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Electron Conformal Radiotherapy Using Bolus and Intensity Modulation

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Electron Conformal Radiotherapy Using Bolus and Intensity Modulation Int. J. Radiation Oncology Biol. Phys., Vol. 53, No. 4, pp. 1023–1037, 2002 Copyright © 2002 Elsevier Science Inc. Printed in the USA. All rights reserved 0360-3016/02/$–see front matter PII S0360-3016(02)02811-0 PHYSICS CONTRIBUTION ELECTRON CONFORMAL RADIOTHERAPY USING BOLUS AND INTENSITY MODULATION † RAJAT J. KUDCHADKER,PH.D.,* KENNETH R. HOGSTROM,PH.D.,* ADAM S. GARDEN, M.D., † MARSHA D. MCNEESE, M.D., ROBERT A. BOYD,PH.D.,* AND JOHN A. ANTOLAK,PH.D.* Departments of *Radiation Physics and †Radiation Oncology, The University of Texas M. D. Anderson Cancer Center, Houston, TX Purpose: Conformal electron beam therapy can be delivered using shaped bolus, which varies the penetration of the electrons across the incident beam so that the 90% isodose surface conforms to the distal surface of the planning target volume (PTV). Previous use of this modality has shown that the irregular proximal surface of the bolus causes the dose heterogeneity in the PTV to increase from 10%, the typical dose spread of a flat-water surface to approximately 20%. The present work evaluates the ability to restore dose homogeneity by varying the incident electron intensity. Methods and Materials: Three patients, one each with chest wall, thorax, and head-and-neck cancer, were planned using electron conformal therapy with bolus, with and without intensity modulation. Resulting dose distributions and dose–volume histograms were compared with non-intensity-modulated bolus plans. ⌬ Results: In all cases, the D90%؊10% for the PTV was reduced; for example, for the head-and-neck case, the ⌬ D90%؊10% for the PTV was reduced from 14.9% to 9.2%. This reduction in dose spread is a direct result of intensity modulation. Conclusions: The results showed that intensity-modulated electron beams could significantly improve the dose homogeneity in the PTV for patients treated with electron conformal therapy using shaped bolus. © 2002 Elsevier Science Inc. Electron conformal therapy, Electron bolus, Intensity modulation. INTRODUCTION The American Association of Physicists in Medicine Task Group 25 recommends that electron beam parameters, Role of bolus in electron therapy To date, conformal radiotherapy has been primarily pho- such as energy, field size, and bolus, be selected so that the ton conformal therapy, and it has been achieved by the use target volume is encompassed within 90% (or any other of multiple shaped fields, with and without intensity mod- appropriate minimum dose) of the prescribed dose (2). At ulation. Photon conformal therapy can be used in treating M. D. Anderson Cancer Center, our goal is to encompass tumors located throughout the body. However, because of the planning target volume (PTV) within 90% of the given the finite range of electrons in tissue, it may be advanta- dose. Given dose is the maximum central axis dose deliv- geous to treat superficial tumors with electron conformal ered in a water phantom at the same source-to-surface radiotherapy (ECRT), which is usually achievable using a distance (SSD) and with the smallest rectangular field that single field. Because of the physical nature of the electron encompasses the irregular field used on the patient (3). In beam, its use for conformal therapy must be considered ECRT, the goal is to select the optimal conditions to con- differently from that of the photon beam. Electron beams of form the 90% dose contour to the distal border of the target energies ranging from 6 to 25 MeV are limited to treating volume, while simultaneously maintaining as uniform dose target volumes within approximately 7.5 cm of the patient within the PTV and as little dose to nearby critical organs surface. Hence, ECRT seems best suited to treating super- and normal tissue as possible. In an actual patient, tumor ficial target volumes in head-and-neck and chest wall sites underdose, normal tissue overdose, or nonuniform dose in in which electron beams can be modified to deliver a uni- the target volume can occur as a result of internal hetero- form dose to the target volume, with a sharp dose falloff geneities, an irregular patient surface, or a highly variable outside the volume. Although ECRT can also be achieved depth of the distal surface of the target volume (4). Hence, with moving electron beams, this paper will be limited to an electron beam with a spatially uniform energy and in- fixed-beam ECRT (1). tensity is not always optimal. The solution to this problem Reprint requests to: Rajat J. Kudchadker, Ph.D., Department of [email protected] Radiation Physics, Box 94, The University of Texas M. D. Ander- Received Aug 17, 2001, and in revised form Feb 19, 2002. son Cancer Center, 1515 Holcombe Boulevard, Houston, TX Accepted for publication Feb 27, 2002. 77030, USA. Tel: (713) 792-3230; Fax: (713) 745-0683; E-mail: 1023 1024 I. J. Radiation Oncology ● Biology ● Physics Volume 53, Number 4, 2002 is a beam that has higher energy where more penetration is that after using bolus to conform the distal 90% dose surface necessary and a lower energy where less penetration is to the distal surface of the PTV, the dose spread in the PTV required. This can be achieved using electron bolus, which increased from the 10% (90%–100%) normally found in a provides spatially dependent energy modulation of the elec- water phantom to as much as 20%. This is attributed to the tron field, thus compensating for the irregularities above. variation in the distance from the virtual source to the bolus Electron bolus has been defined by Hogstrom as “a spe- surface (i.e., irregular shape of the bolus surface). Surface cifically shaped material, which is usually tissue equivalent, irregularities (patient or bolus surface), when compared to that is normally placed either in direct contact with the that for perpendicular incidence to a flat surface, increase patient’s skin surface, close to the patient’s skin surface, or nonuniformity of the dose distribution in the PTV, because inside a body cavity. This material is designed to provide of effects of inverse-square and because of loss of side- extra scattering or energy degradation of the electron beam. scatter equilibrium. It is well known that as the SSD de- Its purpose is usually to shape the dose distribution to creases (increases), the value of maximum dose increases conform to the target volume and/or to provide a more (decreases) (2). For example, if the thickness of a bolus is uniform dose inside the target volume” (5). In its application about 2.5 cm near the central axis, this increases the dose by to conformal therapy, the primary goal is to select the about 5%. Also, it is a known fact that nonperpendicular minimal beam energy and optimal bolus design to conform incidence impacts depth–dose and that an irregular surface to the 90% dose contour to the distal surface of the target leads to hot and cold spots (4). volume so that there is minimal dose to nearby normal structures. In doing this, it is not always possible to maintain Purpose of current work-proof of principle dose uniformity (i.e., 90%–100%) inside the PTV, because It should be possible to eliminate the excess dose non- of multiple Coulomb scattering and variable source-to-sur- uniformity caused by the bolus by modulating the incident face distance (5). electron fluence. It is our hypothesis that the degree of nonuniformity of the dose distribution, caused by the irreg- Use of custom-shaped bolus to date ularly shaped proximal bolus surface, can be minimized by Use of custom-shaped bolus for electron therapy is not a modulating the spatial distribution of the electron beam new concept. Archambeau et al. described a bolus technique intensity. This will be shown possible by demonstrating for to be used with chest wall electrons that controlled the three different patient cases the ability to track the 90% dose penetration of the electron beam over a large area (6). Beach surface to the distal surface of the PTV while simulta- et al. also described a bolus technique based on ultrasound neously maintaining a dose uniformity of nearly 10% and limited computed tomography (CT) images (7). Both of (90%–100%) throughout the PTV. these techniques ignored the effects of electron scatter and Although methods of delivering intensity-modulated also lacked the sophistication of accurate dose calculation, electron beams have been discussed using multileaf colli- automated bolus design, and automated bolus fabrication. In mators (MLCs) (14, 15) or scanned beams (16), this tech- previous work at M. D. Anderson, Low et al. (8) and nology is not presently available for patient use. This work Starkschall et al. (9) introduced a methodology based on will demonstrate a potential use for electron intensity mod- patient CT data that allowed alteration of the dose distribu- ulation using MLC once it becomes available in the clinic. tion using bolus operators and a three-dimensional (3D) implementation of the pencil-beam algorithm. Low et al. METHODS AND MATERIALS also demonstrated the potential of an electron bolus in reducing the dose to uninvolved critical structures in treat- In the present work, the benefit of using intensity modu- ment of the nose, parotid gland, and paraspinal muscles (8, lation in conjunction with energy modulation using bolus 10). Perkins et al., also from M. D. Anderson, demonstrated will be demonstrated for a rectangular-shaped PTV in water the utility of customized 3D electron bolus for optimizing and for three patients selected from a patient database postmastectomy irradiation of two patients with abnormal developed by Bawiec (12). For each patient, two treatment anatomy (11). As part of a project to establish a method for plans are compared, one using bolus (energy modulation) quality assurance of a milled electron bolus, Bawiec studied and one using bolus and incident intensity modulation.
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