Search

Purpose: To quantify and verify the dosimetric impact of high-dose rate (HDR) source positional uncertainty in brachytherapy, and to introduce a model for three-dimensional (3D) position tracking of the HDR source based on a two-dimensional (2D) measurement. This model has been utilized for the development of a comprehensive source quality assurance (QA) method using radiochromic film (RCF) dosimetry including assessment of different digitization uncertainties., Methods: An algorithm was developed and verified to generate 2D dose maps of the mHDR-V2 192 Ir source (Elekta, Veenendaal, Netherlands) based on the AAPM TG-43 formalism. The limits of the dosimetric error associated with source (0.9 mm diameter) positional uncertainty were evaluated and experimentally verified with EBT3 film measurements for 6F (2.0 mm diameter) and 4F (1.3 mm diameter) size catheters at the surface (4F, 6F) and 10 mm further (4F only). To quantify this uncertainty, a source tracking model was developed to incorporate the unique geometric features of all isodose lines (IDLs) within any given 2D dose map away from the source. The tracking model normalized the dose map to its maximum, then quantified the IDLs using blob analysis based on features such as area, perimeter, weighted centroid, elliptic orientation, and circularity. The Pearson correlation coefficients (PCCs) between these features and source coordinates (x, y, z, θ y , θ z ) were calculated. To experimentally verify the accuracy of the tracking model, EBT3 film pieces were positioned within a Solid Water® (SW) phantom above and below the source and they were exposed simultaneously., Results: The maximum measured dosimetric variations on the 6F and 4F catheter surfaces were 39.8% and 36.1%, respectively. At 10 mm further, the variation reduced to 2.6% for the 4F catheter which is in agreement with the calculations. The source center (x, y) was strongly correlated with the low IDL-weighted centroid (PCC = 0.99), while the distance to source (z) was correlated with the IDL areas (PCC = 0.96) and perimeters (PCC = 0.99). The source orientation θ y was correlated with the difference between high and low IDL-weighted centroids (PCC = 0.98), while θ z was correlated with the elliptic orientation of the 60-90% IDLs (PCC = 0.97) for a maximum distance of z = 5 mm. Beyond 5 mm, IDL circularity was significant, therefore limiting the determination of θ z (PCC ≤ 0.48). The measured positional errors from the film sets above and below the source indicated a source position at the bottom of the catheter (-0.24 ± 0.07 mm)., Conclusions: Isodose line features of a 2D dose map away from the HDR source can reveal its spatial coordinates. RCF was shown to be a suitable dosimeter for source tracking and dosimetry. This technique offers a novel source QA method and has the potential to be used for QA of commercial and customized applicators., (© 2020 American Association of Physicists in Medicine.)

Purpose: Organ preservation or nonoperative management of rectal cancer is of growing interest. Image guided adaptive endorectal brachytherapy is a radiation dose escalation modality: we explored its role in elderly patients unfit for surgery and patients refusing surgery., Methods and Materials: In this registry study, patients with rectal cancer who were ineligible for surgery received 40 Gy in 16 fractions of pelvic external beam radiation therapy. They subsequently received 3 weekly image guided adaptive brachytherapy boosts of 10 Gy to the residual tumor, for a total of 30 Gy in 3 fractions. Complete clinical response (cCR) and local control were the primary endpoints., Results: 94 patients were included; the median age was 81.1 years. With a median follow-up of 1.9 years, the proportion of cCR was 86.2%, the tumor regrowth proportion was 13.6%, and the cumulative incidence of local relapse was 2.7% at 1 year and 16.8% at 2 years. When considering responders and nonresponders, the 2-year local control was 71.5%. The overall survival at 2 years was 63.6%. Acute rectal grade 1 to 2 toxicity included all patients: 12.8% of patients had late bleeding requiring iron replacement, blood transfusions, or argon plasma therapy., Conclusions: Results of this registry study, evaluating radiation dose escalation for elderly medically unfit patients with unselected tumors, reveal that a high proportion of patients achieved cCR with a manageable toxicity profile. This technology will likely contribute to the challenging nonoperative management paradigm of rectal cancer., (Copyright © 2019. Published by Elsevier Inc.)

Purpose: We investigate the effect of the GafChromic™ film EBT3 model absorbed dose energy response when used for dose measurements around low-energy photon sources. Monte Carlo based correction procedure in synergy with appropriate calibration curves was shown to provide more accurate absorbed dose (either relative or absolute). An assessment was made of possible dose errors that might be encountered if such energy dependent response is ignored., Methods: We measured PDDs in water from a Xoft 50 kVp source using EBT3 film, and compared to PDD measurements acquired with a PTW-TN34013 parallel-plate ionization chamber. For the x-ray source, we simulated spectra using the EGSnrc (BEAMnrc) Monte Carlo code, and calculated Half Value Layer (HVL) at different distances from the source in water. Measurement strips of EBT3 film were positioned at distances of 2-6 cm from the Xoft source in a water phantom using a custom-made holder and irradiated simultaneously., Results: Our results show that film calibration curves obtained at beam qualities near the effective energy of the Xoft 50 kVp source in water lead to variation in absorbed dose energy dependence of the response of around 5%. However, if the calibration curve was established in an MV beam quality, the error in absorbed dose could be as large as 20%., Conclusion: Accurate dose measurements using radiochromic films at low photon energies require that the radiochromic film dosimetry system be calibrated at appropriate corresponding low energies, as large absorbed dose errors are expected when film calibration is performed in MV beam qualities., (Copyright © 2019 Associazione Italiana di Fisica Medica. Published by Elsevier Ltd. All rights reserved.)

Purpose: We describe methods to improve dose delivery for patients with rectal cancer receiving boost brachytherapy after external beam radiotherapy., Methods and Materials: Patients with rectal cancer who were ineligible or refusing surgery are treated with external beam radiotherapy and subsequently with three weekly image-guided volume-adapted high-dose-rate brachytherapy boosts of 10 Gy to the residual clinical target volume, for a total of 30 Gy in three fractions. Tungsten shielding placed at the center of intracavitary mold applicator and double-balloon technique was used to improve dose conformity to the target., Results: Our results show that the use of tungsten shield and double balloon reduces the dose gradient within the target volume to receive the prescription boost dose of 10 Gy from maximum dose of 60 Gy down to 20 Gy., Conclusions: We outlined two methods for achieving higher high-dose-rate brachytherapy dose conformity using the tungsten shielding rods (to spare contralateral healthy tissues) and double-balloon technique (to decrease dose gradient within the target to minimize dose to the proximal mucosa)., (Copyright © 2019 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.)

Purpose: The Papillon technique using 50-kVp soft X-rays to treat rectal adenocarcinomas was developed and clinically implemented in the 1960s. We describe differences between accurate dosimetry and clinical implementation of this technique that is extending from its very inception to date., Methods and Materials: A renaissance of the Papillon technique occurred with two recently introduced 50-kVp systems: Papillon+ by Ariane and a custom-made rectal applicator (consisting of a surface applicator inserted into a proctoscope) by iCAD's Xoft Axxent Electronic Brachytherapy (eBT) System (iCad, Inc., Sunnyvale, CA). In contrast to the initial design, we investigated the impact of introducing a plastic lid, which would provide more reproducible and more accurate dose delivery across the rectal adenocarcinoma patient population. We use both parallel-plate chamber and radiochromic film dosimeters to determine differences in basic dosimetry characteristics (beam half-value layers, outputs, percent depth doses, and profiles) between the Xoft Electronic Brachytherapy rectal applicator system with and without the plastic lid in place., Results: Compared to the open-cone applicator, the proposed applicator with the plastic lid produces a slightly harder (more penetrating) beam quality (half-value layer of 1.4 vs. 1.3-mm Al), but with reduced output (by 33%), and a slightly broader beam with flatness not worse than 3% and symmetry not worse than 2%., Conclusions: In addition to characterizing beam properties modified by the possible introduction of the plastic cap, we also pointed out and addressed misconceptions in the use of radiochromic films for dose measurements at low-energy photon beams., (Copyright © 2017 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.)

Purpose: Current high-dose-rate brachytherapy skin treatments with the Freiburg flap (FF) applicator are planned with treatment planning systems based on the American Association of Physicists in Medicine TG-43 data sets, which assume full backscatter conditions in dose calculations. The aim of this work is to describe an experimental method based on radiochromic film dosimetry to evaluate dose calculation accuracy during surface treatments with the FF applicator at different depths and bolus thicknesses., Methods and Materials: Absolute doses were measured using a reference EBT3 radiochromic film dosimetry system within a Solid Water phantom at different depths (0, 0.5, 1, 2, and 3 cm) with respect to the phantom surface. The impact of bolus (up to 3-cm thickness) placed on top of the applicator was investigated for two clinical loadings created using Oncentra MasterPlan: 5 cm × 5 cm and 11 cm × 11 cm., Results: For smaller loading and depths beyond 2 cm and for larger loading and depths beyond 1 cm, the dose difference was less than 3% (±4%). At shallower depths, differences of up to 6% (±4%) at the surface were observed if no bolus was added. The addition of 2-cm bolus for the smaller loading and 1 cm for larger loading minimized the difference to less than 3% (±4%)., Conclusions: For typical FF applicator loading sizes, the actual measured dose was 6% (±4%) lower at the skin level when compared with TG-43. Additional bolus above the FF was shown to decrease the dose difference. The consideration of change in clinical practice should be carefully investigated in light of clinical reference data., (Copyright © 2017 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.)

Purpose: In the past, film dosimetry was developed into a powerful tool for external beam radiotherapy treatment verification and quality assurance. The objective of this work was the development and clinical testing of the EBT3 model GafChromic film based brachytherapy quality assurance (QA) system., Methods and Materials: Retrospective dosimetry study was performed to test a patient-specific QA system for preoperative endorectal brachytherapy that uses a radiochromic film dosimetry system. A dedicated phantom for brachytherapy applicator used for rectal cancer treatment was fabricated enabling us to compare calculated-to-measured dose distributions. Starting from the same criteria used for external beam intensity-modulated radiation therapy QA (3%, 3 mm), passing criteria for high- and low-dose gradient regions were subsequently determined. Finally, we investigated the QA system's sensitivity to controlled source positional errors on selected patient plans., Results: In low-dose gradient regions, measured dose distributions with criteria of 3%, 3 mm barely passed the test, as they showed 95% passing pixels. However, in the high-dose gradient region, a more stringent condition could be established. Both criteria of 2%, 3 mm and 3%, 2 mm with gamma function calculated using normalization to the same absolute dose value in both measured and calculated dose distributions, and matrix sizes rescaled to match each other showed more than 95% of pixels passing, on average, for 15 patient plans analyzed., Conclusions: Although the necessity of the patient-specific brachytherapy QA needs yet to be justified, we described a radiochromic film dosimetry-based QA system that can be a part of the brachytherapy commissioning process, as well as yearly QA program., (Copyright © 2015 American Brachytherapy Society. Published by Elsevier Inc. All rights reserved.)

Purpose: To demonstrate that utilization of the direction-modulated brachytherapy (DMBT) concept can significantly improve treatment plan quality in the setting of high-dose-rate (HDR) brachytherapy for cervical cancer., Methods and Materials: The new, MRI-compatible, tandem design has 6 peripheral holes of 1.3-mm diameter, grooved along a nonmagnetic tungsten-alloy rod (ρ = 18.0 g/cm(3)), enclosed in Delrin tubing (polyoxymethylene, ρ = 1.41 g/cm(3)), with a total thickness of 6.4 mm. The Monte Carlo N-Particle code was used to calculate the anisotropic (192)Ir dose distributions. An in-house-developed inverse planning platform, geared with simulated annealing and constrained-gradient optimization algorithms, was used to replan 15 patient cases (total 75 plans) treated with a conventional tandem and ovoids (T&O) applicator. Prescription dose was 6 Gy. For replanning, we replaced the conventional tandem with that of the new DMBT tandem for optimization but left the ovoids in place and kept the dwell positions as originally planned. All DMBT plans were normalized to match the high-risk clinical target volume V100 coverage of the T&O plans., Results: In general there were marked improvements in plan quality for the DMBT plans. On average, D2cc for the bladder, rectum, and sigmoid were reduced by 0.59 ± 0.87 Gy (8.5% ± 28.7%), 0.48 ± 0.55 Gy (21.1% ± 27.2%), and 0.10 ± 0.38 Gy (40.6% ± 214.9%) among the 75 plans, with best single-plan reductions of 3.20 Gy (40.8%), 2.38 Gy (40.07%), and 1.26 Gy (27.5%), respectively. The high-risk clinical target volume D90 was similar, with 6.55 ± 0.96 Gy and 6.59 ± 1.06 Gy for T&O and DMBT, respectively., Conclusions: Application of the DMBT concept to cervical cancer allowed for improved organ at risk sparing while achieving similar target coverage on a sizeable patient population, as intended, by maximally utilizing the anatomic information contained in 3-dimensional imaging. A series of mechanical and clinical validations are to be followed., (Copyright © 2014 Elsevier Inc. All rights reserved.)

Purpose: The aim of this work was to design a novel high-dose rate (HDR) ((192)Ir) brachytherapy applicator for treatment of rectal carcinomas that uses tungsten shielding for possibly improved dosimetric results over commercial brachytherapy applicator(s)., Methods: A set of 15 single-depth applicators and one dual-depth applicator were designed and simulated using Monte Carlo (MCNPX). All applicators simulated were high-density tungsten alloy cylinders, 16-mm in diameter, and 60-mm long, with longitudinal grooves within which an (192)Ir source can be placed. The single-depth designs varied regarding the number and depth of these grooves, ranging from 8 to 16 and 1-mm to 3-mm, respectively. The dual-depth design had ten channels, each of which had two depths at which the source could be placed. Optimized treatment plans were generated for each design on data from 13 treated patients (36 fractions) with asymmetrical clinical target volumes (CTVs). All results were compared against the clinically treated plans which used intracavitary mold applicator (ICMA), as well as a recently designed, highly automated, and collimated intensity modulation device named dynamic modulated brachytherapy (DMBT) device., Results: All applicator designs outperformed the ICMA in every calculated dosimetric criteria, except the total dwell times (∼30% increase). There were clear, but relative, tradeoffs regarding both the number of channels and the depth of each channel. Overall, the 12-channel, 1-mm depth, and 14-channel 2-mm depth designs had the best results of the simpler designs, sparing the healthy rectal tissues the most while achieving comparable CTV coverage with the dose heterogeneity index and lateral spill doses improving by over 10% and the contralateral healthy rectum dose dropping over 30% compared to ICMA. The ten-channel dual-depth design outperformed each single-depth design, yielding the best coverage and sparing., Conclusions: New grooved tungsten HDR-brachytherapy devices have been designed and simulated. The results of this work attest to the capability of these new, highly anisotropic, intelligently shielded applicators to limit dose to healthy tissues while maintaining a conformal prescription dose to the CTV.

Purpose: All forms of past and current high-dose-rate brachytherapy utilize immobile applicators during treatment delivery. The only moving part is the source itself. This paradigm misses an important degree of freedom that, if explored, can in some instances produce previously unachievable dose conformality; that is, the dynamic motion of the applicator itself during treatment delivery. Monte Carlo and treatment planning simulations were used to illustrate the potential benefits of moving applicators for rectal cancer applications in particular. This concept is termed dynamic modulated brachytherapy (DMBT)., Methods: The DMBT system uses a high-density, 18.0 g∕cm(3), 45 mm long tungsten alloy shield, cylindrical in shape, with a small window on one side to encapsulate a (192)Ir source, to create collimation that results in a highly directional beam profile. This shield can be dynamically translated and rotated, using an attached robotic arm, during treatment to create a volumetric modulated arc therapy-type delivery, but from inside the rectal cavity. Monte Carlo simulations and planning optimization algorithms were developed inhouse to evaluate the effectiveness of this new approach using 36 clinical treatment plans comprised of 13 patients each treated using the intracavitary mold applicator (ICMA, Nucletron, The Netherlands) to quantify the potential clinical benefit. The prescription dose was 10 Gy∕fx and the group had an average clinical target volume of 9.0 ± 3.5 cm(3). Ideal phantom geometries were used to evaluate the impact of various shield dimensions and designs on the resulting plan quality., Results: Simulations of ideal phantom geometries found that shields as small as 10 mm in diameter can produce high quality plans. For the clinical patient cases, compared to the ICMA, for equal prescription tumor coverage, the DMBT plans provided >30% decrease in D(5) (high dose volume) resulting in a ∼40% decrease in dose heterogeneity index. In addition, mean dose and D(98) showed a reduction (typically 40%-60%) on all critical structures evaluated. However, for a 10 Gy prescribed dose there was an increase in total treatment time on average from 7.6 to 20.8 min for a source with an air-kerma strength of 40.25 kU (10 Ci)., Conclusions: Dosimetric properties of a novel DMBT system have been described and evaluated. Comparison with the ICMA commercial applicator has shown it to be a prospective step forward in high-dose-rate brachytherapy (192)Ir technology. Dynamic motion of an applicator during treatment, for any applicator and site in general, can provide additional degrees of freedom that, if properly considered, can potentially increase the plan quality significantly.

Purpose: Acute radiation proctitis (ARP) is a common side effect of pelvic radiotherapy, and its management is challenging in daily practice. The present phase I/II study evaluates the safety and efficacy of the botulinum toxin A (BTX-A) in ARP treatment for rectal cancer patients undergoing neoadjuvant high-dose-rate endorectal brachytherapy (HDREBT)., Methods and Materials: Fifteen patients, treated with neoadjuvant HDREBT, 26-Gy in 4 fractions, received the study treatment that consisted of a single injection of BTX-A into the rectal wall. The injection was performed post-HDREBT and prior to the development of ARP. The control group, 20 such patients, did not receive the BTX-A injection. Both groups had access to standard treatment with hydrocortisone rectal aerosol foam (Cortifoam) and anti-inflammatory and narcotic medication. The ARP was clinically evaluated by self-administered daily questionnaires using visual analog scores to document frequency and urgency of bowel movements, rectal burning/tenesmus, and pain symptoms before and after HDREBT., Results: At the time of this analysis, there was no observed systemic toxicity. Patient compliance with the self-administered questionnaire was 100% from week 1 to 4, 70% during week 5, and 40% during week 6. The maximum tolerated dose was established at the 100-U dose level, and noticeable mean differences were observed in bowel frequency (p = 0.016), urgency (p = 0.007), and pain (p = 0.078)., Conclusions: This study confirms the feasibility and efficacy of BTX-A intervention at 100-U dose level for study patients compared to control patients. A phase III study with this dose level is planned to validate these results., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Fractionated high dose rate endorectal brachytherapy (HDR-EBT) using CT-based treatment planning is an alternative method for preoperative down-sizing and down-staging of advanced rectal adeno-carcinomas. The authors present an image guidance procedure that was developed to ensure daily dose reproducibility for the four brachytherapy treatment fractions. Since the applicator might not be placed before each treatment fraction inside the rectal lumen in the same manner as it was placed during the 3D CT volume acquisition used for treatment planning, there is a shift along the catheter axis that may have to be performed. The required shift is determined by comparison of a daily radiograph with the treatment planning digitally-reconstructed radiograph (DRR). A procedure is developed for DRR reconstruction from the 3D data set used for the treatment planning, and two possible daily longitudinal shifts are illustrated: above and below the planning dose distribution. The authors also describe the procedure for rotational alignment illustrated on a clinical case. Reproduction of the treatment planned dose distribution on a daily basis is crucial for the success of fractionated 3D based brachytherapy treatments. Due to the cylindrical symmetry of the applicator used for preoperative HDR-EBT, two types of adjustments are necessary: applicator rotation and dwell position shift along the applicator's longitudinal axis. The impact of the longitudinal applicator shift prior to treatment delivery for 62 patients treated in our institution is also assessed.

The dosimetric properties of a novel intracavitary mold applicator for 192Ir high dose rate (HDR) endorectal cancer treatment have been investigated using Monte Carlo (MC) simulations and experimental methods. The 28 cm long applicator has a flexible structure made of silicone rubber for easy passage into cavities with deep-seated tumors. It consists of eight source catheters arranged around a central cavity for shielding insertion, and is compatible for use with an endocavitary balloon. A phase space model of the HDR source has been validated for dose calculations using the GEANT4 MC code. GAFCHROMIC EBT model film was used to measure dose distributions in water around shielded and unshielded applicators with two loading configurations, and to quantify the shielding effect of a balloon injected with an iodine solution (300 mg I/mL). The film calibration procedure was performed in water using an 192Ir HDR source. Ionization chamber measurements in a Lucite phantom show that placing a tungsten rod in the applicator attenuates the dose in the shielded region by up to 85%. Inserting the shielded applicator into a water-filled balloon pushes the neighboring tissues away from the radiation source, and the resulting geometric displacement reduces the dose by up to 53%; another 8% dose reduction can be achieved when the balloon is injected with an iodine solution. All experimental results agree with the GEANT4 calculations within measurement uncertainties.

Purpose: This study reports the results of a single-institution experience with high-dose-rate brachytherapy (HDRBT) used as a boost in the treatment of esophageal cancer with external beam radiation therapy (ERT) with or without chemotherapy., Methods and Materials: Patients without evidence of metastatic disease were identified. HDRBT was given before ERT with a dose of 20 Gy in 5 fractions. Patients with a Karnofsky performance of more than 70 received treatment with 50 Gy in 25 fractions and concurrent 5-fluorouracil and cis-platinum during Weeks 1 and 5, whereas patients with a Karnofsky performance of less than 70 were treated with radiation therapy alone with 35 Gy in 14 fractions., Results: Fifty-three patients received HDRBT treatment with combined ERT and chemotherapy and 17 patients with ERT alone. The incidence of acute bone marrow toxicity was 55% Grade 2 and 15% Grade 3, and 85% of patients had Grade 2 esophagitis. With a median follow-up time of 26 months, the median survival was 21 months; the 2-year local recurrence was 25%, and the 5-year survival rate was 28%., Conclusion: HDRBT is safe and beneficial for local control in the radical treatment of patients with esophageal cancer.

High-dose rate endorectal brachytherapy (HDR-EBT) is mainly used as a palliative treatment modality. In this paper, we compare dosimetry distributions for a single-channel catheter (Miami) applicator with distributions of the inflatable multichannel (Novi Sad) endorectal applicator. The comparisons were made with respect to dose coverage to the clinical tumor volume as well as to the bladder, rectal wall, prostate, and bone marrow. Our results suggest that a multichannel applicator provides better sparing of the bone marrow by 50%, clinically uninvolved parts of the rectal wall by 70%, and bladder and prostate (in the case of male patients) by 100% in terms of ratio of median doses to critical organ volume for single- and multichannel endorectal applicators. Our results justify the advantage of using a multichannel endorectal brachytherapy applicator as a neo-adjuvant treatment of patients with locally advanced rectal cancer.

Purpose: In this era of new radiation technologies and tumor imaging, a high-dose-rate endorectal brachytherapy has been developed and tested in a phase I/II study of advanced rectal tumors. In this article, we report technical aspects of the treatment modality., Methods and Materials: Forty-nine patients underwent staging with endoscopic endorectal ultrasound, and the tumor dimensions were determined with MRI of the pelvis. Patients with resectable rectal cancer (staged T2, T3, or early T4) were treated with preoperative high-dose-rate endorectal brachytherapy followed by surgery 6-8 weeks later. Under direct rectoscopy, radio-opaque clips were used to mark the tumor margins. The treatment planning was done with the use of a CT simulator, and the treatment was delivered using a flexible endorectal applicator with eight catheters arranged around the circumference of the applicator and a high-dose-rate brachytherapy remote afterloading system with an Iridium-192 source. Isodose distributions were generated by the Plato planning system (Nucletron B.V., Veenendaal, The Netherlands) and digitally reconstructed radiographs were used as references for daily treatment. A tumor dose of 26 Gy in four fractions was prescribed, and intramesorectal deposits were documented on the magnetic resonance images., Results: Forty-nine patients received planned treatment, and all but 2 patients underwent planned surgery. The pathology specimens showed a complete macroscopic response in 64% of the patients and tumor downstaging in 67% of the patients., Conclusions: Advances in tumor imaging and 3D treatment planning systems allow for better tumor mapping and dose planning. The use of a multichannel flexible endorectal applicator leads to tumor downstaging before surgery in patients with resectable locally advanced rectal carcinomas. The technique used in our center was practical and validated by this study.

Simple Summary: This prospective phase II–III randomized trial explores the value of modern technology for prudent radiation dose escalation in the curative management of patients with rectal cancer. It includes 40 operable patients with locally advanced operable rectal cancer. This manuscript illustrates the planned interim analysis, which was intended to confirm safety, including the absence of unexpected toxicities and the inability to perform salvage surgery. The treatment approach involves pelvic chemoradiotherapy followed by either an additional external beam radiation boost or three weekly boosts of brachytherapy, which are adapted in real-time to tumor response. This proof-of-principle study shows that careful radiotherapy dose escalation leads to high proportions of sustained complete clinical response in patients with rectal cancer with acceptable toxicity. Ongoing accrual to the phase III component of the trial is in progress. Background: We explored image-guided adaptive endorectal brachytherapy patients electing non-operative management for rectal cancer. We present the first pre-planned interim analysis. Methods: In this open-label phase II–III randomized study, patients with operable cT2-3ab N0 M0 rectal cancer received 45 Gy in 25 fractions of pelvic external beam radiotherapy (EBRT) with 5-FU/Capecitabine. They were randomized 1:1 to receive either an EBRT boost of 9 Gy in 5 fractions (Arm A) or three weekly adaptive brachytherapy (IGAEBT) boosts totaling 30 Gy (Arm B). Patient characteristics and toxicity are presented using descriptive analyses; TME-free survival between arms with the intention to treat the population is explored using the Kaplan–Meier method. Results: A total of 40 patients were in this analysis. Baseline characteristics were balanced; acute toxicities were similar. Complete clinical response (cCR) was 50% (n = 10/20) in Arm A and 90% in Arm B (n = 18/20). Median follow-up was 1.3 years; 2-year TME-free survival was 38.6% (95% CI: 16.5–60.6%) in the EBRT arm and 76.6% (95% CI: 56.1–97.1%) in the IGAEBT arm. Conclusions: Radiation intensification with IGAEBT is feasible. This interim analysis suggests an improvement in TME-free survival when comparing IGAEBT with EBRT, pending confirmation upon completion of this trial. [ABSTRACT FROM AUTHOR]

Purpose: To quantify and verify the dosimetric impact of high‐dose rate (HDR) source positional uncertainty in brachytherapy, and to introduce a model for three‐dimensional (3D) position tracking of the HDR source based on a two‐dimensional (2D) measurement. This model has been utilized for the development of a comprehensive source quality assurance (QA) method using radiochromic film (RCF) dosimetry including assessment of different digitization uncertainties. Methods: An algorithm was developed and verified to generate 2D dose maps of the mHDR‐V2 192Ir source (Elekta, Veenendaal, Netherlands) based on the AAPM TG‐43 formalism. The limits of the dosimetric error associated with source (0.9 mm diameter) positional uncertainty were evaluated and experimentally verified with EBT3 film measurements for 6F (2.0 mm diameter) and 4F (1.3 mm diameter) size catheters at the surface (4F, 6F) and 10 mm further (4F only). To quantify this uncertainty, a source tracking model was developed to incorporate the unique geometric features of all isodose lines (IDLs) within any given 2D dose map away from the source. The tracking model normalized the dose map to its maximum, then quantified the IDLs using blob analysis based on features such as area, perimeter, weighted centroid, elliptic orientation, and circularity. The Pearson correlation coefficients (PCCs) between these features and source coordinates (x, y, z, θy, θz) were calculated. To experimentally verify the accuracy of the tracking model, EBT3 film pieces were positioned within a Solid Water® (SW) phantom above and below the source and they were exposed simultaneously. Results: The maximum measured dosimetric variations on the 6F and 4F catheter surfaces were 39.8% and 36.1%, respectively. At 10 mm further, the variation reduced to 2.6% for the 4F catheter which is in agreement with the calculations. The source center (x, y) was strongly correlated with the low IDL‐weighted centroid (PCC = 0.99), while the distance to source (z) was correlated with the IDL areas (PCC = 0.96) and perimeters (PCC = 0.99). The source orientation θy was correlated with the difference between high and low IDL‐weighted centroids (PCC = 0.98), while θz was correlated with the elliptic orientation of the 60–90% IDLs (PCC = 0.97) for a maximum distance of z = 5 mm. Beyond 5 mm, IDL circularity was significant, therefore limiting the determination of θz (PCC ≤ 0.48). The measured positional errors from the film sets above and below the source indicated a source position at the bottom of the catheter (−0.24 ± 0.07 mm). Conclusions: Isodose line features of a 2D dose map away from the HDR source can reveal its spatial coordinates. RCF was shown to be a suitable dosimeter for source tracking and dosimetry. This technique offers a novel source QA method and has the potential to be used for QA of commercial and customized applicators. [ABSTRACT FROM AUTHOR]

High-dose-rate endorectal brachytherapy (HDREBT) is an image guided brachytherapy treatment for patients with rectal cancer. It is based on tumor imaging with magnetic resonance in particular, which is used to choose eligible patients and improve tumor visualization. Treatment planning is performed using 3D CT simulation and treatment planning. The treatment is given on an outpatient basis and requires minimal local anesthesia. The validation of the technique was carried out through a preoperative study and is now explored as part of a radical treatment for early rectal cancer or as a boost modality. We describe technical aspects of the HDREBT and we discuss the ongoing institutional review board approved studies exploring the clinical applications of this treatment modality for patients with rectal cancer: 1) as a neoadjuvant treatment for patients with operable rectal tumor; 2) as a option to improve local control in patients with newly diagnosed rectal cancer but with previous pelvic radiation. [ABSTRACT FROM AUTHOR]