 (Pd‐103) was also measured for comparison to the originally published correction factor for validation of the experimental process. When correction factors derived in this work were within experimental uncertainties of those published, no new correction factors were proposed. Results: The three seed models measured herein all demonstrated a similar response to change in pressure as previously documented in the literature with the HDR 1000 Plus well‐type ionization chamber. Correction factors of the functional form Symbol, consistent with those previously published, were found to be appropriate for these seed models. A new correction factor is proposed for the Theragenics AgX100 and Nucletron selectSeed (k1 = 0.0417, k2 = 0.479). The IsoAid Advantage, however, agreed to within uncertainty with the published altitude correction factor for the TheraSeed 200; thus the application of the same correction factor is appropriate (k1 = 0.0241, k2 = 0.562). Symbol. No Caption available. Conclusions: This work presents altitude correction factors for three permanent implant brachytherapy seed models in clinical use. This will allow clinics to utilize model‐specific factors, reducing systematic errors in their air‐kerma strength verifications.

Parameters predicting for prostate specific antigen response rates at one year post low-dose-rate intraoperative prostate brachytherapy

Purpose To develop a model for prostate specific antigen (PSA) values at one year among patients treated with intraoperatively planned 125I prostate brachytherapy (IOPB). Material and methods Four hundred and deven patients treated with IOPB for prostate adenocarcinoma were divided into four groups: those with PSA values ≥ 3 ng/ml; < 3 and ≥ 2; < 2 and ≥ 1 or PSA < 1 between 10.5 and 14.5 months post implantation (1yPSA). Ordinal regression analysis was then performed between patient, tumor, and treatment characteristics. 1yPSA values were also compared with toxicity outcomes. Results Median 1yPSA was 0.77 (0.04-17.36). Thirty-two patients (8%) had a PSA ≥ 3; 35 (9%) had PSA < 3, ≥ 2; 87 (21%) had PSA < 2, ≥ 1, and most patients 254 (62%) had PSA < 1. PSA response was independent of gland volume, Gleason score, clinical stage, seed activity, V90, V200, D90, or number of needles and seeds used. Older patients had significantly lower 1yPSA; median ages 65.1 (46.5-81.0), 62.1 (50.4-79.5), 60.5 (47.1-80.3), and 58.1 (45.1-74.2) years for each of the 1yPSA groups respectively (p < 0.001). Also, both implant V150 (p < 0.001) and initial PSA values (p = 0.04) were predictive of 1yPSA values. There was no correlation between 1yPSA values and toxicity encountered. Conclusions PSA response at 1 year post IOPB appears to be dependent on patient age, initial PSA, and implant V150. Our results provide reassurance that parameters other than biochemical failure influence 1yPSA values.

114: Patient Positioning and Mark-Up Optimization for Permanent Breast Seed Implant (PBSI)

S43 _________________________________________________________________________________________________________ in Ontario (ONT) and to develop recommendations to ensure all patients to have equitable access to MR-guided brachytherapy (MRgBT) for cervical cancer. Methods: A qualitative phone interview was designed by the GYN CoP working group to survey the current state of ccBT in the province. Questions were developed to inquire about the current use of image-guided ccBT and the associated referral processes, the usage of MR imaging in ccBT and the current use of imageguided interstitial GYN BT. All ONT cancer centres offering radiation treatments to GYN cancers were included. Two group members conducted and audio recorded the telephone interviews from May to November 2015 and analyzed all recordings and summarized the data. Results: Thirteen (n = 13) ONT cancer centres were interviewed. Of these, three centres do not offer ccBT, five centres offer CTguided ccBT, four centres offer a combination of CT-MR-guided ccBT and one centre offers strictly MR-guided ccBT. The three centres that do not offer ccBT have established referral processes with three tertiary cancer centres in ONT respectively. However, there is no standardized referral process, referral timing, or method of communication. Other practices vary throughout the centres. Three of 13 centres suggested developing a file portal to standardize and facilitate the sharing of external beam and BT plans, distributions and images. All CTguided ccBT centres except one have plans to develop MRgBT. The tertiary centres mentioned above are also the only centres that offer interstitial GYN BT. They are located in the southwestern part of the province. Of these, one centre offers CT-guided and two centres offer MR-guided interstitial GYN BT. There is currently no standardized guideline to identify patient candidates for interstitial GYN BT. Conclusions: This study demonstrated that models of shared care exist and are functioning in ONT. While referral processes are functioning well, some areas represent opportunities for improvement. Future work is needed by the GYN CoP to improve referral processes and to develop consensus on indications for interstitial brachytherapy. This will ensure all patients in ONT have access to this high quality brachytherapy.

Sci—Fri PM: Topics — 02: Evaluation of Dosimetric Variations in Partial Breast Seed Implant (PBSI) due to Patient Arm Position (Up vs. Down)

The planning for PBSI is done with the patients ipsilateral arm raised, however, anatomical changes and variations are unavoidable as the patient resumes her daily activities, potentially resulting in significant deviations in implant geometry from the treatment plan. This study aims to quantify the impact of the ipsilateral arm position on the geometry and dosimetry of the implant at eight weeks, evaluated on post-plans using the MIM Symphony™ software (MIM Software, Cleveland, OH). The average dose metrics for the three patients treated at the TBCC thus far using rigid fusion and contour transfer for the arms up position were 76% for the CTV V100, 61% for the PTV V100, and 37% for the PTV V200; and for the arms down position 81% for the CTV V100, 64% for the PTV V100, and 42% for the PTV V200. Qualitative analysis of the post-implant CT for one of the three patients showed poor agreement between the seroma contour transferred from the pre-implant CT and the seroma visible on the post-implant CT. To obtain a clinically accurate plan for that patient, contour modifications were used, yielding improved dose metric averages for the arms-up position for all three patients of 87% for the CTV V100, 68% for the PTV V100, and 39% for the PTV V200. Overall, the data available shows that dosimetric parameters increase with the patients arm down, both in terms of coverage and in terms of the hot spot, and accrual of more patients may confirm this in a larger population.

WE-A-17A-11: Implanted Brachytherapy Seed Movement Due to Transrectal Ultrasound Probe-Induced Prostate Deformation

PURPOSE To characterize the movement of implanted brachytherapy seeds due to transrectal ultrasound probe-induced prostate deformation and to estimate the effects on prostate dosimetry. METHODS Implanted probe-in and probe-removed seed distributions were reconstructed for 10 patients using C-arm fluoroscopy imaging. The prostate was delineated on ultrasound and registered to the fluoroscopy seeds using a visible subset of seeds and residual needle tracks. A linear tensor and shearing model correlated the seed movement with position. The seed movement model was used to infer the underlying prostate deformation and to simulate the prostate contour without probe compression. Changes in prostate and surrogate urethra dosimetry were calculated. RESULTS Seed movement patterns reflecting elastic decompression, lateral shearing, and rectal bending were observed. Elastic decompression was characterized by anterior-posterior expansion and superior-inferior and lateral contractions. For lateral shearing, anterior movement up to 6 mm was observed for extraprostatic seeds in the lateral peripheral region. The average intra-prostatic seed movement was 1.3 mm, and the residual after linear modeling was 0.6 mm. Prostate D90 increased by 4 Gy on average (8 Gy max) and was correlated with elastic decompression. For selected patients, lateral shearing resulted in differential change in D90 of 7 Gy between anterior and posterior quadrants, and increase in whole prostate D90 of 4 Gy. Urethra D10 increased by 4 Gy. CONCLUSION Seed movement upon probe removal was characterized. The proposed model captured the linear correlation between seed movement and position. Whole prostate dose coverage increased slightly, due to the small but systematic seed movement associated with elastic decompression. Lateral shearing movement increased dose coverage in the anterior-lateral region, at the expense of the posterior-lateral region. The effect on whole prostate D90 was smaller due to the subset of peripheral seeds involved, but lateral shearing movement can have greater consequences for local dose coverage.

Poster — Thur Eve — 58: Dosimetric validation of electronic compensation for radiotherapy treatment planning

In this study we investigate the deliverability of dosimetric plans generated by the irregular surface compensator (ISCOMP) algorithm for 6 MV photon beams in Eclipse (Varian Medical System, CA). In contrast to physical tissue compensation, the electronic ISCOMP uses MLCs to dynamically modulate the fluence of a photon beam in order to deliver a uniform dose at a user defined plane in tissue. This method can be used to shield critical organs that are located within the treatment portal or improve dose uniformity by tissue compensation in inhomogeneous regions. Three site specific plans and a set of test fields were evaluated using the γ-metric of 3%/ 3 mm on Varian EPID, MapCHECK, and Gafchromic EBT3 film with a clinical tolerance of >95% passing rates. Point dose measurements with an NRCC calibrated ionization chamber were also performed to verify the absolute dose delivered. In all cases the MapCHECK measured plans met the gamma criteria. The mean passing rate for the six EBT3 film field measurements was 96.2%, with only two fields at 93.4 and 94.0% passing rates. The EPID plans passed for fields encompassing the central ∼10 × 10 cm2 region of the detector; however for larger fields and greater off-axis distances discrepancies were observed and attributed to the profile corrections and modeling of backscatter in the portal dose calculation. The magnitude of the average percentage difference for 21 ion chamber point dose measurements and 17 different fields was 1.4 ± 0.9%, and the maximum percentage difference was −3.3%. These measurements qualify the algorithm for routine clinical use subject to the same pre-treatment patient specific QA as IMRT.