Introduction Cervical cancer brachytherapy requires highly conformal dose delivery while minimizing radiation exposure to surrounding organs at risk. However, conventional tandem and ring applicators are limited by fixed geometry that may not optimally accommodate patient-specific anatomy. The purpose of this work was to evaluate the dosimetric performance of a novel tandem and ring applicator (RTA) featuring adjustable tandem translation relative to the ring, intended to improve organ-at-risk (OAR) sparing while preserving target coverage in high-dose-rate (HDR) brachytherapy for cervical cancer. Methods and Materials A novel RTA allowing linear tandem translation within the ring plane was developed to enable patient-specific alignment without altering ring position. Two cervical cancer patients treated with external beam radiotherapy (45Gy/25 fractions) followed by HDR brachytherapy (27.5Gy/5 fractions) were analyzed. Conventional and novel RTAs were sequentially applied to acquire CT-based datasets. Plans were optimized for adequate HR-CTV D90 coverage while minimizing D2cm 3 doses to bladder, rectum, sigmoid, and bowel. EQD2 values (α/β = 3 for OARs; 10 for HR-CTV) were compared. Five retrospective cases were replanned using the novel RTA. Point A was redefined as 2cm superior along the tandem from the cervical os and 2cm lateral perpendicular to the tandem axis. Results For patient 1, the novel RTA reduced average daily D2cm 3 by 0.54Gy (rectum), 0.78Gy (bladder), and 0.18Gy (sigmoid), with cumulative EQD2 reductions of 2.23Gy, 4.88Gy, and 1.05Gy, respectively. Patient 2 showed daily reductions of 0.52Gy (rectum) and 0.44Gy (bladder), with EQD2 decreases of 3.20Gy and 4.55Gy. HR-CTV EQD2 increased by 1.67Gy. Retrospective plans demonstrated consistent reductions in OAR doses. Conclusion The novel RTA demonstrated favorable dosimetric trends for OAR sparing while maintaining target coverage in cervical HDR brachytherapy. The revised Point A definition provides a preliminary framework for anatomically consistent dose reporting in flexible tandem configurations.

In recent years, various tracking technologies that work independently of imaging systems have been proposed to automate, simplify, and enhance various tasks in the brachytherapy treatment workflow. These tasks, critical to the overall accuracy of the therapeutic dose delivery, include applicator, catheter and needle insertion guidance, and reconstruction as well as transfer tube connection in afterloading technique. Task Group 317 was established as a joint American Association of Physicists in Medicine (AAPM) and European Society for Radiotherapy and Oncology (ESTRO) committee to review: the current state-of-the-art scientific literature as it pertains to tracking technology in the field of brachytherapy; the benefits and issues related to the use of the technology for automated reconstruction of brachytherapy implants, quality control (QC) tasks such as channel path and tip reconstruction, and real-time guidance tasks; their limitations, in particular in the clinical environment and, finally, to develop recommendations related to commissioning, quality assurance (QA) and clinical use. The Task Group has looked in detail at key tracking technologies in advanced brachytherapy applications: infrared, electromagnetic, fiber optic shape sensing (fiber Bragg grating), and active radiofrequency coil tracking. For each, the performance and accuracy in well-controlled conditions as well as in clinically relevant environments are provided. Guidelines for clinical implementations, including target accuracy and performance needed for critical tasks, are summarized. Risk-based analysis is discussed in the context of an electromagnetic-based tracking system used as part of a clinical trial. The report concludes with the essential elements of an effective quality management program dedicated to the advanced features enabled by the above-described technology.

Purpose: Although quality assurance (QA) is crucial in radiation oncology departments, substantial efforts are required to monitor and ensure compliance to high standards. This work aims to analyze the impact of implementing a centralized and automated tracking dashboard on compliance and variance observed in radiation therapy QA results for linear accelerators. Methods and Materials: The study was performed in a large academic center with 7 linear accelerators. An in-house QA Dashboard was implemented in 2019 with design specifications including automated monitoring and visualization of QA progress, ease of use and accessibility, ease of integration, adaptability to new technologies, and facilitation of automated reminders. Monthly QA data were collected between 2016 and 2022 to analyze compliance pre- and post-dashboard implementation. Compliance was characterized as the percentage of tests completed on time. In addition, variance trends for linear accelerator dosimetry measurements and imaging were analyzed over 7 years. Results: In total, 76,066 records were analyzed. Of these records, 73,187 QA measurements were completed on time. Overall QA compliance increased from 75% in 2016 to >99% in 2019 after successful implementation of the QA Dashboard. The main improvement was observed for tests that were implemented more recently, eg, imaging QA and formal recording of daily QA review by physicists. The coefficient of variation was reduced by approximately a factor of 2 for imaging QA after the implementation of the QA Dashboard. For recorded dosimetry measurements, no substantial change in variance was observed. Conclusions: Implementation of the QA Dashboard resulted in a distinct increase in QA compliance. Reduction in the variance in QA measurements was observed for all imaging modalities. These findings demonstrate high impact of automated tracking QA tools on improved compliance and accuracy of QA.

Purpose: Best practices for high-dose-rate surface applicator brachytherapy treatment (SABT) have long relied on computed tomography (CT)-based imaging to visualize diseased sites for treatment planning. Compared with magnetic resonance (MR)-based imaging, CT provides insufficient soft tissue contrast. This work described the feasibility of clinical implementation of MR-based imaging in SABT planning to provide individualized treatment optimization. Material and methods: A 3D-printed phantom was used to fit Freiberg flap-style (Elekta, The Netherlands) ap- plicator. Images were taken using an optimized pointwise encoding time reduction with radial acquisition (PETRA) MR sequence for catheter visualization, and a helical CT scan to generate parallel treatment plans. This clinical study included three patients undergoing SABT for Dupuytren's contracture/palmar fascial fibromatosis imaged with the same modalities. SABT planning was performed in Oncentra Brachy (Elekta Brachytherapy, The Netherlands) treatment planning software. A geometric analysis was conducted by comparing CT-based digitization with MR-based digitization. CT and MR dwell positions underwent a rigid registration, and average Euclidean distances between dwell positions were calculated. A dosimetric comparison was performed, including point-based dose difference calculations and volumetric segmentations with Dice similarity coefficient (DSC) calculations. Results: Euclidean distances between dwell positions from CT-based and MR-based plans were on average 0.68 +/- 0.05 mm and 1.35 +/- 0.17 mm for the phantom and patients, respectively. The point dose difference calculations were on average 0.92% for the phantom and 1.98% for the patients. The D-95 and D-90 DSC calculations were both 97.9% for the phantom, and on average 93.6% and 94.2%, respectively, for the patients. Conclusions: The sub-millimeter accuracy of dwell positions and high DSC's (> 0.95) of the phantom demonstrat- ed that digitization was clinically acceptable, and accurate treatment plans were produced using MR-only imaging. This novel approach, MRI-guided SABT, will lead to individualized prescriptions for potentially improved patient outcomes.

Objective . The feasibility of MRI-only treatment planning (MRTP) for interstitial high-dose rate (HDR) brachytherapy (BT) was investigated for patients diagnosed with gynecologic cancer. Approach . A clinical MRTP workflow utilizing a ‘pointwise encoding time reduction with radial acquisition (PETRA)’ sequence was proposed. This is a clinically available MRI sequence optimized to improve interstitial catheter-tissue contrast. Interstitial needles outside the obturator region were reconstructed using MR images only. For catheters penetrating through the obturator, a library-based reconstruction was proposed. In this work, dwell coordinates from the clinical CT-based reconstruction were used as the surrogate for the library-based approach. For MR-only plan, dwell times were activated and assigned as in the clinical plans. The catheter reconstruction was assessed by comparing dwell position coordinates. The dosimetric comparisons between a clinical plan and MR-only plan were assessed for physical and EQD2 dose and volume parameters for D 90 , D 50 and D 98 for clinical target volume (CTV) and D 2cc , D 0.1cc and D 5cc for OARs. Main results . Catheter reconstruction was possible using the optimized PETRA sequence on MR images. An overall reconstruction difference of 1.7 ± 0.5 mm, attributed to registration-based errors, was found compared to the CT-based reconstruction. The MRTP workflow has the potential to generate a treatment plan with an equivalent dosimetric quality compared to the conventional CT/MRI-based approach. For CTV D 90 , physical and EQD2 dose and volume parameter differences were 1.5 ± 1.9% and 0.7 ± 1.0 Gy, respectively. For D 2cc OARs, DVH (EQD2) differences were −0.4 ± 1.1% (−0.2 ± 0.5 Gy), 0.5 ± 2.8% (0.2 ± 1.3 Gy) and −0.5 ± 1.4% (−0.2 ± 0.5 Gy) for rectum, bladder, and sigmoid, respectively. Significance . With the proposed MRTP approach, CT imaging may no longer be needed in HDR BT for interstitial gynecologic treatment. A proof-of-concept study was conducted to demonstrated that MRTP using PETRA is feasible, with comparable dosimetric results to the conventional CT/MRI-based approach.

Purpose Clinical implementation of MRI-only treatment planning (MRTP) for HDR brachytherapy can eliminate the need for CT from the current CT/MRI-based treatment planning process. Removal of CT from the workflow can potentially reduce the error and time associated with image registration and improve the overall planning process. An optimized 'Pointwise Encoding Time Reduction with Radial Acquisition (PETRA)' MR sequence has been shown to allow an accurate reconstruction of interstitial catheters, without relying on CT. The contrast between the applicator and surrounding tissues, used in combination with an applicator library database which is a feature available in many treatment planning systems (TPS), enables the reconstruction of catheters within the applicator and allows implementation of the fully MRTP workflow. This pilot study demonstrates the clinical implementation of MRTP using PETRA MRI in a cervical cancer patient, treated with a Venezia applicator. Materials and Methods The clinical workflow for MRTP was designed and implemented for a patient with cervical squamous cell carcinoma (SCC) using an optimized PETRA MRI. A Venezia applicator (3 cm lunar ovoids, 7 cm/30º tandem) was placed in the patient with four interstitial catheters. The optimized PETRA images (TR\TE 3.3\0.7ms) were acquired on a 3T MAGNETOM Vida (Siemens). An MRI-only treatment plan was generated on Oncentra Brachy TPS (Elekta Brachytherapy), using the PETRA MRI. A Venezia applicator model available from the Advanced gynecological applicator set in the TPS library database was used to perform a model-based catheter reconstruction on the PETRA image set for two lunar ovoids of the Venezia applicator. The tandem was manually reconstructed, instead of using the library-based reconstruction. The remaining four interstitial needles were also manually reconstructed on PETRA MRI. The treatment plan was generated for 550 cGy x 5 fractions, prescribed at 100% isodose lines with 0.25 cm step sizes. EQD2 to target (HR-CTV D90, GTVres and IR-CTV D98) and OARs D2cc (rectum, bladder, sigmoid and bowel) was reported. Results The catheter reconstruction workflow was designed to incorporate the reconstruction with the applicator model available in the TPS library database and manual reconstruction for catheters within the applicator and interstitial needles, respectively. Using this workflow, a clinical treatment plan that provided HR-CTV D90 coverage of 133% (96.9 Gy total EQD2) was generated, without requiring CT in the entire planning process. DVH (and total EQD2) parameters for GTVres and IR-CTV D98 were 134% (97.6 Gy) and 79% (76Gy), respectively. For OAR D2cc, DVH (and total EQD2) parameters were 16% (48.2Gy), 82% (78.7Gy), 57% (63.8Gy) and 11% (47.6Gy) for rectum, bladder, sigmoid and bowel, respectively. Conclusions This pilot study validates the clinical implementation of MRTP workflow using PETRA MRI in a clinical case for a Venezia applicator. The MRI-only workflow successfully generated a clinical treatment plan that was used to treat the cervical SCC patient without CT. Clinical implementation of MRI-only treatment planning (MRTP) for HDR brachytherapy can eliminate the need for CT from the current CT/MRI-based treatment planning process. Removal of CT from the workflow can potentially reduce the error and time associated with image registration and improve the overall planning process. An optimized 'Pointwise Encoding Time Reduction with Radial Acquisition (PETRA)' MR sequence has been shown to allow an accurate reconstruction of interstitial catheters, without relying on CT. The contrast between the applicator and surrounding tissues, used in combination with an applicator library database which is a feature available in many treatment planning systems (TPS), enables the reconstruction of catheters within the applicator and allows implementation of the fully MRTP workflow. This pilot study demonstrates the clinical implementation of MRTP using PETRA MRI in a cervical cancer patient, treated with a Venezia applicator. The clinical workflow for MRTP was designed and implemented for a patient with cervical squamous cell carcinoma (SCC) using an optimized PETRA MRI. A Venezia applicator (3 cm lunar ovoids, 7 cm/30º tandem) was placed in the patient with four interstitial catheters. The optimized PETRA images (TR\TE 3.3\0.7ms) were acquired on a 3T MAGNETOM Vida (Siemens). An MRI-only treatment plan was generated on Oncentra Brachy TPS (Elekta Brachytherapy), using the PETRA MRI. A Venezia applicator model available from the Advanced gynecological applicator set in the TPS library database was used to perform a model-based catheter reconstruction on the PETRA image set for two lunar ovoids of the Venezia applicator. The tandem was manually reconstructed, instead of using the library-based reconstruction. The remaining four interstitial needles were also manually reconstructed on PETRA MRI. The treatment plan was generated for 550 cGy x 5 fractions, prescribed at 100% isodose lines with 0.25 cm step sizes. EQD2 to target (HR-CTV D90, GTVres and IR-CTV D98) and OARs D2cc (rectum, bladder, sigmoid and bowel) was reported. The catheter reconstruction workflow was designed to incorporate the reconstruction with the applicator model available in the TPS library database and manual reconstruction for catheters within the applicator and interstitial needles, respectively. Using this workflow, a clinical treatment plan that provided HR-CTV D90 coverage of 133% (96.9 Gy total EQD2) was generated, without requiring CT in the entire planning process. DVH (and total EQD2) parameters for GTVres and IR-CTV D98 were 134% (97.6 Gy) and 79% (76Gy), respectively. For OAR D2cc, DVH (and total EQD2) parameters were 16% (48.2Gy), 82% (78.7Gy), 57% (63.8Gy) and 11% (47.6Gy) for rectum, bladder, sigmoid and bowel, respectively. This pilot study validates the clinical implementation of MRTP workflow using PETRA MRI in a clinical case for a Venezia applicator. The MRI-only workflow successfully generated a clinical treatment plan that was used to treat the cervical SCC patient without CT.

Purpose For trans-rectal ultrasound (TRUS)-based high dose-rate (HDR) prostate brachytherapy, contouring the prostate can be challenging due to poor soft-tissue contrast, implanted needle artifacts, and bleeding. This project aimed to quantify the percentage of cases in which observers preferred the artificial intelligence (AI)-generated contours over the clinical contours, and to assess the potential of AI-assisted contouring as an educational tool for residents. Materials and Methods We conducted a single retrospective study of 150 patients, who underwent HDR brachytherapy as a monotherapy, boost, or salvage with a single radiation oncologist (MK). We downloaded TRUS images after needle-insertion and clinical target volume (CTV) contours utilized for treatment planning onto a research workstation. Since the CTV for high-risk disease often included a portion of the seminal vesicle, we truncated the CTV to include only the prostate on the research workstation, and labeled the CTV as the reference contour. We prepared a UNet to provide prostate contours from TRUS images utilizing 100 patients in the training set and 20 patients in the validation set. We then applied the UNet to a testing dataset of 20 patients. We calculated the Dice coefficient between the AI and the reference contours. Then, we recruited 8 observers, 3 of whom were attending physicians experienced in prostate brachytherapy (PN, PO, MD), and 5 of whom were residents (TC, AM, DR, TW, and AM) with prostate brachytherapy experience. For each observer, we presented TRUS images with both reference and AI contours for the 20 test cases. The contours were labeled as 'R' or 'B' and were scrambled with respect to their source (reference or AI). We asked the observer to choose which contour set (R or B) was preferred for clinical use. We also asked the observer to rate the contour corresponding to the AI contour on a 4-point rating scale (0 = no edits, 1 = minimal edits, 2 = moderate edits, 3 major edits). We calculated the percentage of cases, in which the observer preferred the AI contour. We also calculated the percentage of AI contours requiring no or minimal edits. Finally, we asked 5 residents to provide prostate contours for 10 cases prior to visualizing any AI contours. In a subsequent session, we asked residents to edit AI contours. For each session, we computed the average percentage of cases in which the Dice coefficient between the AI and resident contours was greater than 0.85. Results The median Dice coefficient between AI and reference contours was 0.93 (inter-quartile range (IQR): 0.91, 0.94). Observers preferred AI contours for a median of 57.5% (IQR: 47.5, 65.0) of the test cases. Six of 8 observers preferred AI contours in the majority (>=10 of the 20 test cases), while the remaining two preferred AI contours in 35% and 45% of the cases. The mean percentage of cases, in which observers requested no or minimal edits, was 72.5% (IQR: 66.3, 86.3). Among the 5 resident observers, the average percentages of cases with Dice coefficients >= 0.85 were 17% without and 100% with AI contours available (p < 0.01). Conclusions Observers preferred the AI contours to the clinical contours in the majority of cases. AI algorithms have the potential to aid radiation oncologists, including residents, in defining the prostate CTV prior to treatment planning. Further prospective studies are needed to identify how to best integrate AI prostate contours into the HDR brachytherapy workflow. For trans-rectal ultrasound (TRUS)-based high dose-rate (HDR) prostate brachytherapy, contouring the prostate can be challenging due to poor soft-tissue contrast, implanted needle artifacts, and bleeding. This project aimed to quantify the percentage of cases in which observers preferred the artificial intelligence (AI)-generated contours over the clinical contours, and to assess the potential of AI-assisted contouring as an educational tool for residents. We conducted a single retrospective study of 150 patients, who underwent HDR brachytherapy as a monotherapy, boost, or salvage with a single radiation oncologist (MK). We downloaded TRUS images after needle-insertion and clinical target volume (CTV) contours utilized for treatment planning onto a research workstation. Since the CTV for high-risk disease often included a portion of the seminal vesicle, we truncated the CTV to include only the prostate on the research workstation, and labeled the CTV as the reference contour. We prepared a UNet to provide prostate contours from TRUS images utilizing 100 patients in the training set and 20 patients in the validation set. We then applied the UNet to a testing dataset of 20 patients. We calculated the Dice coefficient between the AI and the reference contours. Then, we recruited 8 observers, 3 of whom were attending physicians experienced in prostate brachytherapy (PN, PO, MD), and 5 of whom were residents (TC, AM, DR, TW, and AM) with prostate brachytherapy experience. For each observer, we presented TRUS images with both reference and AI contours for the 20 test cases. The contours were labeled as 'R' or 'B' and were scrambled with respect to their source (reference or AI). We asked the observer to choose which contour set (R or B) was preferred for clinical use. We also asked the observer to rate the contour corresponding to the AI contour on a 4-point rating scale (0 = no edits, 1 = minimal edits, 2 = moderate edits, 3 major edits). We calculated the percentage of cases, in which the observer preferred the AI contour. We also calculated the percentage of AI contours requiring no or minimal edits. Finally, we asked 5 residents to provide prostate contours for 10 cases prior to visualizing any AI contours. In a subsequent session, we asked residents to edit AI contours. For each session, we computed the average percentage of cases in which the Dice coefficient between the AI and resident contours was greater than 0.85. The median Dice coefficient between AI and reference contours was 0.93 (inter-quartile range (IQR): 0.91, 0.94). Observers preferred AI contours for a median of 57.5% (IQR: 47.5, 65.0) of the test cases. Six of 8 observers preferred AI contours in the majority (>=10 of the 20 test cases), while the remaining two preferred AI contours in 35% and 45% of the cases. The mean percentage of cases, in which observers requested no or minimal edits, was 72.5% (IQR: 66.3, 86.3). Among the 5 resident observers, the average percentages of cases with Dice coefficients >= 0.85 were 17% without and 100% with AI contours available (p < 0.01). Observers preferred the AI contours to the clinical contours in the majority of cases. AI algorithms have the potential to aid radiation oncologists, including residents, in defining the prostate CTV prior to treatment planning. Further prospective studies are needed to identify how to best integrate AI prostate contours into the HDR brachytherapy workflow.

Purpose Current practice for interstitial high-dose-rate (HDR) brachytherapy treatment planning involves manual tracking of needle positions on CT images. Recently, PETRA MRI showed the potential to detect empty interstitial gynecological brachytherapy needles owing to low needle signal, intermediate tissue contrast and high spatial resolution. We aimed to develop an algorithm for automatic interstitial needle tracking on PETRA images by employing their negative contrast against surrounding tissue. Materials and Methods Fourteen ProGuide Sharp Needles 6Fx294mm were implanted in an endometrial adenocarcinoma patient using a Syed-Neblett template. One of the needles lay in the obturator center, and six inside the obturator slots. The patient underwent standard CT imaging with copper-filled needles. After marker removal, 3D PETRA images of the patient's pelvis (TR\TE 3.32\0.07 ms, voxel size 0.9 mm3 isotropic, 416 slices) were acquired on a 3T Siemens Vida MR scanner. PETRA MR images were analyzed for interstitial needle detection using custom code in MATLAB R2022a. A rectangular part of all axial slices where needles may be contained (slices 149-310) was selected. For each slice part, corner points were extracted from the original part and from its sharpened gradient magnitude image by detecting Harris features. Points found within Euclidean distance (ED) of 3 pixels on both images were matched, and the pixel with minimum signal intensity closest to each such point on the original part was selected. Minimum intensity points lying within 2 pixels ED between subsequent slices were considered as possible catheter coordinates. Such points appearing on at least 10 continuous slices were designated as true catheters, and their positions were compared to those visible on CT. Results Figure 1a) displays the selected MR image parts for two example slices. Pixels with detected Harris features and their closest signal minima are marked by red and green crosses, respectively, in b). Pixels accepted as needle positions are shown as magenta crosses in c). Matching CT image parts with bright marker-filled needles are displayed in d). Empty interstitial catheters on PETRA images corresponded to local minima of signal intensity, located near Harris features. The assumption of needle position continuity within an allowed distance along subsequent slices helped avoid spurious signal minima, while following needle position variations caused by different deflection angles. Needles guided through the obturator grooves and the central needle were detected only after they became detached from the obturator (ex. figure left). All 14 needles were tracked along their visible length inside tissue (ex. figure right). Visual comparison to CT images confirmed interstitial catheter detection on MRI. Selecting a range of slices and zooming on a part of each slice for analysis reduced computational time and output data size. Conclusions Automated detection of interstitial needles is feasible on PETRA MR images and may facilitate MR-only treatment planning in interstitial HDR brachytherapy. The newly developed algorithm was able to track all needles inside tissue based on their signal intensity characteristics, without requiring manual seed position placement.

PURPOSE:Cardiac toxicity is a well-recognized risk after radiation therapy (RT) in patients with non-small cell lung cancer (NSCLC). However, the extent to which treatment planning optimization can reduce mean heart dose (MHD) without untoward increases in lung dose is unknown. METHODS AND MATERIALS:Retrospective analysis of RT plans from 353 consecutive patients with locally advanced NSCLC treated with intensity modulated RT (IMRT) or 3-dimensional conformal RT. Commercially available machine learning-guided clinical decision support software was used to match RT plans. A leave-one-out predictive model was used to examine lung dosimetric tradeoffs necessary to achieve a MHD reduction. RESULTS:Of all 232 patients, 91 patients (39%) had RT plan matches showing potential MHD reductions of >4 to 8 Gy without violating the upper limit of lung dose constraints (lung volume [V] receiving 20 Gy (V20 Gy) <37%, V5 Gy <70%, and mean lung dose [MLD] <20 Gy). When switching to IMRT, 75 of 103 patients (72.8%) had plan matches demonstrating improved MHD (average 2.0 Gy reduction, P < .0001) without violating lung constraints. Examining specific lung dose tradeoffs, a mean ≥3.7 Gy MHD reduction was achieved with corresponding absolute increases in lung V20 Gy, V5 Gy, and MLD of 3.3%, 5.0%, and 1.0 Gy, respectively. CONCLUSIONS:Nearly 40% of RT plans overall, and 73% when switched to IMRT, were predicted to have reductions in MHD >4 Gy with potentially clinically acceptable tradeoffs in lung dose. These observations demonstrate that decision support software for optimizing heart-lung dosimetric tradeoffs is feasible and may identify patients who might benefit most from more advanced RT technologies.

Purpose The vaginal cuff is the main location of relapse after curative surgery for early-stage endometrial cancer. Adjuvant vaginal cylinder brachytherapy is increasingly used to decrease the risk of postoperative recurrence due to low vaginal cuff recurrence rates and low gastrointestinal toxicity. Current clinical treatment planning follows TG-43 guidance, which assumes that all material is water. Cylinder composition can vary substantially from water equivalent materials, highlighting the importance of investigations into the dosimetric impact of cylinder composition. In the present study, we evaluated dosimetric characteristics of plans utilizing TG-186, which provides a model-based calculation approach to account for heterogeneities in cylinder configuration. Materials and Methods Ten graphically planned vaginal cylinder plans were retrospectively evaluated in this study. Each patient was fitted with an Elekta cylinder (part# 084350) comprised primarily of polysulfone (PSU, 1.29g/cc) and secondarily of stainless steel (8.00g/cc) for the stem and rings. Cylinder diameters ranged from 2.5cm to 3.5cm, and treatment lengths from 4.0cm to 7.5cm. Dose prescription was 7Gy/fx x 3fx at 5mm depth for the most proximal 2cm of the cylinder and at cylinder surface for the remainder of the treatment length. Treatment planning was conducted in Oncentra Brachy 4.5.3. The original TG-43 plans (which assume a water-equivalent material, 1.00g/cc) were recalculated using Oncentra's Advanced Collapsed-Cone Engine (ACE) according to TG-186's guidance to account for differences in density. Dwell times were unchanged, so heterogeneity correction accounts for any dosimetric changes. Points placed at the middle of the 5mm depth portion and the middle of the surface depth portion - prescription dose for TG-43 - were compared, as were rectum points. In addition, all isodose lines were converted to contours and compared for the following metrics: percent volume change, Hausdorff distance (HD), mean distance to agreement (MDA), and Dice coefficient. A paired Wilcoxon signed-rank test assessed for significance. Results Recalculated TG-186 plans were found to have consistently and statistically significantly (p<0.05) less dose: 5mm depth: -10.5% +/- 2.4%, surface: -15.9% +/- 3.5%, and rectum: -12.1% +/- 3.4% average dose reduction. Compared to TG-43, the TG-186 100% isodose lines as contours show an average volume decrease of 22.2% +/- 2.5%, HD 4.6mm +/- 1.2mm, MDA 1.5mm +/- 0.3mm, and Dice 0.87 +/- 0.02. The high dose region as defined by 150% of prescription (25.7% volume decrease, HD 4.4mm, MDA 1.3mm) and the low dose region of 50% prescription (19.5% volume decrease, HD 8.2mm, MDA 1.9mm) show similar trends. Conclusions Ten TG-43 graphical vaginal cylinder plans were recalculated using TG-186 to account for the material composition of the cylinders. ACE calculations show that the dose delivered to target prescription points and to OARs may be less than originally planned, although the impact on treatment efficacy is presently unclear. Further studies on other cylinder models will be useful to evaluate the generalizability of these findings, as well as any potential effect on treatment.

<h3>Purpose</h3> In skin High-Dose-Rate (HDR) brachytherapy, CT is commonly used for surface applicator imaging and treatment planning. However, since CT does not clearly depict skin, lesion depth is estimated from additional ultrasound examinations. MRI offers better soft tissue contrast than CT, and a PETRA MR sequence has recently shown the potential to visualize surface applicators and catheters, producing treatment plans comparable to CT-based ones. Additional skin detection on MR images would provide a complete information set for MR-only treatment planning. We investigated the feasibility of simultaneous skin and surface applicator imaging using MRI. <h3>Materials and Methods</h3> A Freiburg flap applicator (14 catheters with 24 beads each) was wrapped around the right shin of a healthy female volunteer. The volunteer was scanned in a 3T Siemens Vida using an UltraFlexLarge18 coil around and a Spine32 coil under her lower right leg. A 3D Dixon VIBE sequence (TR\TE 4.02\1.32 ms, field-of-view (FOV) 350 × 350 mm², 1.09 × 1.09 × 1.1 mm³ voxels, bandwidth 1116 Hz/pixel, acquisition time 1min 38s) was acquired coronally. Obtained in-phase, opposed-phase, water-only and fat-only image series were assessed for applicator and skin detectability in all three orientations. The series providing best visibility for both features was selected, and skin thickness was measured at five points around the leg (figure top left), on five reconstructed axial slices with distance 17.5 mm from each other, using a linear distance tool in the MIM software. <h3>Results</h3> The 3D properties of the Dixon VIBE sequence allowed for coronal acquisition orientation to best cover the area of interest, with reliable reconstruction in the axial plane employed for treatment planning. Both skin and applicator spheres were visible on the opposed-phase (figure top) and water-only (figure bottom) images in the three orthogonal planes. Opposed-phase images were selected for further analysis due to the consistent sphere signal and clear demarcation of the skin from the underlying bright subcutaneous fat. All applicator beads contained inside the FOV were visualized. Empty plastic catheters produced no signal, but their position in the center of the spheres could be determined by their negative contrast. Average skin thickness from assessed points and slices of the opposed-phase images was 0.18 ± 0.02 mm (mean ± standard deviation). These results lie inside the range of dermal and total skin thickness values reported for extremities of female subjects. The small standard deviation of the mean suggests that measured skin thickness was consistent among measurement locations. <h3>Conclusions</h3> An optimized 3D Dixon VIBE MR sequence offered simultaneous visualization of human skin and a surface brachytherapy applicator for the first time. While PETRA sequences can provide positive catheter contrast, the lack of catheter signal on Dixon VIBE images can also be used to determine their position inside the brighter applicator spheres. Dixon VIBE offered the additional advantage of skin visualization on opposed-phase and water-only series with spatial resolution around 1 mm, providing reasonable skin thickness assessments. As its acquisition time is short, Dixon VIBE can be combined with T2-weighted MR sequences for contouring organs at risk, presenting the possibility to achieve surface brachytherapy treatment simulations with a single imaging modality. Its implementation may pave the path towards MR-only treatment planning in surface HDR brachytherapy.