Dosimetry of a prototype retractable eMLC for fixed-beam electron therapy

Abstract

An electron multileaf collimator (eMLC) has been designed that is unique in that it retracts to 37 cm from the isocenter [63-cm source-to-collimator distance (SCD)] and can be deployed to distances of 20 and 10 cm from the isocenter (80 and 90 cm SCD, respectively). It is expected to be capable of arc therapy at 63 cm SCD; isocentric, fixed-beam therapy at 80 cm SCD; and source-to-surface distance (SSD), fixed-beam therapy at 90 cm SCD. In all positions, its leaves could be used for unmodulated or intensity-modulated therapy. Our goal in the present work is to describe the general characteristics of the eMLC and to demonstrate that its leakage characteristics and dosimetry are adequate for SSD, fixed-beam therapy as an alternative to Cerrobend cutouts with applicators once the prototype's leaves are motorized. Our eMLC data showed interleaf electron leakage at 15 MeV to be less than 0.1% based on a 0.0025 cm manufacturing tolerance, and lateral electron leakage at 5 and 15 MeV to be less than 2%. X-ray leakage through the leaves was 1.6% at 15 MeV. Our data showed that beam penumbra was independent of direction and leaf position. The dosimetric properties of square fields formed by the eMLC were very consistent with those formed by Cerrobend inserts in the 20 x 20 cm2 applicator. Output factors exhibited similar field-size dependence. Airgap factors exhibited almost identical field-size dependence at two SSDs (105 and 110 cm), consistent with the common assumption that airgap factors are applicator independent. Percent depth-dose curves were similar, but showed variations up to 3% in the buildup region. The pencil-beam algorithm (PBA) fit measured data from the eMLC and applicator-cutout systems equally well, and the resulting two-dimensional (2-D) dose distributions, as predicted by the PBA, agreed well at common airgap distance. Simulating patient setups for breast and head and neck treatments showed that almost all fields could be treated using similar SSDs as when using applicators, although head and neck treatments require placing the patient's head on a head-holder treatment table extension. The results of this work confirmed our design goals and support the potential use of the eMLC design in the clinical setting. The eMLC should allow the same treatments as are typically delivered with the electron applicator-cutout system currently used for fixed-beam therapy.

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

“…Improved plan quality can be achieved using modulated electron radiation therapy (MERT) 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 . MERT can be accomplished by sophisticated techniques based on modulation of multiple electron energies and electron beam weights or intensities, which is further classified into two categories: segmented‐field electron conformal therapy (ECT) and intensity‐modulated electron therapy (IMET).…”

Introduction to passive electron intensity modulation

“…Some envisioned intensity modulation using scanned electron beams 9 , which requires helium in the treatment head to reduce multiple Coulomb scattering (MCS), as air produces spot beams that are too broad 10 . This led to work on electron MLC (eMLC) designs by Hogstrom et al 11 , Gauer et al 12,13 , and others, which resulted in a commercially available eMLC provided by a third party (Euromechanics, Schwarzenbruck, Germany; see http://english.euromechanics.de/electron-multileaf-collimator-emlc/). However, this technology has not resulted in widely available electron IM, possibly due to the high cost of an add-on eMLC, low fraction of electron patients, lack of its ability to deploy/retract, need for its integration into commercially available treatment planning systems (TPS), and other reasons.…”

Image‐guided bolus electron conformal therapy – a case study

“…The bolus is patient‐specific and is designed for shaping the distal 90% dose surface to conform and contain the planning target volume (PTV) while delivering minimal dose to adjacent, underlying critical structures and normal tissues. The delivery of ECT can be in principle achieved through a variety of approaches such as the use of a retractable electron multileaf collimator (eMLC), ( 1 ) modulation of energy and intensity of electron beams ( 2 ) and, most recently, use of existing photon‐based MLCs for electron fluence modulation. ( 3 ) However, these approaches are far from being mainstream for ECT delivery due to the fact that they exist primarily as prototypes in academic institutions or because commercially available treatment planning systems (TPS) do not support them.…”