Bolus Electron Conformal Therapy Research Articles

Defining the role of intensity modulation in electron conformal therapy for the treatment of head and neck cancer

Bolus electron conformal therapy (BECT) in the treatment of cancers of the head and neck is often limited by an inability to reduce dosimetric hot spots resulting from surface irregularity or tissue heterogeneity. We examined the potential benefits of using intensity modulation for electron therapy (IM-BECT) to reduce hotspots in patients undergoing electron beam therapy for superficial cancers of the head and neck (HN). Twenty patients with HN cancer previously treated with BECT were identified. Each case included the treatment targets and a primary organ at risk (OAR) that were defined by the radiation oncologist. A target +2 cm rind structure was created for analysis of the dose deposition in areas surrounding the target volume as a measure of conformality. Each patient plan was transferred into the novel IM-BECT planning software and each case was recomputed as per the original parameters. Next, each case was replanned with the inclusion of intensity modulation, as well as a new custom conformal bolus that was redesigned for optimized range compensation when paired with an intensity modulator. The plans were then normalized to prescription dose and compared for target coverage/dose and OAR dose. For patients who had a hotspot of 125% or greater, the hotspot was on average reduced by 13.1% with IM-BECT. For IM-BECT, the average primary OAR means dose and target+2cm rind mean dose increased slightly by 10.6% and 6.4%, respectively (primary OAR mean [p = 0.0001], and Target+2cm rind mean [p = 0.0001], paired t-test). IM-BECT is an effective method of reducing hotspots in patients with superficial HN cancer. Improvements came at the expense of slight increases in dose to the underlying tissues. This retrospective planning study represents the first example of IM-BECT to actual HN patient cases. Expanding the role of IM-BECT in other disease sites could potentially compared to conventional BECT.

Dosimetric validation of intensity-modulated bolus electron conformal therapy planning and delivery using an anthropomorphic cylindrical head phantom.

This work investigated the dosimetric accuracy of the intensity-modulated bolus electron conformal therapy (IM-BECT) planning and delivery process using the decimal ElectronRT (eRT) treatment planning system. An IM-BECT treatment plan was designed using eRT for a cylindrical, anthropomorphic retromolar trigone phantom. Treatment planning involved specification of beam parameters and design of a variable thickness wax bolus and Passive Radiotherapy Intensity Modulator for Electrons (PRIME) device, which was comprised of 33 tungsten island blocks of discrete diameters from 0.158 to 0.223cm (Intensity Reduction Factors from 0.937 to 0.875, respectively) inside a 10.1×6.7 cm2 copper cutout. For comparison of calculation accuracy, a BECT plan was generated by copying the IM-BECT plan and removing the intensity modulation. For both plans, a 16 MeV electron beam was used with 104.7cm source-to-surface distance to bolus. In-phantom TLD-100 measurements (N=47) were compared with both eRT planned dose distributions, which used the pencil beam redefinition algorithm with modifications for passive electron intensity modulation (IM-PBRA). Dose difference and distance to agreement (DTA) metrics were computed for each measurement point. Comparison of measured dose distributions with planned dose distributions yielded dose differences (calculated minus measured) characterized by a mean and standard deviation of -0.36%±1.64% for the IM-BECT plan, which was similar to -0.36%±1.90% for the BECT plan. All dose measurements were within 5% of the planned dose distribution, with both the BECT and IM-BECT measurement sets having 46/47 (97.8%) points within 3% or within 3mm of the respective treatment plans. It was found that the IM-BECT treatment plan generated using eRT was sufficiently accurate for clinical use when compared to TLD measurements in a cylindrical, anthropomorphic phantom, and was similarly accurate to the BECT treatment plan in the same phantom.

Defining the Role of Intensity Modulation in Electron Conformal Therapy for the Treatment of Head and Neck Cancer

The use of bolus electron conformal therapy (BECT) in the treatment of cancers of the head and neck is often limited by an inability to reduce dosimetric hot spots resulting from surface or tissue heterogeneity. In this study, we examined the potential benefits of using a recently patented, low-cost form of Intensity Modulation for electron therapy (IM-BECT) to reduce treatment hotspots in patients undergoing electron beam therapy for cancer of the Head and Neck (HN). The treatment plans from twelve patients with HN cancer previously treated with BECT were identified (treatment energies ranged from 6-18 MeV and field sizes of 36-625 cm2). Each case included the treatment targets and at least one primary OAR that were defined by the original treating radiation oncologist. Additionally, a target + 2 cm rind structure was created for analysis of the dose deposition in areas immediately surrounding the target volume as a measure of conformality. Each patient plan was transferred into a novel IM-BECT planning software and each case was recomputed as per the original prescription, gantry, couch, collimator angles, and original clinically used bolus. Next, each case was replanned with the inclusion of intensity modulation, as well as a new custom conformal bolus that was designed for optimized range compensation. The patient plans were then normalized such that 100% equals the prescription dose value and then transferred to a plan analysis software for comparison of the target coverage/dose and OAR dose. Comparison of the BECT and IM-BECT treatment plans demonstrated that IM-BECT was able to significantly reduce dosimetric hotspots for this cohort of patients undergoing radiation therapy for HN cancer, bringing the average maximum dose down from 130.6% to only 120.6% (p = 0.044, paired t-test). Moreover, the impact of IM-BECT appeared to be most substantial in the patients with the highest baseline maximum dose. For patients who had a hotspot of 125% or greater, the hotspot was on average reduced by 19%. Further dosimetric analysis demonstrated a small resultant increase in the low dose deposition to the surrounding normal tissues. For BECT, the average primary OAR mean dose and Target+2cm rind mean dose were 27.5% and 60.0%, respectively. For IM-BECT, the average primary OAR mean dose and Target+2cm rind mean dose increased slightly to 30.9% and 64.6%, respectively [Primary OAR mean (P = 0.0008), and Target+2cm rind mean (P = 0.0001), paired t-test]. IM-BECT is an effective method of reducing dosimetric hotspots in patients undergoing radiation therapy for cancer of the HN. This improvement came at the expense of a small increase in dose to the underlying tissues. This retrospective planning study represents the first example of IM-BECT being applied to real patient cases and suggests further development of IM-BECT is warranted.

Modeling scatter through sides of island blocks used for intensity-modulated bolus electron conformal therapy.

Passive Radiotherapy Intensity Modulators for Electrons (PRIME) devices are comprised of cylindrical tungsten island blocks imbedded in a machinable foam slab within the patient's cutout. Intensity-modulated bolus electron conformal therapy (IM-BECT) uses PRIME devices to reduce dose heterogeneity caused by the irregular bolus surface. Heretofore, IM-BECT dose calculations used the pencil beam redefinition algorithm (PBRA) assuming perfect collimation. This study investigates modeling electron scatter into and out the sides of island blocks. Dose distributions were measured in a water phantom at 7, 13, and 20 MeV for devices having nominal intensity reduction factors of 1.000 (foam only), 0.937, 0.812, and 0.688, corresponding to nominal island block diameters (dnom ) of 0.158, 0.273, and 0.352cm, respectively. Pencil beam theory derived an effective diameter (dIS ) to account for in-scattered electrons as a function of dnom and beam energy (Ep,0 ). However, for out-scattered electrons, an effective diameter (dmod ) was estimated by best fitting measured data. In the modulated region (under island blocks, depth<R90 ), modified PBRA-calculated dose distributions showed 2%/2mm passing rates for dnom =0.158, 0.273, and 0.352cm of (100%, 100%, 100%) at 7 MeV, (100%, 100%, 93.5%) at 13 MeV, and (99.8%, 85.4%, and 71.5%) at 20 MeV. The largest dose differences (≤6%) occurred at the highest energy (20 MeV), largest dnom , shallowest depths (≤2cm), and on central axis. An equation for modeling island block scatter, dmod (dnom , Ep,0 ), has been developed for use in the PBRA, insignificantly impacting calculation time. Although inaccuracy sometimes exceeded our 2%/2mm criteria, it could be clinically acceptable, as superficial dose differences often fall inside the bolus. Also, patient PRIME devices are expected to have fewer large diameter island blocks than did test devices. Inaccuracies are attributed to out-scattered electrons having energy spectra different than the primary beams.

Planning and delivery of intensity modulated bolus electron conformal therapy.

PurposeBolus electron conformal therapy (BECT) is a clinically useful, well‐documented, and available technology. The addition of intensity modulation (IM) to BECT reduces volumes of high dose and dose spread in the planning target volume (PTV). This paper demonstrates new techniques for a process that should be suitable for planning and delivering IM‐BECT using passive radiotherapy intensity modulation for electrons (PRIME) devices.MethodsThe IM‐BECT planning and delivery process is an addition to the BECT process that includes intensity modulator design, fabrication, and quality assurance. The intensity modulator (PRIME device) is a hexagonal matrix of small island blocks (tungsten pins of varying diameter) placed inside the patient beam‐defining collimator (cutout). Its design process determines a desirable intensity‐modulated electron beam during the planning process, then determines the island block configuration to deliver that intensity distribution (segmentation). The intensity modulator is fabricated and quality assurance performed at the factory (.decimal, LLC, Sanford, FL). Clinical quality assurance consists of measuring a fluence distribution in a plane perpendicular to the beam in a water or water‐equivalent phantom. This IM‐BECT process is described and demonstrated for two sites, postmastectomy chest wall and temple. Dose plans, intensity distributions, fabricated intensity modulators, and quality assurance results are presented.ResultsIM‐BECT plans showed improved D90‐10 over BECT plans, 6.4% versus 7.3% and 8.4% versus 11.0% for the postmastectomy chest wall and temple, respectively. Their intensity modulators utilized 61 (single diameter) and 246 (five diameters) tungsten pins, respectively. Dose comparisons for clinical quality assurance showed that for doses greater than 10%, measured agreed with calculated dose within 3% or 0.3 cm distance‐to‐agreement (DTA) for 99.9% and 100% of points, respectively.ConclusionThese results demonstrated the feasibility of translating IM‐BECT to the clinic using the techniques presented for treatment planning, intensity modulator design and fabrication, and quality assurance processes.

Useful island block geometries of a passive intensity modulator used for intensity-modulated bolus electron conformal therapy.

PurposeThis project determined the range of island block geometric configurations useful for the clinical utilization of intensity‐modulated bolus electron conformal therapy (IM‐BECT).MethodsMultiple half‐beam island block geometries were studied for seven electron energies 7‐20 MeV at 100 and 103 cm source‐to‐surface distance (SSD). We studied relative fluence distributions at 0.5 cm and 2.0 cm depths in water, resulting in 28 unique beam conditions. For each beam condition, we studied intensity reduction factor (IRF) values of 0.70, 0.75, 0.80, 0.85, 0.90, and 0.95, and hexagonal packing separations for the island blocks of 0.50, 0.75, 1.00, 1.25, and 1.50 cm, that is, 30 unique IM configurations and 840 unique beam‐IM combinations. A combination was deemed acceptable if the average intensity downstream of the intensity modulator agreed within 2% of that intended and the variation in fluence was less than ±2%.ResultsFor 100 cm SSD, and for 0.5 cm depth, results showed that beam energies above 13 MeV did not exhibit sufficient scatter to produce clinically acceptable fluence (intensity) distributions for all IRF values (0.70–0.95). In particular, 20 MeV fluence distributions were unacceptable for any values, and acceptable 16 MeV fluence distributions were limited to a minimum IRF of 0.85. For the 2.0 cm depth, beam energies up to and including 20 MeV had acceptable fluence distributions. For 103 cm SSD and for 0.5 cm and 2.0 cm depths, results showed that all beam energies (7–20 MeV) had clinically acceptable fluence distributions for all IRF values (0.70–0.95). In general, the more clinically likely 103 cm SSD had acceptable fluence distributions with larger separations (r), which allow larger block diameters.ConclusionThe geometric operating range of island block separations and IRF values (block diameters) producing clinically appropriate IM electron beams has been determined.

Introduction to passive electron intensity modulation.

This work introduces a new technology for electron intensity modulation, which uses small area island blocks within the collimating aperture and small area island apertures in the collimating insert. Due to multiple Coulomb scattering, electrons contribute dose under island blocks and lateral to island apertures. By selecting appropriate lateral positions and diameters of a set of island blocks and island apertures, for example, a hexagonal grid with variable diameter circular island blocks, intensity modulated beams can be produced for appropriate air gaps between the intensity modulator (position of collimating insert) and the patient. Such a passive radiotherapy intensity modulator for electrons (PRIME) is analogous to using physical attenuators (metal compensators) for intensity modulated x‐ray therapy (IMXT). For hexagonal spacing, the relationship between block (aperture) separation (r) and diameter (d) and the local intensity reduction factor (IRF) is discussed. The PRIME principle is illustrated using pencil beam calculations for select beam geometries in water with half beams modulated by 70%–95% and for one head and neck field of a patient treated with bolus electron conformal therapy. Proof of principle is further illustrated by showing agreement between measurement and calculation for a prototype PRIME. Potential utilization of PRIME for bolus electron conformal therapy, segmented‐field electron conformal therapy, modulated electron radiation therapy, and variable surface geometries is discussed. Further research and development of technology for the various applications is discussed. In summary, this paper introduces a practical, new technology for electron intensity modulation in the clinic, demonstrates proof of principle, discusses potential clinical applications, and suggests areas of further research and development.

SU‐F‐T‐87: Comparison of Advanced Radiotherapy Techniques for Post‐ Mastectomy Breast Cancer Patients

Purpose:To determine if bolus electron conformal therapy (Bolus‐ECT) combined with intensity modulated x‐ray therapy (IMXT) and flattening filter free volumetric modulated arc therapy (FFF‐VMAT (6x and 10x)) can maintain equal or better dose coverage than standard volumetric modulated arc therapy (Std‐VMAT) while reducing doses to organs at risk (OARs).Methods:Bolus‐ECT with IMXT, FFF‐VMAT, and Std‐VMAT treatment plans were produced for ten post‐mastectomy radiotherapy (PMRT) patients previously treated at our clinic. The treatment plans were created on commercially available treatment planning system (TPS) and all completed treatment plans were reviewed and approved by a radiation oncologist. The plans were evaluated based on planning target volume (PTV) coverage, tumor control probability (TCP), dose homogeneity index (DHI), conformity index (CI), and dose to organs at risk (OAR).Results:All techniques produced clinically acceptable PMRT plans. Overall, Bolus‐ECT with IMXT exhibited higher maximum dose compared to all VMAT techniques. Bolus‐ECT with IMXT and FFF‐VMAT10x had slightly improved TCP over FFF‐VMAT6x and Std‐VMAT. However, all VMAT techniques showed improved CI and DHI over Bolus‐ECT with IMXT. All techniques showed very similar mean lung dose. Bolus‐ECT with IMXT exhibited a reduced mean heart dose over Std‐VMAT. Both FFF‐VMAT techniques had higher mean heart dose compared to Std‐VMAT. In addition, Bolus‐ECT with IMXT was able to reduce mean dose to the contralateral breast compared to Std‐VMAT and both FFF‐VMAT techniques had comparable but slightly reduced dose compared to Std‐VMAT.Conclusion:This work has shown that Bolus‐ECT with IMXT produces clinically acceptable plans while reducing OAR doses. Both FFF‐VMAT techniques are comparable to Std‐VMAT with slight improvements. Even though all VMAT techniques produce more homogenous and conformal dose distributions, Bolus‐ECT with IMXT is a viable option for treating post‐mastectomy patients possibly leading to reduced risks of normal tissue complications.

Evaluation of the Eclipse eMC algorithm for bolus electron conformal therapy using a standard verification dataset

The purpose of this study was to evaluate the accuracy and calculation speed of electron dose distributions calculated by the Eclipse electron Monte Carlo (eMC) algorithm for use with bolus electron conformal therapy (ECT). The recent commercial availability of bolus ECT technology requires further validation of the eMC dose calculation algorithm. eMC‐calculated electron dose distributions for bolus ECT have been compared to previously measured TLD‐dose points throughout patient‐based cylindrical phantoms (retromolar trigone and nose), whose axial cross sections were based on the mid‐PTV (planning treatment volume) CT anatomy. The phantoms consisted of SR4 muscle substitute, SR4 bone substitute, and air. The treatment plans were imported into the Eclipse treatment planning system, and electron dose distributions calculated using 1% and <0.2% statistical uncertainties. The accuracy of the dose calculations using moderate smoothing and no smoothing were evaluated. Dose differences (eMC‐calculated less measured dose) were evaluated in terms of absolute dose difference, where 100% equals the given dose, as well as distance to agreement (DTA). Dose calculations were also evaluated for calculation speed. Results from the eMC for the retromolar trigone phantom using 1% statistical uncertainty without smoothing showed calculated dose at 89% (41/46) of the measured TLD‐dose points was within 3% dose difference or 3 mm DTA of the measured value. The average dose difference was −0.21%, and the net standard deviation was 2.32%. Differences as large as 3.7% occurred immediately distal to the mandible bone. Results for the nose phantom, using 1% statistical uncertainty without smoothing, showed calculated dose at 93% (53/57) of the measured TLD‐dose points within 3% dose difference or 3 mm DTA. The average dose difference was 1.08%, and the net standard deviation was 3.17%. Differences as large as 10% occurred lateral to the nasal air cavities. Including smoothing had insignificant effects on the accuracy of the retromolar trigone phantom calculations, but reduced the accuracy of the nose phantom calculations in the high‐gradient dose areas. Dose calculation times with 1% statistical uncertainty for the retromolar trigone and nose treatment plans were 30 s and 24 s, respectively, using 16 processors (Intel Xeon E5‐2690, 2.9 GHz) on a framework agent server (FAS). In comparison, the eMC was significantly more accurate than the pencil beam algorithm (PBA). The eMC has comparable accuracy to the pencil beam redefinition algorithm (PBRA) used for bolus ECT planning and has acceptably low dose calculation times. The eMC accuracy decreased when smoothing was used in high‐gradient dose regions. The eMC accuracy was consistent with that previously reported for accuracy of the eMC electron dose algorithm and shows that the algorithm is suitable for clinical implementation of bolus ECT.PACS number(s): 87.55.kd

Comparison of bolus electron conformal therapy plans to traditional electron and proton therapy to treat melanoma in the medial canthus

Electrons can be used to treat superficial skin lesions given their finite depth; however, treating irregular surfaces with varying depths may be challenging. Traditionally a machinist can design customized devices to improve the dose coverage and shield nearby organs; however, the precision of handmade devices can be worrisome when treating nearby critical structures. Bolus electron conformal therapy (BolusECT) was introduced in the 1990s using an automated process of computer numerical control (CNC) milling machines to form a variable-thickness bolus from machinable wax using coordinate specification from computed tomography (CT) scans.1,2 The increased speed of CNC milling technology and advent of 3-dimensional (3D) printing have expanded the number of options for treating superficial lesions.1–4 Proton therapy can also treat variable depth lesions, but is not widely available. We present 3 treatment options available at our institution to treat a challenging case involving the medial canthus: handmade uniform bolus with skin collimator, automated variable-thickness bolus with skin collimator, and proton therapy.

SU-E-T-356: Accuracy of Eclipse Electron Macro Monte Carlo Dose Algorithm for Use in Bolus Electron Conformal Therapy

Purpose: To evaluate the accuracy of electron dose distribution calculated by the Varian Eclipse electron Monte Carlo (eMC) algorithm for use with recent commercially available bolus electron conformal therapy (ECT). Methods: eMC-calculated electron dose distributions for bolus ECT have been compared to those previously measured for cylindrical phantoms (retromolar trigone and nose), whose axial cross sections were based on the mid-PTV CT anatomy for each site. The phantoms consisted of SR4 muscle substitute, SR4 bone substitute, and air. The bolus ECT treatment plans were imported into the Eclipse treatment planning system and calculated using the maximum allowable histories (2×10^9), resulting in a statistical error of <0.2%. Smoothing was not used for these calculations. Differences between eMC-calculated and measured dose distributions were evaluated in terms of absolute dose difference as well as distance to agreement (DTA). Results: Results from the eMC for the retromolar trigone phantom showed 89% (41/46) of dose points within 3% dose difference or 3 mm DTA. There was an average dose difference of −0.12% with a standard deviation of 2.56%. Results for the nose phantom showed 95% (54/57) of dose points within 3% dose difference or 3 mm DTA. There was an average dose difference of 1.12% with a standard deviation of 3.03%. Dose calculation times for the retromolar trigone and nose treatment plans were 15 min and 22 min, respectively, using 16 processors (Intel Xeon E5-2690, 2.9 GHz) on a Varian Eclipse framework agent server (FAS). Results of this study were consistent with those previously reported for accuracy of the eMC electron dose algorithm and for the .decimal, Inc. pencil beam redefinition algorithm used to plan the bolus. Conclusion: These results show that the accuracy of the Eclipse eMC algorithm is suitable for clinical implementation of bolus ECT.

Investigation of the clinical potential of scattering foil free electron beams

Electron beam therapy has been an important radiation therapy modality for many decades. Studies have been conducted recently for more efficient and advanced delivery of electron beam radiation therapy. X-ray contamination is a common problem that exists with all of the advanced electron beam therapy techniques such as Bolus Electron conformal therapy, segmented electron conformal therapy, and modulated electron arc therapy. X-ray contamination could add some limitations to the advancement and clinical utility of those electron modalities. It was previously shown in the literature that the scattering foil is one of the major accelerator parts contributing to the generation of bremsstrahlung photons. Thus, in this work we investigate the dosimetric characteristics of scattering foil free (SFF) electron beams and the feasibility of using those beams for breast cancer boosts. The SFF electron beams were modeled and simulated using the Monte Carlo method. CT scans of six previously treated breast patients were used for the treatment plan generation utilizing our in-house Monte Carlo-based treatment planning system. Electron boost plans with conventional beams and the SFF beams were generated, respectively, for all patients. A significant reduction of the photon component was observed with the removal of the primary scattering foil for beam energies higher than 12 MeV. Flatness was greatly affected but the difference in flatness between conventional and SFF beams was much reduced for small cone sizes, which were often used clinically for breast boosts. It was found that the SFF electron beams could deliver high-quality dose distributions as conventional electron beams for boost treatments of the breast with an added advantage of a further reduced dose to the lung and the heart.

Evaluation of bolus electron conformal therapy compared with conventional techniques for the treatment of left chest wall postmastectomy in patients with breast cancer

Postmastectomy radiation (PMRT) lowers local-regional recurrence risk and improves survival in selected patients with breast cancer. The chest wall and lower axilla are technically challenging areas to treat with homogenous doses and normal tissue sparing. This study compares several techniques for PMRT to provide data to guide selection of optimal treatment techniques. Twenty-five consecutive left-sided patients treated postmastectomy were contoured using Radiation Therapy Oncology Group (RTOG) atlas guidelines then planned using 4 different PMRT techniques: opposed tangents with wedges (3-dimensional [3D] wedges), opposed tangents with field-in-field (FiF) modulation, 8-field intensity modulation radiotherapy (IMRT), and custom bolus electron conformal therapy (BolusECT, .decimal, Inc., Sanford, FL). Required planning target volume (PTV) coverage was held constant, and then dose homogeneity and normal tissue dose parameters were compared among the 4 techniques. BolusECT achieved clincally acceptable PTV coverage for 22 out of 25 cases. Compared with either tangential technique, IMRT and BolusECT provided the lowest heart V25 doses (3.3% ± 0.9% and 6.6% ± 3.2%, respectively with p < 0.0001). FiF had the lowest mean total lung dose (7.3 ± 1.1Gy, with p = 0.0013), IMRT had the lowest total lung V20 (10.3% ± 1.6%, p < 0.0001), and BolusECT had the lowest mean heart dose (7.3 ± 2.0Gy, p = 0.0002). IMRT provided the optimal dose homogeneity and normal tissue sparing compared with all other techniques for the cases in which BolusECT could not achieve acceptable PTV coverage. IMRT generally exposes contralateral breast and lung to slightly higher doses. Optimal PMRT technique depends upon patient anatomy. Patients whose maximal target volume depth is about 5.7cm or less can be treated with BolusECT-assisted 12 or 15MeV electron beams. At these energies, BolusECT has comparable dose-volume statistics as IMRT and lower heart V25 than opposed tangential beams. Patients with larger depths are best treated with IMRT, which provides significant advantages in both dose homogeneity and normal tissue sparing compared with all other techniques.

Accuracy of pencil‐beam redefinition algorithm dose calculations in patient‐like cylindrical phantoms for bolus electron conformal therapy

The purpose of this study was to document the improved accuracy of the pencil beam redefinition algorithm (PBRA) compared to the pencil beam algorithm (PBA) for bolus electron conformal therapy using cylindrical patient phantoms based on patient computed tomography (CT) scans of retromolar trigone and nose cancer. PBRA and PBA electron dose calculations were compared with measured dose in retromolar trigone and nose phantoms both with and without bolus. For the bolus treatment plans, a radiation oncologist outlined a planning target volume (PTV) on the central axis slice of the CT scan for each phantom. A bolus was designed using the planning.decimal(®) (p.d) software (.decimal, Inc., Sanford, FL) to conform the 90% dose line to the distal surface of the PTV. Dose measurements were taken with thermoluminescent dosimeters placed into predrilled holes. The Pinnacle(3) (Philips Healthcare, Andover, MD) treatment planning system was used to calculate PBA dose distributions. The PBRA dose distributions were calculated with an in-house C++ program. In order to accurately account for the phantom materials a table correlating CT number to relative electron stopping and scattering powers was compiled and used for both PBA and PBRA dose calculations. Accuracy was determined by comparing differences in measured and calculated dose, as well as distance to agreement for each measurement point. The measured doses had an average precision of 0.9%. For the retromolar trigone phantom, the PBRA dose calculations had an average ± 1σ dose difference (calculated - measured) of -0.65% ± 1.62% without the bolus and -0.20% ± 1.54% with the bolus. The PBA dose calculation had an average dose difference of 0.19% ± 3.27% without the bolus and -0.05% ± 3.14% with the bolus. For the nose phantom, the PBRA dose calculations had an average dose difference of 0.50% ± 3.06% without bolus and -0.18% ± 1.22% with the bolus. The PBA dose calculations had an average dose difference of 0.65% ± 6.21% without bolus and 1.75% ± 5.94% with the bolus. From a clinical perspective an agreement of 5% or better between planned (calculated) and delivered (measured) dose is desired. Statistically, this was true for 99% (± 2σ) of the dose points for three of the four cases for the PBRA dose calculations, the exception being the nose without bolus for which this was true for 89% (± 1.6σ) of the dose points. For the retromolar trigone, with and without bolus, the PBA showed agreement of 5% or better for approximately 86% (± 1.5σ) of the dose points. For the nose, with and without bolus, the PBA showed agreement of 5% or better for only approximately 58% (± 0.8σ) of the dose points. The measured data, whose high precision makes them useful for evaluation of the accuracy of electron dose algorithms, will be made publicly available. Based on the spread in dose differences, the PBRA has at least twice the accuracy of the PBA. From a clinical perspective the PBRA accuracy is acceptable in the retromolar trigone and nose for electron therapy with and without bolus.

Delivery confirmation of bolus electron conformal therapy combined with intensity modulated x-ray therapy

The purpose of this study was to demonstrate that a bolus electron conformal therapy (ECT) dose plan and a mixed beam plan, composed of an intensity modulated x-ray therapy (IMXT) dose plan optimized on top of the bolus ECT plan, can be accurately delivered. Calculated dose distributions were compared with measured dose distributions for parotid and chest wall (CW) bolus ECT and mixed beam plans, each simulated in a cylindrical polystyrene phantom that allowed film dose measurements. Bolus ECT plans were created for both parotid and CW PTVs (planning target volumes) using 20 and 16 MeV beams, respectively, whose 90% dose surface conformed to the PTV. Mixed beam plans consisted of an IMXT dose plan optimized on top of the bolus ECT dose plan. The bolus ECT, IMXT, and mixed beam dose distributions were measured using radiographic films in five transverse and one sagittal planes for a total of 36 measurement conditions. Corrections for film dose response, effects of edge-on photon irradiation, and effects of irregular phantom optical properties on the Cerenkov component of the film signal resulted in high precision measurements. Data set consistency was verified by agreement of depth dose at the intersections of the sagittal plane with the five measured transverse planes. For these same depth doses, results for the mixed beam plan agreed with the sum of the individual depth doses for the bolus ECT and IMXT plans. The six mean measured planar dose distributions were compared with those calculated by the treatment planning system for all modalities. Dose agreement was assessed using the 4% dose difference and 0.2 cm distance to agreement. For the combined high-dose region and low-dose region, pass rates for the parotid and CW plans were 98.7% and 96.2%, respectively, for the bolus ECT plans and 97.9% and 97.4%, respectively, for the mixed beam plans. For the high-dose gradient region, pass rates for the parotid and CW plans were 93.1% and 94.62%, respectively, for the bolus ECT plans and 89.2% and 95.1%, respectively, for the mixed beam plans. For all regions, pass rates for the parotid and CW plans were 98.8% and 97.3%, respectively, for the bolus ECT plans and 97.5% and 95.9%, respectively, for the mixed beam plans. For the IMXT component of the mixed beam plans, pass rates for the parotid and CW plans were 93.7% and 95.8%. Bolus ECT and mixed beam therapy dose delivery to the phantom were more accurate than IMXT delivery, adding confidence to the use of planning, fabrication, and delivery for bolus ECT tools either alone or as part of mixed beam therapy. The methodology reported in this work could serve as a basis for future standardization of the commissioning of bolus ECT or mixed beam therapy. When applying this technology to patients, it is recommended that an electron dose algorithm more accurate than the pencil beam algorithm, e.g., a Monte Carlo algorithm or analytical transport such as the pencil beam redefinition algorithm, be used for planning to ensure the desired accuracy.

Is wax equivalent to tissue in electron conformal therapy planning? A Monte Carlo study of material approximation introduced dose difference

With CT‐based Monte Carlo (MC) dose calculations, material composition is often assigned based on the standard Hounsfield unit ranges. This is known as the density threshold method. In bolus electron conformal therapy (BolusECT), the bolus material, machineable wax, would be assigned as soft tissue and the electron density is assumed equivalent to soft tissue based on its Hounsfield unit. This study investigates the dose errors introduced by this material assignment. BEAMnrc was used to simulate electron beams from a Trilogy accelerator. SPRRZnrc was used to calculate stopping power ratios (SPR) of tissue to wax, SPRwaxtissue, and tissue to water, SPRwatertissue, for 6, 9, 12, 15, and 18 MeV electron beams, of which 12 and 15 MeV beams are the most commonly used energies in BolusECT. DOSXYZnrc was applied in dose distribution calculations in a tissue phantom with either flat wax slabs of various thicknesses or a wedge‐shaped bolus on top. Dose distribution for two clinical cases, a chest wall and a head and neck, were compared with the bolus material treated as wax or tissue. The SPRwaxtissue values for 12 and 15 MeV beams are between 0.935 and 0.945, while the SPRwatertissue values are between 0.990 and 0.991. For a 12 MeV beam, the dose in tissue immediately under the bolus is overestimated by 2.5% for a 3 cm bolus thickness if the wax bolus is treated as tissue. For 15 MeV beams, the error is 1.4%. However, in both clinical cases the differences in the PTV DVH is negligible. Due to stopping power differences, dose differences of up to 2.5% are observed in MC simulations if the bolus material is misassigned as tissue in BolusECT dose calculations. However, for boluses thinner than 2 cm that are more likely encountered in practice, the error is within clinical tolerance.PACS numbers: 87.55.km, 87.56.ng

Bolus electron conformal therapy for the treatment of recurrent inflammatory breast cancer: a case report

The treatment of locoregionally recurrent breast cancer in patients who have previously undergone radiation therapy is challenging. Special techniques are often required that both eradicate the disease and minimize the risks of retreatment. We report the case of a patient with an early-stage left breast cancer who developed inflammatory-type recurrence requiring re-irradiation of the chest wall using bolus electron conformal therapy with image-guided treatment delivery. The patient was a 51-year-old woman who had undergone lumpectomy, axillary lymph node dissection, and adjuvant whole-breast radiation therapy for a stage I left breast cancer in June 1998. In March 2009, she presented at our institution with biopsy-proven recurrent inflammatory carcinoma and was aggressively treated with multi-agent chemotherapy followed by mastectomy that left a positive surgical margin. Given the patient's prior irradiation and irregular chest wall anatomy, bolus electron conformal therapy was used to treat her chest wall and draining lymphatics while sparing the underlying soft tissue. The patient still had no evidence of disease 21 months after treatment. Our results indicate that bolus electron conformal therapy is an accessible, effective radiation treatment approach for recurrent breast cancer in patients with irregular chest wall anatomy as a result of surgery. This approach may complement standard techniques used to reduce locoregional recurrence in the postmastectomy setting.

Evaluation of Bolus Electron Conformal Therapy in Post Mastectomy Chestwall Irradiation Including Simultaneous Irradiation of the Internal Mammary Nodal Volume

To compare bolus electron conformal therapy (ECT) with tangent photon beams (3D) and photon-based intensity modulated radiation therapy (IMRT) as a treatment modality in post mastectomy chest wall irradiation (PMRT) including the internal mammary nodes.

SU‐E‐T‐686: Monte Carlo Study of Material Approximation Introduced Dose Difference in Electron Conformal Therapy Planning

Purpose: When Monte Carlo (MC) is used in dose calculation with CT data, materials are often assigned based on the local Houndsfield units if the density threshold method is applied. When bolus is used to compensate for body surface irregularities and/or to modulate the therapeutic depth in bolus electron conformal therapy (bolusECT), the bolus material, which is machineable wax, is treated as soft tissue, since the density of bolus overlaps with soft tissue. This study investigates the dose errors introduced by this material misassignment.Methods: BEAMnrc was used to simulate electron beams from a Trilogy accelerator. SPRRZnrc was used to calculate stopping power ratios (SPR) of tissue to wax, SPR(tissue/wax), and tissue to water, SPR(tissue/water), for 12 and 15 MeV electron beams which are the most commonly used energies in bolusECT. DOSXYZnrc was applied in dose distribution calculation in tissue with wax slabs of various thicknesses and a wedge‐bolus on top respectively. The MC dose distributions calculated using different bolus materials were compared.Results: With the same density, the SPR(tissue/wax) for 12 and 15 MeV beams ranges from 0.934 to 0.944, while the SPR(tissue/water) values are between 0.990 and 0.991, indicating higher stopping powers of wax. The 12 MeV beam range is 6.5 cm in wax and 6.7 cm in tissue. For a 12 MeV beam, the dose in tissue immediate under the bolus is overestimated by about 2.5% of maximum dose at 3 cm bolus thickness if wax bolus is treated as tissue. For 15 MeV beams, the error is about 1.4% Conclusions: Due to stopping power differences, dose differences are observed if the bolus material is misassigned in bolusECT dose calculations. Large dose overestimations occur at thick bolus region. However, the error is mostly within clinical tolerance, especially for thin bolus regions.

SU-E-T-750: Interfacing the Pencil Beam Redefinition Algorithm with a Commercial Treatment Planning System

Purpose: The purpose of this work was to interface the pencil beam redefinition algorithm (PBRA) dose engine with the Pinnacle treatment planning system (TPS) for use in bolus electron conformal therapy (ECT) dose planning. Currently Pinnacle TPS uses the Hogstrom pencil beam algorithm (PBA). Methods: A C program that calculates electron dose using the PBRA has been developed. This program inputs the Pinnacle files for the CT data, the user‐generated plan data, and the PBA beam data. The 3D calculated dose outputs a binary file that is uploaded into Pinnacle. This method allows the Pinnacle TPS to be used for plan analysis, e.g. dose display and dose volume histograms of the planning target volume and normal tissues Results: To date our program has been validated by demonstrating, for each beam energy, that agreements of PBRA calculated with measured dose distributions in a water phantom, as a function of field size and SSD, are consistent with previously published results. Also, the accuracy of the PBRA in heterogeneous phantoms agrees with previously published geometric phantom measurements and with patient based phantoms measurements (subject of companion paper). The PBRA is advantageous because it requires no additional beam commissioning data and its accuracy in patients is comparable to Monte Carlo. Calculation times in water for 4×4cm and 20×20cm fields for the 9 MeV beam are 5sec and 120sec respectively on a 2.93Ghz processor using a non‐optimized code. Using the 20 MeV beam these times are 14sec and 290sec, respectively. Conclusion: Our stand alone C program demonstrates a reasonable way to perform patient dose calculations using the PBRA with the Pinnacle TPS. Use of the PBRA might be beneficial for some sites when planning bolus ECT. This work was supported in part by a research agreement with .decimal Inc.