Energy- and intensity-modulated electron beams for radiotherapy

129

124

This is a proof‐of‐concept study demonstrating the capacity for modulated electron radiation therapy (MERT) dose distributions using 3D printed bolus. Previous reports have involved bolus design using an electron pencil beam model and fabrication using a milling machine. In this study, an in‐house algorithm is presented that optimizes the dose distribution with regard to dose coverage, conformity, and homogeneity within the planning target volume (PTV). The algorithm takes advantage of a commercial electron Monte Carlo dose calculation and uses the calculated result as input. Distances along ray lines from the distal side of 90% isodose line to distal surface of the PTV are used to estimate the bolus thickness. Inhomogeneities within the calculation volume are accounted for using the coefficient of equivalent thickness method. Several regional modulation operators are applied to improve the dose coverage and uniformity. The process is iterated (usually twice) until an acceptable MERT plan is realized, and the final bolus is printed using solid polylactic acid. The method is evaluated with regular geometric phantoms, anthropomorphic phantoms, and a clinical rhabdomyosarcoma pediatric case. In all cases the dose conformity are improved compared to that with uniform bolus. For geometric phantoms with air or bone inhomogeneities, the dose homogeneity is markedly improved. The actual printed boluses conform well to the surface of complex anthropomorphic phantoms. The correspondence of the dose distribution between the calculated synthetic bolus and the actual manufactured bolus is shown. For the rhabdomyosarcoma patient, the MERT plan yields a reduction of mean dose by 38.2% in left kidney relative to uniform bolus. MERT using 3D printed bolus appears to be a practical, low‐cost approach to generating optimized bolus for electron therapy. The method is effective in improving conformity of the prescription isodose surface and in sparing immediately adjacent normal tissues.PACS number: 81.40.Wx

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Design and production of 3D printed bolus for electron radiation therapy

Moran

Robar

2014

129

124

“…MERT utilizes multiple intensity modulated electron fields of varying angle and energy 35–39 . Ma et al reports MERT being delivered using an eMLC 39 ; however, PRIME might be able to be used for MERT.…”

Section: Discussionmentioning

confidence: 99%

Introduction to passive electron intensity modulation

Hogstrom

Carver

Chambers

et al. 2017

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, e.g. 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.

“…Modulation of the electron fields may improve the dose conformity 3 and has been extensively investigated using the photon multi‐leaf collimator (MLC) 4, 5, 6 or a dedicated electron MLC 7, 8. Nevertheless, the use of static electron fields remains common 9 and collimation is typically performed using aperture cut‐outs placed in the electron applicator at a small distance from the patient's skin 10…”

Section: Introductionmentioning

confidence: 99%

Production of patient‐specific electron beam aperture cut‐outs using a low‐cost, multi‐purpose 3D printer

Michiels

Mangelschots

Roover

et al. 2018

Electron beam collimators for non‐standard field sizes and shapes are typically fabricated using Styrofoam molds to cast the aperture cut‐out. These molds are often produced using a dedicated foam cutter, which may be expensive and only serves a single purpose. An increasing number of radiotherapy departments, however, has a 3D printer on‐site, to create a wide range of custom‐made treatment auxiliaries, such as bolus and dosimetry phantoms. The 3D printer can also be used to produce patient‐specific aperture cut‐outs, as elaborated in this note. Open‐source programming language was used to automatically generate the mold's shape in a generic digital file format readable by 3D printer software. The geometric mold model has the patient's identification number integrated and is to be mounted on a uniquely fitting, reusable positioning device, which can be 3D printed as well. This assembly likewise fits uniquely onto the applicator tray, ensuring correct and error‐free alignment of the mold during casting of the aperture. For dosimetric verification, two aperture cut‐outs were cast, one using a conventionally cut Styrofoam mold and one using a 3D printed mold. Using these cut‐outs, the clinical plan was delivered onto a phantom, for which the transversal dose distributions were measured at 2 cm depth using radiochromic film and compared using gamma‐index analysis. An agreement score of 99.9% between the measured 2D dose distributions was found in the (10%–80%) dose region, using 1% (local) dose‐difference and 1.0 mm distance‐to‐agreement acceptance criteria. The workflow using 3D printing has been clinically implemented and is in routine use at the author's institute for all patient‐specific electron beam aperture cut‐outs. It allows for a standardized, cost‐effective, and operator‐friendly workflow without the need for dedicated equipment. In addition, it offers possibilities to increase safety and quality of the process including patient identification and methods for accurate mold alignment.