Dosimetry and Monte Carlo modelling of the Papillon+ contact X-ray brachytherapy device.

PURPOSE: This study aimed to develop and validate a Monte Carlo (MC) model for the Papillon+ contact x-ray brachytherapy (CXB) device, producing 50 kilovolt (kV) X-rays, specifically focusing on its application with a 25 mm diameter rectal applicator for contact therapy. MATERIAL AND METHODS: The validation process involved depth dose and transverse dose profile measurements using EBT3 gafchromic films positioned in a plastic water low energy range phantom. The half-value layer (HVL) was further measured and derived from the simulated X-ray spectra. RESULTS: Excellent agreement within ±2% was achieved between the measured and simulated on-axis depth dose curves for the 25 mm rectal applicator. Transverse dose profile measurements showed a high level of agreement between the simulation and measurements, on average 3.1% in contact with the applicator at the surface of the phantom and on average 1.7% at 10 mm depth. A close agreement within 5.5% was noticed concerning the HVL between the measurement and simulation. The simulated gamma spectra and 2D-dose distribution demonstrated a soft X-ray energy spectrum and a uniform dose distribution in contact with the applicator. CONCLUSIONS: An MC model was successfully developed for the Papillon+ eBT device with a 25 mm diameter rectal applicator. The validated model, with its demonstrated accuracy in depth dose and transverse dose profile simulations, is a valuable tool for quality assurance and patient safety and, in a later phase, may be used for treatment planning, dose calculations and tissue inhomogeneity corrections.

Breast conserving surgery with intraoperative electron beam radiation therapy for low-risk breast cancer: Five-year follow-up of 306 patients.

Recent studies have reported a higher than expected risk of ipsilateral breast tumor recurrence (IBTR) after breast conserving surgery (BCS) and a single dose of electron beam intra-operative radiotherapy (IORT). This finding was the rationale to perform a retrospective single center cohort study evaluating the oncologic results of consecutive patients treated with BCS and IORT. Women were eligible if they had clinical low-risk (N0, ≤2 cm unifocal, Bloom and Richardson grade 1-2), estrogen receptor-positive and human-epidermal-growth-factor-receptor-2-negative breast cancer. Prior to BCS, pN0 status was determined by sentinel lymph node biopsy. Data on oncologic follow-up were analyzed. Between 2012 and 2019, 306 consecutive patients were treated and analyzed, with a median age of 67 (50-86) years at diagnosis. Median follow-up was 60 (8-120) months. Five-year cumulative risk of IBTR was 13.4% (95% confidence interval [CI] 9.4-17.4). True in field recurrence was present in 3.9% of the patients. In 4.6% of the patients, the IBRT was classified as a local recurrence due to seeding of tumor cells in the cutis or subcutis most likely related to percutaneous biopsy. In 2.9% of the patients, the IBRT was a new outfield primary tumor. Three patients had a regional lymph node recurrence and two had distant metastases as first event. One breast cancer-related death was observed. Estimated 5-year overall survival was 89.8% (95% CI 86.0-93.6). In conclusion, although some of IBTR cases could have been prevented by adaptations in biopsy techniques and patient selection, BCS followed by IORT was associated with a substantial risk of IBTR.

Post-operative complications following dose adaptation of intra-operative electron beam radiation therapy in locally advanced or recurrent rectal cancer.

Purpose: The benefit of intra-operative radiotherapy (IORT) in the treatment of locally advanced rectal cancer (LARC) or locally recurrent rectal cancer (LRRC) lie in its ability to provide high-dose of radiation to limited at-risk volume, thereby eliminating microscopic disease and decreasing toxicity. A comparative study between high-dose-rate (HDR) brachytherapy, named intra-operative brachytherapy (IOBT), and intra-operative electron radiotherapy (IOERT) was performed showing favorable LRFS after IOBT, possibly due to a higher surface dose that is inherent in IOBT technique. The IOERT technique in Catharina Hospital Eindhoven was adapted to increase the surface dose, aiming to improve local control. Post-operative complications due to an increased radiation dose remain the matter of concern. This retrospective study was performed to compare complication rates before and after adapted IOERT dose. Material and methods: All patients undergoing surgery with IOERT for LARC or LRRC from September 2019 until July 2023, were considered. Patients selected until August 31, 2021 were included in control cohort (n = 108), and those chosen from September 1, 2021 onwards were included in intervention cohort (n = 92). Perioperative and (major) post-operative complications were classified retrospectively, during admission, at 30 days, and at 90 days. Results: In LARC patients, a decrease in post-operative complications was observed (p = 0.009). 19% of LARC patients experienced major post-operative surgical complications, i.e., Clavien-Dindo grade 3b-5, regardless of treatment group. No difference in major 90-day complications was noted (p = 0.142). In LRRC patients, the use of induction chemotherapy decreased from 78% to 29% (p < 0.001), which complicated comparison. However, no difference in major post-operative complications was observed at 30 days (p = 0.222) or 90 days (p = 0.977) after surgery. Conclusions: Increased surface dose of IOERT does not seem to lead to an increase in post-operative complications. Further research is needed to evaluate the efficacy of dose adaptation in IOERT to improve local oncological control rates. Routine evaluation of CTCAE scores in follow-up will help uncover possible long-term radiation-induced toxicity.

Optimisation of Organ Preservation treatment strategies in patients with rectal cancer with a good clinical response after neoadjuvant (chemo)radiotherapy: Additional contact X-ray brachytherapy versus eXtending the observation period and local excision (OPAXX) - protocol for two multicentre, parallel, single-arm, phase II studies.

INTRODUCTION: Standard treatment for patients with intermediate or locally advanced rectal cancer is (chemo)radiotherapy followed by total mesorectal excision (TME) surgery. In recent years, organ preservation aiming at improving quality of life has been explored. Patients with a complete clinical response to (chemo)radiotherapy can be managed safely with a watch-and-wait approach. However, the optimal organ-preserving treatment strategy for patients with a good, but not complete clinical response remains unclear. The aim of the OPAXX study is to determine the rate of organ preservation that can be achieved in patients with rectal cancer with a good clinical response after neoadjuvant (chemo)radiotherapy by additional local treatment options. METHODS AND ANALYSIS: The OPAXX study is a Dutch multicentre study that investigates the efficacy of two additional local treatments aiming at organ preservation in patients with a good, but not complete response to neoadjuvant treatment (ie near-complete response or a small residual tumour mass <3 cm). The sample size will be 168 patients in total. Patients will be randomised (1:1) between two parallel single-arm phase II studies: study arm 1 involves additional contact X-ray brachytherapy (an intraluminal radiation boost), while in study arm 2 the observation period is extended followed by a second response evaluation and optional transanal local excision. The primary endpoint of the study is the rate of successful organ preservation at 1 year following randomisation. Secondary endpoints include toxicity, morbidity, oncological and functional outcomes at 1 and 2 years of follow-up. Finally, an observational cohort study for patients who are not eligible for randomisation is conducted. ETHICS AND DISSEMINATION: The trial protocol has been approved by the medical ethics committee of the Netherlands Cancer Institute (METC20.1276/M20PAX). Informed consent will be obtained from all participants. The trial results will be published in an international peer-reviewed journal. TRIAL REGISTRATION NUMBER: NCT05772923.

Inter-observer variability in library plan selection on iterative CBCT and synthetic CT images of cervical cancer patients.

INTRODUCTION: In the Library-of-Plans (LoP) approach, correct plan selection is essential for delivering radiotherapy treatment accurately. However, poor image quality of the cone-beam computed tomography (CBCT) may introduce inter-observer variability and thereby hamper accurate plan selection. In this study, we investigated whether new techniques to improve the CBCT image quality and improve consistency in plan selection, affects the accuracy of LoP selection in cervical cancer patients. MATERIALS AND METHODS: CBCT images of 12 patients were used to investigate the inter-observer variability of plan selection based on different CBCT image types. Six observers were asked to individually select a plan based on clinical X-ray Volumetric Imaging (XVI) CBCT, iterative reconstructed CBCT (iCBCT) and synthetic CTs (sCT). Selections were performed before and after a consensus meeting with the entire group, in which guidelines were created. A scoring by all observers on the image quality and plan selection procedure was also included. For plan selection, Fleiss' kappa (κ) statistical test was used to determine the inter-observer variability within one image type. RESULTS: The agreement between observers was significantly higher on sCT compared to CBCT. The consensus meeting improved the duration and inter-observer variability. In this manuscript, the guidelines attributed the overall results in the plan selection. Before the meeting, the gold standard was selected in 76% of the cases on XVI CBCT, 74% on iCBCT, and 76% on sCT. After the meeting, the gold standard was selected in 83% of the cases on XVI CBCT, 81% on iCBCT, and 90% on sCT. CONCLUSION: The use of sCTs can increase the agreement of plan selection among observers and the gold standard was indicated to be selected more often. It is important that clear guidelines for plan selection are implemented in order to benefit from the increased image quality, accurate selection, and decrease inter-observer variability.

Clinical evaluation of a deep learning segmentation model including manual adjustments afterwards for locally advanced breast cancer.

Introduction: Deep learning (DL) models are increasingly developed for auto-segmentation in radiotherapy. Qualitative analysis is of great importance for clinical implementation, next to quantitative. This study evaluates a DL segmentation model for left- and right-sided locally advanced breast cancer both quantitatively and qualitatively. Methods: For each side a DL model was trained, including primary breast CTV (CTVp), lymph node levels 1-4, heart, lungs, humeral head, thyroid and esophagus. For evaluation, both automatic segmentation, including correction of contours when needed, and manual delineation was performed and both processes were timed. Quantitative scoring with dice-similarity coefficient (DSC), 95% Hausdorff Distance (95%HD) and surface DSC (sDSC) was used to compare both the automatic (not-corrected) and corrected contours with the manual contours. Qualitative scoring was performed by five radiotherapy technologists and five radiation oncologists using a 3-point Likert scale. Results: Time reduction was achieved using auto-segmentation in 95% of the cases, including correction. The time reduction (mean ± std) was 42.4% ± 26.5% and 58.5% ± 19.1% for OARs and CTVs, respectively, corresponding to an absolute mean reduction (hh:mm:ss) of 00:08:51 and 00:25:38. Good quantitative results were achieved before correction, e.g. mean DSC for the right-sided CTVp was 0.92 ± 0.06, whereas correction statistically significantly improved this contour by only 0.02 ± 0.05, respectively. In 92% of the cases, auto-contours were scored as clinically acceptable, with or without corrections. Conclusions: A DL segmentation model was trained and was shown to be a time-efficient way to generate clinically acceptable contours for locally advanced breast cancer.

Coverage of Lateral Lymph Nodes in Rectal Cancer Patients with Routine Radiation Therapy Practice and Associated Locoregional Recurrence Rates.

PURPOSE: Involved internal iliac and obturator lateral lymph nodes (LLNs) are a known risk factor for the occurrence of ipsilateral local recurrences (LLR) in rectal cancer. This study examined coverage of LLNs with routine radiation therapy practice in the Netherlands and associated LLR rates. METHODS AND MATERIALS: Patients with a primary tumor ≤8 cm of the anorectal junction, cT3-4 stage, and at least 1 internal iliac or obturator LLN with short axis ≥5 mm who received neoadjuvant (chemo)radiation therapy, were selected from a national, cross-sectional study of patients with rectal cancer treated in the Netherlands in 2016. Magnetic resonance images and radiation therapy treatment plans were reviewed regarding segmented LLNs as gross tumor volume (GTV), location of LLNs within clinical target volume (CTV), and received proportion of the planned radiation therapy dose. RESULTS: A total of 223 out of 3057 patients with at least 1 LLN ≥5 mm were selected. Of those, 180 (80.7%) LLNs were inside the CTV, of which 60 (33.3%) were segmented as GTV. Overall, 202 LLNs (90.6%) received ≥95% of the planned dose. Four-year LLR rates were not significantly higher for LLNs situated outside the CTV compared with those inside (4.0% vs 12.5%, P = .092) or when receiving <95% versus ≥95% of the planned radiation therapy dose (7.1% vs 11.3%, P = .843), respectively. Two of 7 patients who received a dose escalation of 60 Gy developed an LLR (4-year LLR rate of 28.6%). CONCLUSIONS: This evaluation of routine radiation therapy practice showed that adequate coverage of LLNs was still associated with considerable 4-year LLR rates. Techniques resulting in better local control for patients with involved LLNs need to be explored further.

Cytoreductive Surgery and Hyperthermic Intraperitoneal Chemotherapy with Intra-Operative Radiotherapy for Patients with Locally Advanced or Locally Recurrent Rectal Cancer and Peritoneal Metastases.

Purpose: To assess the safety and long-term outcome of a multimodality treatment consisting of radical surgery, intra-operative radiotherapy (IORT), and cytoreductive surgery with hyperthermic intraperitoneal chemotherapy (CRS-HIPEC) for patients with locally advanced rectal cancer (LARC) or locally recurrent rectal carcinoma (LRRC) and peritoneal metastases (PM). Methods: The present study was a single-center cohort study, including all consecutive patients undergoing this treatment in a tertiary referral center for LARC, LRRC, and PM. Postoperative complications, intensive care stay (ICU stay), and re-admission rates were assessed as well as disease-free survival (DFS) and overall survival (OS). Results: A total of 14 LARC and 16 LRRC patients with PM were included in the study. The median ICU stay was 1 day, and 57% of patients developed a severe postoperative complication. No 90-day mortality was observed. Median DFS was 10.0 months (Interquartile Range 7.1-38.7), and median OS was 31.0 months (Interquartile Range 15.9-144.3). Conclusions: As postoperative complications and survival were in line with treatments that are accepted for LARC or LRRC and PM as separate procedures, we conclude that combined treatment with IORT and CRS-HIPEC should be considered as a treatment option for selected patients with LARC or LRRC and peritoneal metastases in tertiary referral centers.

Narrowing the difference in dose delivery for IOERT and IOBT for locally advanced and locally recurrent rectal cancer.

Purpose: Intra-operative radiotherapy (IORT) has been used as a tool to provide a high-dose radiation boost to a limited volume of patients with fixed tumors with a likelihood of microscopically involved resection margins, in order to improve local control. Two main techniques to deliver IORT include high-dose-rate (HDR) brachytherapy, termed 'intra-operative brachytherapy' (IOBT), and electrons, termed 'intra-operative electron radiotherapy' (IOERT), both having very different dose distributions. A recent paper described an improved local recurrence-free survival favoring IOBT over IOERT for patients with locally advanced or recurrent rectal cancer and microscopically irradical resections. Although several factors may have contributed to this result, an important difference between the two techniques was the higher surface dose delivered by IOBT. This article described an adaptation of IOERT technique to achieve a comparable surface dose as dose delivered by IOBT. Material and methods: Two steps were taken to increase the surface dose for IOERT: 1. Introducing a bolus to achieve a maximum dose on the surface, and 2. Re-normalizing to allow for the same prescribed dose at reference depth. Conclusions: We describe and propose an adaptation of IOERT technique to increase surface dose, decreasing the differences between these two techniques, with the aim of further improving local control. In addition, an alternative method of dose prescription is suggested, to consider improved comparison with other techniques in the future.

Endorectal contact radiation boosting: Making the case for dose AND volume reporting.

INTRODUCTION: The various rectal endoluminal radiation techniques all have steep, but different, dose gradients. In rectal contact brachytherapy (CXB) doses are typically prescribed and reported to the applicator surface and not to the gross tumor volume (GTV), clinical target volume (CTV) or organs at risk (OAR), which is crucial to understand tumor response and toxicity rates. To quantify the above-described problem, we performed a dose modeling study using a fixed prescription dose at the surface of the applicator and varied tumor response scenarios. METHODS: Endorectal ultrasound-based 3D-volume-models of rectal tumors and the rectal wall were used to simulate the delivered dose to GTV, CTV and the rectal wall layers, assuming treatment with Maastro HDR contact applicator for rectal cancer with a fixed prescription dose to the applicator surface (equivalent to 3 × 30 Gy CXB) and various response scenarios. RESULTS: An identical prescribed dose to the surface of the applicator resulted in a broad range of doses delivered to the GTV, CTV and the uninvolved intestinal wall. For example, the equieffective dose in 2 Gy per fraction (EQD2) D90% of the GTV varied between 63 and 231 Gy, whereas the EQD2 D2cc of the rectal wall varied between 97 and 165 Gy. CONCLUSION: Doses prescribed at the surface are not representative of the dose received by the tumor and the bowel wall. This stresses the relevance of dose reporting and prescription to GTV and CTV volumes and OAR in order to gain insight between delivered dose, local control and toxicity and to optimize treatment protocols.

A Multidisciplinary Approach for the Personalised Non-Operative Management of Elderly and Frail Rectal Cancer Patients Unable to Undergo TME Surgery.

Despite it being the optimal curative approach, elderly and frail rectal cancer patients may not be able to undergo a total mesorectal excision. Frequently, no treatment is offered at all and the natural course of the disease is allowed to unfold. These patients are at risk for developing debilitating symptoms that impair quality of life and require palliative treatment. Recent advancements in non-operative treatment modalities have enhanced the toolbox of alternative treatment strategies in patients unable to undergo surgery. Therefore, a proposed strategy is to aim for the maximal non-operative treatment, in an effort to avoid the onset of debilitating symptoms, improve quality of life, and prolong survival. The complexity of treating elderly and frail patients requires a patient-centred approach to personalise treatment. The main challenge is to optimise the balance between local control of disease, patient preferences, and the burden of treatment. A comprehensive geriatric assessment is a crucial element within the multidisciplinary dialogue. Since limited knowledge is available on the optimal non-operative treatment strategy, these patients should be treated by dedicated multidisciplinary rectal cancer experts with special interest in the elderly and frail. The aim of this narrative review was to discuss a multidisciplinary patient-centred treatment approach and provide a practical suggestion of a successfully implemented clinical care pathway.

Advanced design, simulation, and dosimetry of a novel rectal applicator for contact brachytherapy with a conventional HDR 192Ir source.

PURPOSE: Dose escalation yields higher complete response to rectal tumors, which may enable the omission of surgery. Dose escalation using 50 kVp contact x-ray brachytherapy (CXB) allow the treatment of a selective volume, resulting in low toxicity and organs-at-risk preservation. However, the use of CXB devices is limited because of its high cost and lack of treatment planning tools. Hence, the MAASTRO applicator (for HDR 192Ir sources) was developed and characterized by measurements and Monte Carlo simulations to be a cost-effective alternative to CXB devices. METHODS AND MATERIALS: A cylindrical applicator with lateral shielding was designed to be used with a rectoscope using its tip as treatment surface. Both the applicator and the rectoscope have a slanted edge to potentially allow easier placement against tumors. The applicator design was achieved by Monte Carlo modeling and validated experimentally with film dosimetry, using the Papillon 50 (P50) device as reference. RESULTS: The applicator delivers CXB doses in less than 9 min using a 20375 U source for a treatment area of approximately 20 × 20 mm2 at 2 mm depth. Normalized at 2 mm, the dose falloff for depths of 0 mm, 5 mm, and 10 mm are 130%, 70%, and 43% for the P50 and 140%, 67%, and 38% for the MAASTRO applicator, respectively. CONCLUSIONS: The MAASTRO applicator was designed to use HDR 192Ir sources to deliver a dose distribution similar to those of CXB devices. The applicator may provide a cost-effective solution for endoluminal boosting with clinical treatment planning system integration.

Microscopic intramural extension of rectal cancer after neoadjuvant chemoradiation: A meta-analysis based on individual patient data.

OBJECTIVE: In selected rectal cancer patients with residual local disease following neoadjuvant chemoradiation (CRT) and the preference of an organ preservation pathway, additional treatment with dose escalation by endoluminal radiotherapy (RT) may ultimately result in a clinical complete response. To date, the widespread introduction of selective endoluminal radiation techniques is hampered by a lack of evidence-based guidelines that describe the radiation treatment volume in relation to the residual tumor mass. In order to convert an incomplete response into a complete one with additional treatment such as dose-escalation with endoluminal RT from a theoretical perspective, it seems important to treat all remaining microscopic tumor cells after CRT. In this setting, residual tumor extension beneath normal appearing mucosa (microscopic intramural spread - MIS) becomes relevant for accurate tumor volume and margin estimation. With the goal of providing evidence-based guidelines that define an appropriate treatment volume and patient selection, we present results from a meta-analysis based on individual patient data of studies that have assessed the extent or range of MIS of rectal cancers after neoadjuvant CRT. This meta-analysis should provide an estimate of the residual tumor volume/extension that needs to be targeted by any additional radiation therapy boost in order to achieve complete tumor eradication after initial incomplete or near-complete response following standard CRT. METHODS AND MATERIALS: A PubMed search was performed. Additional articles were selected based on identification from reference lists. Papers were eligible when reporting MIS in patients who were treated by total mesorectal excision or local excision/transanal endoscopic microsurgery (TEM) after neo-adjuvant long-course CRT. The mean MIS was calculated for the entire group along with the 70th until 95th percentiles. Additional exploratory subgroup analyses were performed. RESULTS: Individual patient data from 349 patients with residual disease from five studies were analyzed. 80% of tumors showed no MIS. In order to appropriately treat MIS in 95% of rectal cancer patients after CRT, a margin of 5.5 mm around the macroscopic tumor would suffice. An exploratory subgroup analysis showed that T-stage after CRT (ypT) and time interval between neoadjuvant CRT and surgery are significant factors predicting the extent of MIS (p < 0.001.) The group of ypT1 had the smallest MIS, followed by the ypT3-4 group, while the ypT2 group had the largest MIS (p < 0.001). Regarding time interval between CRT and surgery, a statistically significant difference was seen when comparing the three time-interval groups (less than 8 weeks, 8-12 weeks, and more than 12 weeks), where waiting more than 12 weeks after CRT resulted in the largest MIS (p < 0.0001). CONCLUSION: Based on this meta-analysis, in order to treat the MIS for 95% of rectal cancer patients after CRT, a Clinical Target Volume (CTV) margin of 5.5 mm from the lateral most edge of the macroscopic tumor would suffice. 80% of tumors showed no MIS and would not require an extra CTV margin for treatment. These findings support the feasibility of localized radiotherapy boosts for dose-escalation to improve response among patients with incomplete response after standard CRT and can also be applied in the surgical setting.

A systematic review comparing radiation toxicity after various endorectal techniques.

PURPOSE: A clinical complete response is seen after neoadjuvant chemoradiation for rectal tumors in 15%-20% of patients. These patients can potentially be spared mutilating total mesorectal excision surgery through a watch-and-wait policy. Recent studies show that dose escalation by a radiation boost increases the clinical complete response rate. The boost dose to the tumor can be administered through external beam radiotherapy or through internal radiotherapy using techniques like contact therapy, low-dose-rate or high-dose-rate brachytherapy (BT). However, limited information is available concerning treatment-related toxicity of these techniques. With this systematic review, we aim to summarize and compare published data concerning acute and late toxicity after contact X-ray therapy (CXT) and BT for rectal cancer. METHODS AND MATERIALS/RESULTS: Thirty-eight studies reporting toxicity after endorectal radiation techniques for rectal cancer were included, resulting in 3682 patients for analysis. Direct comparison of toxicity by the different radiation modes was hampered by various combinations of endorectal techniques, a lack of clear reporting of toxicity scores, dose prescription, technique used, and treated volumes. ≥ Grade 3 rectal toxicity was reported in 2.9% of patients having received only CXT; 6.3% of patients who received only BT had Grade 3 rectal toxicity, and BT also caused Grade 3 urinary toxicity in 1 patient. CONCLUSION: All techniques reported some ≥ Grade 3 toxicity. Toxicity after CXT was confined to the rectum, whereas after BT, urogenital toxicity and skin toxicity were seen as well. Unfortunately, few specific conclusions could be drawn regarding the dose-related risk of toxicity for the various techniques due to nonuniform reporting strategies and missing information. To enable future comparisons and improvements, the endorectal radiation field urgently needs consensus guidelines on dose reporting, dose prescription, treatment volume specification, and toxicity reporting.

A novel rectal applicator for contact radiotherapy with HDR 192Ir sources.

PURPOSE: Dose escalation to rectal tumors leads to higher complete response rates and may thereby enable omission of surgery. Important advantages of endoluminal boosting techniques include the possibility to apply a more selective/localized boost than using external beam radiotherapy. A novel brachytherapy (BT) rectal applicator with lateral shielding was designed to be used with a rectoscope for eye-guided positioning to deliver a dose distribution similar to the one of contact x-ray radiotherapy devices, using commonly available high-dose-rate 192Ir BT sources. METHODS AND MATERIALS: A cylindrical multichannel BT applicator with lateral shielding was designed by Monte Carlo modeling, validated experimentally with film dosimetry and compared with results found in the literature for the Papillon 50 (P50) contact x-ray radiotherapy device regarding rectoscope dimensions, radiation beam shape, dose fall-off, and treatment time. RESULTS: The multichannel applicator designed is able to deliver 30 Gy under 13 min with a 20350 U (5 Ci) source. The use of multiple channels and lateral shielding provide a uniform circular treatment surface with 22 mm in diameter. The resulting dose fall-off is slightly steeper (maximum difference of 5%) than the one generated by the P50 device with the 22 mm applicator. CONCLUSIONS: A novel multichannel rectal applicator for contact radiotherapy with high-dose-rate 192Ir sources that can be integrated with commercially available treatment planning systems was designed to produce a dose distribution similar to the one obtained by the P50 device.