Matthew J. Webster

Liver motion during cone beam computed tomography guided stereotactic body radiation therapy

PURPOSE Understanding motion characteristics of liver such as, interfractional and intrafractional motion variability, difference in motion within different locations in the organ, and their complex relationship with the breathing cycles are particularly important for image-guided liver SBRT. The purpose of this study was to investigate such motion characteristics based on fiducial markers tracked with the x-ray projections of the CBCT scans, taken immediately prior to the treatments. METHODS Twenty liver SBRT patients were analyzed. Each patient had three fiducial markers (2 × 5-mm gold) percutaneously implanted around the gross tumor. The prescription ranged from 2 to 8 fractions per patient. The CBCT projections data for each fraction (∼650 projections∕scan), for each patient, were analyzed and the 2D positions of the markers were extracted using an in-house algorithm. In total, >55 000 x-ray projections were analyzed from 85 CBCT scans. From the 2D extracted positions, a 3D motion trajectory of the markers was constructed, from each CBCT scans, resulting in left-right (LR), anterior-posterior (AP), and cranio-caudal (CC) location information of the markers with >55 000 data points. The authors then analyzed the interfraction and intrafraction liver motion variability, within different locations in the organ, and as a function of the breathing cycle. The authors also compared the motion characteristics against the planning 4DCT and the RPM™ (Varian Medical Systems, Palo Alto, CA) breathing traces. Variations in the appropriate gating window (defined as the percent of the maximum range at which 50% of the marker positions are contained), between fractions were calculated as well. RESULTS The range of motion for the 20 patients were 3.0 ± 2.0 mm, 5.1 ± 3.1 mm, and 17.9 ± 5.1 mm in the planning 4DCT, and 2.8 ± 1.6 mm, 5.3 ± 3.1 mm, and 16.5 ± 5.7 mm in the treatment CBCT, for LR, AP, and CC directions, respectively. The range of respiratory period was 3.9 ± 0.7 and 4.2 ± 0.8 s during the 4DCT simulation and the CBCT scans, respectively. The authors found that breathing-induced AP and CC motions are highly correlated. That is, all markers moved cranially also moved posteriorly and vice versa, irrespective of the location. The LR motion had a more variable relationship with the AP∕CC motions, and appeared random with respect to the location. That is, when the markers moved toward cranial-posterior direction, 58% of the markers moved to the patient-right, 22% of the markers moved to the patient-left, and 20% of the markers had minimal∕none motion. The absolute difference in the motion magnitude between the markers, in different locations within the liver, had a positive correlation with the absolute distance between the markers (R(2) = 0.69, linear-fit). The interfractional gating window varied significantly for some patients, with the largest having 29.4%-56.4% range between fractions. CONCLUSIONS This study analyzed the liver motion characteristics of 20 patients undergoing SBRT. A large variation in motion was observed, interfractionally and intrafractionally, and that as the distance between the markers increased, the difference in the absolute range of motion also increased. This suggests that marker(s) in closest proximity to the target be used.

Dynamic modulated brachytherapy (DMBT) for rectal cancer

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.

Direction-Modulated Brachytherapy for High-Dose-Rate Treatment of Cervical Cancer. I: Theoretical Design

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.

HDR brachytherapy of rectal cancer using a novel grooved‐shielding applicator design

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.

WE‐A‐108‐07: Dynamic Modulated Brachytherapy for Accelerated Partial Breast Irradiation

PURPOSE To develop a novel brachytherapy applicator for HDR-based accelerated partial breast irradiation (APBI) that significantly improves upon those commercially available, thereby reducing acute and late side effects seen with current APBI techniques. METHODS First, the limitations of adjusting the channel placements or placing more channels in a balloon-like multi-catheter ABPI device were explored. Then, a novel tungsten-shielded applicator design was evaluated. The applicator design consists of a 9-mm diameter tungsten rod, density=19.0gm/cm3 , with 8 sunken grooves along the edge to allow passage of the 192 Ir source. Using dose distributions calculated with Monte Carlo (MCNPX), optimized plans were simulated for our devices, as well as the MammoSite and SAVI, for a set of realistic phantom geometries. These geometries, based on patient cases, were ellipsoids of varying sizes with different PTV orientations and a surrounding rind of healthy tissue to be spared. RESULTS The benefits of adding more channels become insignificant after about 7 channels. Calculations were done for up to 361 channels, but it was found that this was insufficient to create any noticeable improvement. However, the novel shielded design significantly decreased the V150 and V200, while maintaining the target coverage (V90). All planning variables were superior for this design, except for a modest increase in the total dwell time. CONCLUSIONS The introduction of additional channels did little to aid the performance of a balloon-like design. There was a very slight benefit to the dose distributions, but not nearly enough to warrant further investigations. Based on these findings, any device based on open isotropic radiation source modulation cannot be improved much further. Therefore, it is necessary to consider designs with directional radiation profiles, which we have shown to give superior coverage. Our new tungsten shielded applicators are a large step forward along the path of brachytherapy applicator development.

SU‐F‐BRA‐11: Dynamic Modulated Brachytherapy (DMBT): Concept, Design, and Application

Purpose: To build, from grounds up, a novel dynamic modulated brachytherapy (DMBT) system for treating intracavitary tumors, such as rectal, breast, and vaginal cancers. The key component of the innovation is in enabling “dynamic” modulation (i.e., Ir‐192 source is highly collimated and dynamically moved during delivery) to create an unparalleled dose conformality compared with the currently available commercial systems/applicators. We show that our proposed prototype system (both soft‐ and hard‐ware) is extremely efficient in maximizing target volume coverage while minimizing dose to healthy tissues. Methods: Our system uses a 1.9‐cm diameter tungsten‐shield, with density of 18.5g/cc, to create a highly collimated Ir‐192 source radiation profiles. Various window openings in the shield have been simulated with the MCNP code for planning. The code itself was validated against TG43 parameters. The shield is controlled by an in‐house built, computer‐controlled, robotic arm which allows for 360° rotations and 1‐D translational motions. An in‐house coded planning system with the simulated annealing algorithm was used to design optimal plans on ideal rectal anatomies and patient cases. The optimal DMBT plans were then compared with the state‐of‐the‐art commercial Intracavitary Mold Applicator (ICMA)‐based plans. Results: The various simulations encompassing target sizes up to 5‐cm longitudinal and radial directions, with circumferential volumes from 45‐degree to 360‐degree were simulated. Results consistently show that the DMBT system yields broader shoulder past prescription dose in target DVH initially and drops off quicker at higher doses, compared with ICMA, which reflects better dose conformality. More spectacularly, however, up to 50% decrease in normal tissuedose across all dose range were observed. This will translate into clinically observable reduction in acute/late toxicities. Conclusions: Our preliminary results show that DMBT offers exceptional degrees of dynamics for achieving unparalleled dose conformality in brachytherapy. Further research is needed to validate our work in various clinical sites.

SU-C-16A-03: Direction Modulated Brachytherapy for HDR Treatment of Cervical Cancer

PURPOSE To investigate a new Directional Modulated Brachytherapy (DMBT) intra-uterine tandem using various 192-Ir after-loaders. METHODS Dose distributions from the 192-Ir sources were modulated using a 6.3mm diameter tungsten shield (18.0g/cm3). The source moved along 6 longitudinal grooves, each 1.3mm in diameter, evenly spaced along periphery of the shield, The tungsten rod was enclosqed by 0.5mm thick Delrin (1.41g/cc). Monte Carlo N particle (MCNPX) was used to calculate dose distributions. 51million particles were calculated on 504 cores of a supercomputer. Fifteen different patients originally treated with a traditional tandem-and-ovoid applicator, with 5 fractions each, (15 patients X 5 fxs = 75 plans) were re-planned with the DMBT applicator combined with traditional ovoids, on an in-house developed HDR brachytherapy planning platform, which used intensity modulated planning capabilities using a constrained gradient optimization algorithm. For all plans the prescription dose was 6 Gy and they were normalized to match the clinical treated V100. RESULTS Generally, the DMBT plan quality was a remarkable improvement from conventional T&O plans because of the anisotropic dose distribution of DMBT. The largest difference was to the bladder which had a 0.59±0.87 Gy (8.5±28.7%) reduction in dose. This was because of the the horseshoe shape (U-shape) of the bladder. The dose reduction to rectum and sigmoid were 0.48±0.55 Gy (21.1±27.2%) and 0.10±0.38 Gy (40.6±214.9%), respectively. The D90 to the HRCTV was 6.55±0.96 Gy (conventional T&O) and 6.59±1.06 Gy (DMBT). CONCLUSION For image guided adaptive brachytherapy, greater flexibility of radiation intensity is essential and DMBT can be the solution.

WE‐A‐108‐06: Dynamic Modulated Brachytherapy for Cervical Cancer

PURPOSE We propose two new intra-uterine tandem designs that are capable of creating non-isotropic 192-Ir dose distributions and give unprecedented dose conformality for treatment of cervical cancer. METHODS The first tandem design, MC6, has 6 peripheral holes of 1-mm diameter, and the in-between-space filled with tungsten (18.0 g/cc), wrapped in delrin (1.41 g/cc). The second design, DMBT, has 19 holes of 1-mm diameter, each occupied by gold wires (19.3 g/cc) that can slide in and out (with motors), and the in-between-space filled with silicon (1.14 g/cc). MCNPX Monte Carlo was used to simulate the resulting non-isotropic dose distributions. An in-house developed HDR brachytherapy planning platform, with intensity modulated planning capability using Simulated Annealing and Constrained-Gradient Optimization algorithms, was used to plan 36 patient cases and compare with the clinically treated, conventional T&O applicator-based plans. For the proposed tandem designs, the plans were optimized with the same ovoids in place, as the conventional T&O plans. RESULTS All 36 plans were prescribed 6Gy and normalized to D90=6Gy and V100=95%. Generally, the plan qualities were markedly better using MC6 and DMBT, with DMBT usually performing the best overall. Mean 2cc/0.1cc doses to the bladder were 9.14±0.78/9.32± 0.80Gy, 6.87±0.57/7.05±0.59Gy, and 6.61±0.51/6.74±0.52Gy, for T&O, MC6, and DMBT, respectively. For the rectum, they were 5.78± 0.69/5.92±0.70Gy, 5.17±0.52/5.30±0.52Gy, and 4.92±0.50/5.04± 0.50Gy. For the sigmoid, they were 5.23±0.58/5.34±0.59Gy, 5.04± 0.56/5.21±0.57Gy, and 4.86±0.51/5.03±0.52Gy. The most improvement was in the bladder dose and this was due to the horseshoe-like wrapping of the bladder around the CTV, thus benefitting the most with the dose conformation enhancements achieved by the proposed designs. The CTV dose heterogeneity index (DHI) was 2.43±0.01, 2.30±0.01, and 2.26±0.01, respectively. CONCLUSIONS We have shown two tandem designs that advance the conformality of image-guided cervix HDR, in congruence with the current trend of 3D image based planning to maximize the therapeutic ratio.

SU‐E‐T‐337: Intensity Modulated Brachytherapy for Rectal Cancer Using A Novel Grooved Shielding Design

PURPOSE To create a novel HDR (192-Ir) brachytherapy applicator for treatment of rectal carcinomas that uses tungsten shielding to make it dosimetrically superior to all commercially available applicators, despite being no larger than them. METHODS A set of 16 new applicators were designed and simulated using Monte Carlo (MCNPX). All designs were made of a 16-mm diameter, high density tungsten alloy cylinder with inset grooves running along its length for the source to travel along. The designs varied regarding the number and depth of these grooves. Each design was optimized on 36 clinically treated plans (using 8-channel Intracavitary Mold Applicator (Nucletron)) with asymmetrical CTVs, on in-house written intensity modulated brachytherapy planning optimization software. Additionally, a 10 channel device with two channels per groove at varying depths was considered. All results were compared against the clinically treated plans. RESULTS First, all device designs outperformed the Intracavitary Mold Applicator in EVERY metric, except the total dwell times (about 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 design had the best results of the simpler designs, sparing the healthy rectal tissues the most while achieving comparable CTV coverage. All designs significantly outperformed the clinically treated plans using the Intracavitary Mold Applicator. CONCLUSIONS Our extensive simulations and planning showed that our device designs have the ability to conform to the CTV and spare OAR with previously unmatched quality compared to existing commercial brachytherapy devices.