J Cunha

Evaluation of PC-ISO for Customized, 3D Printed, Gynecologic 192-Ir HDR Brachytherapy Applicators

The purpose of this study was to evaluate the radiation attenuation properties of PC‐ISO, a commercially available, biocompatible, sterilizable 3D printing material, and its suitability for customized, single‐use gynecologic (GYN) brachytherapy applicators that have the potential for accurate guiding of seeds through linear and curved internal channels. A custom radiochromic film dosimetry apparatus was 3D‐printed in PC‐ISO with a single catheter channel and a slit to hold a film segment. The apparatus was designed specifically to test geometry pertinent for use of this material in a clinical setting. A brachytherapy dose plan was computed to deliver a cylindrical dose distribution to the film. The dose plan used an 192Ir source and was normalized to 1500 cGy at 1 cm from the channel. The material was evaluated by comparing the film exposure to an identical test done in water. The Hounsfield unit (HU) distributions were computed from a CT scan of the apparatus and compared to the HU distribution of water and the HU distribution of a commercial GYN cylinder applicator. The dose depth curve of PC‐ISO as measured by the radiochromic film was within 1% of water between 1 cm and 6 cm from the channel. The mean HU was ‐10 for PC‐ISO and ‐1 for water. As expected, the honeycombed structure of the PC‐ISO 3D printing process created a moderate spread of HU values, but the mean was comparable to water. PC‐ISO is sufficiently water‐equivalent to be compatible with our HDR brachytherapy planning system and clinical workflow and, therefore, it is suitable for creating custom GYN brachytherapy applicators. Our current clinical practice includes the use of custom GYN applicators made of commercially available PC‐ISO when doing so can improve the patients treatment. PACS number: none

Dosimetric equivalence of nonstandard HDR brachytherapy catheter patterns

PURPOSE To determine whether alternative high dose rate prostate brachytherapy catheter patterns can result in similar or improved dose distributions while providing better access and reducing trauma. MATERIALS AND METHODS Standard prostate cancer high dose rate brachytherapy uses a regular grid of parallel needle positions to guide the catheter insertion. This geometry does not easily allow the physician to avoid piercing the critical structures near the penile bulb nor does it provide position flexibility in the case of pubic arch interference. This study used CT datasets with 3 mm slice spacing from ten previously treated patients and digitized new catheters following three hypothetical catheter patterns: conical, bi-conical, and fireworks. The conical patterns were used to accommodate a robotic delivery using a single entry point. The bi-conical and fireworks patterns were specifically designed to avoid the critical structures near the penile bulb. For each catheter distribution, a plan was optimized with the inverse planning algorithm, IPSA, and compared with the plan used for treatment. Irrelevant of catheter geometry, a plan must fulfill the RTOG-0321 dose criteria for target dose coverage (V100(Prostate) > 90%) and organ-at-risk dose sparing (V75(Bladder) < 1 cc, V75(Rectum) < 1 cc, V125(Urethra) << 1cc). RESULTS The three nonstandard catheter patterns used 16 nonparallel, straight divergent catheters, with entry points in the perineum. Thirty plans from ten patients with prostate sizes ranging from 26 to 89 cc were optimized. All nonstandard patterns fulfilled the RTOG criteria when the clinical plan did. In some cases, the dose distribution was improved by better sparing the organs-at-risk. CONCLUSION Alternative catheter patterns can provide the physician with additional ways to treat patients previously considered unsuited for brachytherapy treatment (pubic arch interference) and facilitate robotic guidance of catheter insertion. In addition, alternative catheter patterns may decrease toxicity by avoidance of the critical structures near the penile bulb while still fulfilling the RTOG criteria.

WE-F-16A-01: Commissioning and Clinical Use of PC-ISO for Customized, 3D Printed, Gynecological Brachytherapy Applicators

PURPOSE (1) Evaluate the safety and radiation attenuation properties of PCISO, a bio-compatible, sterilizable 3D printing material by Stratasys, (2) establish a method for commissioning customized multi- and single-use 3D printed applicators, (3) report on use of customized vaginal cylinders used to treat a series of serous endometrial cancer patient. METHODS A custom film dosimetry apparatus was designed to hold a Gafchromic radio film segment between two blocks of PC-ISO and 3D-printed using a Fortus 400mc (StrataSys). A dose plan was computed using 13 dwell positions at 2.5 mm spacing and normalized to 1500 cGy at 1 cm. Film exposure was compared to control tests in only air and only water. The average Hounsfield Unit (HU) was computed and used to verify water equivalency. For the clinical use cases, the physician specifies the dimensions and geometry of a custom applicator from which a CAD model is designed and printed. RESULTS The doses measured from the PC-ISO Gafchromic film test were within 1% of the dose measured in only water between 1cm and 6cm from the channel. Doses increased 7-4% measured in only air. HU range was 11-43. The applicators were sterilized using the Sterrad system multiple times without damage. As of submission 3 unique cylinders have been designed, printed, and used in the clinic. A standardizable workflow for commissioning custom 3D printed applicators was codified and will be reported. CONCLUSIONS Quality assurance (QA) evaluation of the PC-ISO 3D-printing material showed that PC-ISO is a suitable material for a gynecological brachytherapy vaginal cylinder in a clinical setting. With the material commissioning completed, if the physician determines that a better treatment would Result, a customized design is fabricated with limited additional QA necessary. Although this study was specific to PC-ISO, the same setup can be used to evaluate other 3D-printing materials.

WE‐E‐108‐10: Validating a 192Ir‐Based Small Animal Irradiation Apparatus Using a 3D‐Printed Applicator: Comparison Between TG‐43, Monte Carlo and Films Dosimetry

PURPOSE Accurate assessment of dose delivered is key to early prediction of radiation-induced anatomic and physiologic changes vital in providing the most accurate patient-specific treatments. We are conducting a translational small-animal study using functional Hyperpolarized-13C-Urea with DCE-MRI to evaluate tumor perfusion changes following local targeted Ir-192 irradiation using the Leipzig applicator. The purpose of this study is to present and evaluate a novel Monte Carlo (MC) tool, which provides heterogeneous dose predictions for this specific application, and to compare the results against the TG-43 dose calculation. METHODS CT scans were obtained with a Leipzig applicator mold (fabricated using a 3D-printer to avoid metal artifact) centrally placed on top of the CyberKnife Ball Cube QA phantom . The CT density of the mold was overridden to be that of the applicator material, Tungsten. A Monte Carlo platform, ALGEBRA (ALgorithm for heterogeneous dosimetry based on GEANT4 for BRAchytherapy) was used for simulation. Dose measurements were done using Gafchromic EBT2 film placed orthogonally inside the Ball Cube exposed to Ir-192 with the Leipzig applicator in place. Simple TG-43 calculations for a free source in water was done using the Oncentra treatment planning system. RESULTS The two-dimensional planar dose distributions obtained from MC simulation showed strong agreement (within 4-5%) with dose measurements while TG-43 calculations showed differences up to 20%. Compared to TG-43, the MC Result was attenuated less at the surface because of the Leipzig air cavity, but penetrated less due to the collimation effect. CONCLUSION We have validated a new MC simulation tool using a Leipzig applicator for small animal irradiation. This tool will provide the basis for studies associating tumor response with the actual dose delivered. The tool can be further used to construct dosimetry information for clinical treatments using the Leipzig applicator as an alternative to superficial/orthovoltage radiation treatment.

TU‐D‐BRB‐03: Enforcing Maximum Dwell Times in High Dose Rate Brachytherapy Highlights the Tradeoff between Small Dwell Time Gradients and Dose Coverage

Purpose Applicator‐based dose optimization (forward planning) emphasizes dwell time homogeneity to avoid regions of exceptionally high dose concentration. Anatomy‐based dose optimization (inverse planning) relaxes this constraint in order to obtain a dose distribution that is more conformal the anatomy. We developed a version of inverse planning constrained by maximum dwell times. The relationship between dwell time homogeneity and the dose delivered to cancerous and healthy organs is examined. Methods and Materials A software method was implemented within the IPSA inverse planning algorithm to allow for (1) imposition of a user‐defined global maximum dwell time at any given dwell position and (2) homogeneity of dwell times within any given catheter. In‐silico studies were performed on six previously‐treated‐patient image datasets using the contours and catheter digitization of the clinically‐used plan. Two each of tandem‐and‐ring gynecological, tandem‐and‐ovoid gynecological, and prostate were examined. The plan used for treatment served as the control and had no restriction on dwell times. New plans were optimized by imposing either a global (Study 1) or catheter‐specific maximum dwell times (Study 2) for different time limits. Dosimetric indices were compared with the control plans. Results Clinically acceptable plans were achievable with modest restriction of the dwell times, but severe restriction lead to unacceptable plans. Across all studies, restriction of dwell times correlated with an increase of the final objective function value. A higher objective function means less adherence of the optimization to the dosimetric requirements imposed on the optimization by the user. Conclusion Imposing restrictions on a global maximum dwell and intra‐catheter dwell time homogeneity necessarily results in a worsening of the dosimetric optimization objective function. However, less severe restrictions can allow for some tailoring of the dose distribution to avoid intra‐catheter dwell time heterogeneity with only a small decrease in target coverage. Research sponsored in part by Nucletron.

TU-FG-202-03: Recent Technology Innovations in Prostate Brachytherapy

The greater use of magnetic resonance imaging (MR) in brachytherapy, for both image based planning and image guided procedures, presents a number of challenges for the physicist involved. MR is widely acknowledged as the imaging modality of choice to appreciate a patients pelvic anatomy and is thus used more often for brachytherapy of gynecologic and prostate cancers. Incorporating MR in brachytherapy treatments requires changes throughout the clinical workflow from implantation through target definition, applicator digitization and post-implant evaluation. These presentations will review the some of the concerns encountered in image based brachytherapy environment, highlight the challenges to incorporating more information from MR, and discuss some of the technical developments underway to facilitate the further use of MR in brachytherapy. LEARNING OBJECTIVES 1. To review the rationale for incorporating MR in image based brachytherapy for treatment of gynecologic and prostate cancers. 2. To review the technical challenges encountered when using brachytherapy devices in an MR environment. 3. To discuss some developments to facilitate MR based planning and guidance. J. Cunha, Phillips Healthcare.

SU-G-JeP1-10: Feasibility of CyberKnife Tracking Using the Previously-Implanted Permanent Brachytherapy Seed Cloud

PURPOSE Robotic radiosurgery is a salvage treatment option for patients with recurrent prostate cancer. We explored the feasibility of tracking the bolus of permanent prostate implants (PPI) using image recognition software optimized to track spinal anatomy. METHODS Forty-five inert iodine seeds were implanted into a gelatin-based prostate phantom. Four superficial gold seeds were inserted to provide ground-truth alignment. A CT scan of the phantom (120 kVp, 1 mm slice thickness) was acquired and a single-energy iterative metal artifact reduction (MAR) algorithm was used to enhance the quality of the DRR used for tracking. CyberKnife treatment plans were generated from the MAR CT and regular CT (no-MAR) using spine tracking. The spine-tracking grid was centered on the bolus of seeds and resized to encompass the full seed cloud. A third plan was created from the regular CT scan, using fiducial tracking based on the 4 superficial gold seeds with identical align-center coordinates. The phantom was initially aligned using the fiducial-tracking plan. Then the MAR and no-MAR spine-tracking plans were loaded without moving the phantom. Differences in couch correction parameters were recorded in the case of perfect alignment and after the application of known rotations and translations (roll/pitch of 2 degrees; translations XYZ of 2 cm). RESULTS The spine tracking software was able to lock on to the bolus of seeds and provide couch corrections both in the MAR and no-MAR plans. In all cases, differences in the couch correction parameters from fiducial alignment were <0.5 mm in translations and <1 degree in rotations. CONCLUSION We were able to successfully track the bolus of seeds with the spine-tracking grid in phantom experiments. For clinical applications, further investigation and developments to adapt the spine-tracking algorithm to optimize for PPI seed cloud tracking is needed to provide reliable tracking in patients. One of the authors (MD) has received research support and speaker honoraria from Accuray.

SU-C-16A-05: OAR Dose Tolerance Recommendations for Prostate and Cervical HDR Brachytherapy: Dose Versus Volume Metrics

PURPOSE HDR brachytherapy consensus dose tolerance recommendations for organs at risk (OARs) remain widely debated. Prospective trials reporting metrics must be sufficiently data-dense to assess adverse affects and identify optimally predictive tolerances. We explore the tradeoffs between reporting dose-metrics versus volume-metrics and the potential impact on trial outcome analysis and tolerance recommendations. METHODS We analyzed 26 prostate patients receiving 15 Gy HDR single-fraction brachytherapy boost to 45 Gy external beam radiation therapy and 28 cervical patients receiving 28 Gy HDR brachytherapy monotherapy in 4 fractions using 2 implants. For each OAR structure, a robust linear regression fit was performed for the dose-metrics as a function of the volume-metrics. The plan quality information provided by recommended dose-metric and volume-metric values were compared. RESULTS For prostate rectal dose, D2cc and V75 lie close to the regression line, indicating they are similarly informative. Two outliers for prostate urethral dose are substantially different from the remaining cohort in terms of D0.1cc and V75, but not D1cc, suggesting the choice of reporting dose metric is essential. For prostate bladder and cervical bladder, rectum, and bowel, dose outliers are more apparent via V75 than recommended dose-metrics. This suggests that for prostate bladder dose and all cervical OAR doses, the recommended volume-metrics may be better predictors of clinical outcome than dose-metrics. CONCLUSION For plan acceptance criteria, dose and volume-metrics are reciprocally equivalent. However, reporting dosemetrics or volume-metrics alone provides substantially different information. Our results suggest that volume-metrics may be more sensitive to differences in planned dose, and if one metric must be chosen, volumemetrics are preferable. However, reporting discrete DVH points severely limits the ability to identify planning tolerances most predictive of adverse effects. Thus, we recommend that full OAR DVH reporting be required for future prospective trials.

MO‐D‐144‐01: Ultrasound Guided RT Intervention & Novel Technologies

Significant advances were made over the past decade in ultrasound (US) imaging and new therapeutic applications are emerging. Used alone or in combination with other imaging modalities, US imaging is also well suited for robot-assisted interventions. This session will feature invited presentations on (1) the use of US and robot for interstitial breast implant high dose rate (HDR) brachytherapy, (2) the possibilities of electromagnetic tracking system for interactive needle navigation, 3D-printed patient-specific skewed-needle template, and co-robots for interventional brachytherapy, (3), the real time US guidance, dosimetry and treatment optimization for novel HDR brachytherapy such as single fraction partial prostate treatment for early stage disease, and (4) the development of an integrated 3D x-ray/ultrasound imaging system for on-board guidance of soft tissue targets for external beam radiation therapy. LEARNING OBJECTIVES 1. Understand the improvements made on ultrasound imaging technology 2. Understand the possibilities of real-time imaging for a wide range of applications 3. Identify new applications for medical physicists of ultrasound imaging for image-guided interventions. Part of this work is supported by NCI R01 CA 161613, and another part by a research contract with Philips Medical Systems.

TU-C-141-07: Limitations of Summing Dose Across Fractions: A Simple Test to Identify Failure

Purpose: With the increased availability of clinical deformable image registration tools, summing dose distributions calculated over different CT volumes is a tempting option. However, visual inspections of image registrations are insufficient to assess accuracy, and no patient‐specific QA methods exist to‐date. If the plan quality of the resulting summed dose is poor, it is difficult to discern whether the degradation results from organ motion or registration inaccuracies. We present a straightforward patient‐specific test that identifies invalid dose registrations. Methods: We applied the proposed test to assess dose registration accuracy for an illustrative 2‐fraction HDR brachytherapy prostate case. One plan per fraction was optimized using a unique CT acquired and contoured for each fraction and MIMVista was used to deformably register the clinical dose and contours. The resulting deformed and target CTs were compared. DVHs for the original dose distributions were calculated on their corresponding contours and for the deformed dose using both the target and warped contours. The four resulting DVH sets were compared and analyzed. Results: Though the deformed and target CTs match closely, V100 for the deformed dose plan is 42.0% as compared to 88.6% and 89.6% for the first and second day plans. Computing the deformed dose prostate DVH on the warped contours instead produces a target DVH with V100 of 82.6%, which differs greatly from the deformed V100 calculated on the target contours. Conclusion: When assessing a deformed dose, it is impossible to determine a priori whether degradation of the plan DVHs has resulted from organ motion or registration inaccuracies. If contoured and registered properly, the DVHs of the deformed dose should be similar regardless of whether they are computed over the deformed or target CT contours. Large differences between the two are a strong indication that the warped dose is unusable.