British Journal of Radiology 75 (2002),670-677 © 2002 The British Institute of Radiology
Improved dose homogeneity in scalp irradiation using a single set-up point and different energy electron beams
R Yaparpalvi, MS,
D P Fontenla, PhD and
J J Beitler, MD, MBA
Department of Radiation Oncology, Montefiore Medical Center and the Albert Einstein College of Medicine, Bronx, New York, USA
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Abstract
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Homogeneous irradiation of the entire or a large portion of the superficial scalp poses both technical and dosimetric challenges. Some techniques will irradiate too much of the underlying normal brain while other techniques are either complex and involve field matching problems or may require sophisticated linear accelerator (linac) add-ons such as intensity modulated radiation therapy (IMRT)/electron multileaf collimation. However, many radiotherapy facilities are not equipped with such treatment modalities. We propose a practical treatment technique that can be delivered with a standard linac capable of producing high energy electrons. The proposed technique offers a simple alternative for achieving results equivalent to IMRT. Dose homogeneity throughout the treatment volume is achieved by aiming different energy electron beams at differential areas of the treatment surface to achieve improved dosimetry and rapid treatment delivery, while using a single set-up point. We introduced this treatment technique at our institution to treat superficial cancers of the scalp and other irregular surfaces.
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Introduction
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Homogeneous irradiation of the entire or a large portion of the superficial scalp poses both technical and dosimetric challenges. Some techniques will irradiate too much of the underlying normal brain while other techniques are either complex and involve field matching problems or may require sophisticated linear accelerator (linac) add-ons such as intensity modulated radiation therapy (IMRT)/electron multileaf collimator [1–9]. However, many radiotherapy facilities are not equipped with such treatment modalities. We propose a practical treatment technique that can be delivered with a standard linac capable of producing high-energy electrons. The proposed technique offers a simple alternative for achieving results equivalent to IMRT.
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Treatment technique
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Dose homogeneity throughout the treatment volume is achieved by aiming different energy electron beams at differential areas of the treatment surface to achieve improved dosimetry and rapid treatment delivery, while using a single set-up point. Ideally, a prone patient position is used and the curved treatment surface is envisaged as a set of concentric circles (Figure 1
). In general, the lowest electron energies are used to treat the surface represented by the innermost circle (circle 1), and the treatment surfaces represented by the outer concentric circles (circles 2 and 3) are treated with higher energy electrons. The surface treated by the first electron beam is blocked for treatment by the second electron beam, and so on. Figure 1 illustrates the proposed technique, showing the three differential treatment surfaces (and the corresponding electron beam energy changes). However, depending on clinical indications, target depth and target volume, different energy electron beams are combined, which may necessitate changes to the electron beam energy and the corresponding number of treatment surfaces. Typically, an electron applicator large enough to encompass the entire treatment surface plus a margin is used, and applicator inserts defining the treatment surface of each beam are changed between beam treatments. This technique was used on a patient with extensive squamous cell carcinoma of the scalp and skull.
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Figure 1. Idealistic beam's-eye-view of the treatment surface and conceptual description of the proposed treatment technique.
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Materials and methods
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Patient
A 62-year-old Black female presented with a history of squamous cell carcinoma of the scalp. CT of the head showed a very large multilobulated cystic and/or partially necrotic mass destroying the outer table of the right frontal calvarium with excessive involvement of the soft tissue and overlying skin (Figure 2). The plan was to deliver the most homogeneous dose possible to the large curved treatment volume with minimal dose to the underlying brain.
Treatment planning and dosimetry
CT was performed with the patient in the supine treatment position. While a prone patient positioning is preferred, owing to medical reasons a supine treatment position was used for this patient. Before CT scanning, orthogonal radiographs (anteroposterior and lateral) were taken on a simulator with fluoroscopy capability in order to determine reference points to assist in daily patient positioning. These reference points were visualized on the CT images using radio-opaque markers. The patient was immobilized in a comfortable yet rigid fashion using a thin clear plastic head and neck support (MT-Silver; Med-Tec, Orange City, IA) and a Velcro chin strap (03-1930; JRT Associates, Elmsford, NY), with a major emphasis on straight head positioning. CT images of the entire head were obtained using a 5 mm slice thickness at 5 mm intervals. The CT images were then transferred directly via Ethernet to the radiation treatment planning (RTP) system. The clinical target volume (CTV) and appropriate structures such as the brain and eyes were outlined on the CT images.
Dose calculations were performed on a commercial three-dimensional (3D) RTP system (Render plan 3-D; Elekta Oncology Systems, Atlanta, GA). Render plan 3-D uses a modified Hogstrom pencil beam (full summation) algorithm for its electron beam calculations [10]. The dosimetry process is started by creating an electron beam and choosing a 25 x 25 cm2 electron applicator. Utilizing beam's-eye-view (BEV) and beam interactive controls, and simultaneously viewing the patient's CT images in coronal, sagittal and transverse planes, the electron beam was oriented to centre the CTV in the open area of the 25 x 25 cm2 electron applicator. For this patient a posterior superior oblique field was used by rotating the treatment couch 90° from its standard position and using a 50° gantry angle. To assist in patient treatment set-up, the necessary displacements from the simulation reference points to accomplish this beam orientation were noted.
Based on the target depth along the beam central axis, an appropriate energy electron beam (12 MeV) was selected and an isodose distribution calculation for the 25 x 25 cm2 open beam configuration was performed. As expected, the curved treatment surface resulted in a non-uniform dose distribution. The dose coverage varied with the radius of curvature of the treatment surface and the complementary source-to-skin distance (SSD). Laterally, the target received a lower dose farther from the beam central axis compared with the target near the beam central axis, which showed uniform dose coverage. The area enclosed by this uniform dose region was mapped and a margin of 1–2 cm was added to this area. A custom beam shaped to the 25 x 25 cm2 electron applicator, with an opening equal to this area, was then designed and this formed the innermost treatment circle (circle 1 in Figure 1
).
A second electron beam with the same beam orientation and accessory as the first (25 x 25 cm2 electron applicator) was added to irradiate the target surface not treated by the first electron beam. Based on the target depth we selected a 16 MeV energy electron beam. The surface treated by the 12 MeV electron beam was blocked for treatment by the 16 MeV beam. A custom beam shape was designed for the 16 MeV electron beam using automatic margin generation (a 2 cm margin around the CTV was given) and the digitally reconstructed radiograph capabilities of the planning system. Ideally, the treatment surface irradiated by the 16 MeV electron beam should resemble a concentric circle such as represented by area 2 in Figure 1. The actual applicator insert used for the 16 MeV electron beam is shown in Figure 3. The central electron shield represents the surface treated by the 12 MeV electron beam and was supported on a 3.5 mm thick polystyrene slab custom designed to fit the 25 x 25 cm2 electron applicator.
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Figure 3. The applicator insert used for the 16 MeV electron beam. The central block represents the surface treated by the 12 MeV electron beam.
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The electron beam weights and the central shield dimension were manually optimized. The central shield dimension was designed to obtain uniform dose coverage and to minimize hot/cold spots at the junction of the two electron energies used. The optimal beam weights were selected to minimize the dose variation in radial direction across the entire target volume. Initially, a 1:1 beam weighting and a central shield equal in dimension to the 12 MeV beam size were selected and a dose distribution calculation was performed. Based on the hot/cold spots at the junction of electron energy changes and the target dose coverage, either the central shield dimension was changed or the beam weights were altered, and the calculation process was repeated. In this manner, the dose distributions were optimized to restrict hot spots to 
120% of the prescribed dose to a 1–2 ml volume while obtaining adequate dose coverage of the CTV. After an acceptable treatment plan was generated, hard copies of the dosimetric data, isodose distributions, as well as the BEV projections of the custom beam shapes and central electron shield were printed out. A commercial shielding alloy (Cerrobend, cadmium free; Med-Tec, Orange City, IA) was used to make the electron applicator inserts and the central shielding used in this study. Accurate positioning of the central electron shield on the polystyrene slab was accomplished using the BEV hardcopy printout at a 95 cm source-to-electron-cone-insert distance. Velcro sticky tape was used to secure the central shield on the polystyrene tray.
Treatment
The patient was treated on a 2100C/D Varian linear accelerator (linac) (Varian Oncology Systems, Palo Alto, CA) at a set-up distance of 100 cm. The couch was rotated 90° from its standard position and the patient was positioned on the treatment couch in such a way that the patient's head extended as far out beyond the treatment table edge as safely possible in order to avoid treatment table in the beam's path. The eyes were protected by combined use of beam shaping and external eye shields. Combining set-up instructions from the simulation and the treatment plan, gantry, table and collimator angles were selected and the 25 x 25 cm2 electron applicator was attached. Using the applicator inserts for the 12 MeV and 16 MeV beam treatment surfaces, light fields were marked on the patient's scalp to assist in daily positioning and verification. Additional verification of electron beam fields was accomplished using a technique published in literature [11]. This verification method consists of superimposing the central plane image of the posterior superior oblique field on a right or left lateral port film. Port film verifications were performed for the first treatment fraction and on a weekly basis thereafter. Because use of 12 MeV and 16 MeV electron beams already provided adequate surface dose, no additional surface bolus material was used. The prescribed dose was 60 Gy delivered in 2 Gy fractions. Patient dose was verified using diode dosimetry. During patient dosimetry, the patient was set-up as usual by the therapists. Before treatment, the diode was placed on the patient's scalp surface along the beam central axis or in the centre of the open portion of the electron light field and was secured with hypoallergenic tape. No attempt was made to estimate the junction doses from diode measurements. Additional details are available in our recent publication [12].
Phantom measurements
In-phantom measurements were performed to verify the accuracy of electron dose calculations by our 3D RTP system. Phantom measurements were made on our linac. Radiation therapy verification film (Kodak X-Omat V-film; Eastman Kodak, Rochester, NY) was used for the measurements. The films were sandwiched between 25 x 25 cm2 polystyrene slabs of desired thickness and were exposed perpendicular to the incident electron beam. The measurement geometry consisted of a 25 cm phantom thickness, 100 cm SSD and a 20 x 20 cm2 electron applicator. Calculated doses were compared with measured doses for the following scenarios: (a) a circular field using a 12 MeV electron beam (a 9 cm diameter circle); (b) a centrally shielded concentric circular field using a 16 MeV electron beam (a 15 cm diameter circular field with a 9 cm diameter central shield); and (c) a combination of fields (a) and (b). The Cerrobend applicator inserts and central electron shield used for phantom studies are shown in Figures 4 and 5. In each case, the phantom was irradiated to a known dose based on the monitor units obtained from the treatment planning system. A Macbeth densitometer (Model TD-502; Macbeth, Newburgh, NY) and a film dosimetry system (RIT Inc., Colorado Springs, CO) were used to analyse the exposed films. Prior to film analysis, a plot of the optical density vs dose for the XV films was established. Film dose–response data were collected both for 12 MeV and 16 MeV electron beams and were found to be independent of energy.
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Figure 5. The centrally shielded concentric circular 16 MeV electron beam field used in the phantom study.
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Results
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Figure 6 shows the patient isodose distributions in the sagittal and coronal planes. The dose was prescribed to the 90% isodose line. Patient dose verification using diodes showed a concordance within ±6% between the expected dose and that measured with diodes. In-phantom results for the three scenarios considered in this study are presented in Figures 7–9. Measured and calculated doses are compared for: (a) the open circular 12 MeV electron beam (Figure 7); (b) for the centrally shielded concentric circular 16 MeV electron beam (Figure 8); and (c) for the combined 12 MeV and 16 MeV electron beam fields (Figure 9). In each case the doses were normalized with respect to the measured central axis dose. Patient assessment 4 months following completion of radiation therapy revealed complete resolution of disease (Figure 10). No evidence of hot or cold spot problems at the junction of electron energy changes was observed.
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Figure 7. A comparison of in-phantom calculated and measured dose profile for the open circular 12 MeV electron beam field.
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Figure 8. A comparison of calculated and measured dose profiles at different depths in the phantom for the centrally shielded concentric circular 16 MeV electron beam field.
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Figure 9. A comparison of calculated and measured dose profiles at different depths in the phantom for a combination of 12 MeV and 16 MeV electron beam fields.
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Discussion
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Centrally shielded beams are more commonly employed in photon beam therapy compared with electron beam therapy [13]. The RTP system used for electron beam dose calculation in this study does not simulate the effects of the presence of a tray to hold blocks on the electron beams. The presence of the tray in the electron beam path may decrease the range of electrons and increase scatter. In our dose distributions we have accounted for the presence of 3.5 mm thick polystyrene tray holding the central shield (at the end of the electron applicator) by approximating it to be the same as adding a 3.5 mm thick water-equivalent bolus on the patient's scal psurface. In-phantom measured dose profiles obtained by positioning the 3.5 mm thick polystyrene tray on the electron applicator compared with on the phantom surface, respectively, are compared in Figure 11. The measured data reasonably support our approximation.
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Figure 11. A comparison of in-phantom measured dose profiles obtained by positioning the 3.5 mm thick polystyrene tray on the electron applicator compared with on the phantom.
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For centrally shielded electron beams, Gosselin et al [14] have shown that the dose under the shield, depending on lateral shield dimensions and the depth in the phantom, does not become negligible even for a shielding thickness greater than the range of electrons in the shield material. Electron transmission through the central shield is considered to be zero by our RTP system. Based on our phantom data for a centrally shielded electron beam, it is evident that while calculated dose profiles follow measured dose profiles at different depths in the phantom, considerable differences between calculated and measured point doses exist, especially at the central shield–open beam interface (Figure 8). As a clinical consequence, it can be inferred that additional hot/cold spots (up to 10% in magnitude) to those calculated by the treatment planning system may exist within the target volume, especially at the junction of electron energy changes (Figure 9). The magnitude of these dose discrepancies, however, may also vary depending on factors such as electron beam energies, size and thickness of the electron shield, depth etc. Since direct patient dose distribution measurements are not possible and because electron dose calculation algorithms on most RTP systems are relatively weak [15], we concur that the dose distributions in centrally shielded electron beams are best determined by phantom measurements under clinical conditions [14]. These dose calculation inaccuracies may be further affected in this patient by the presence of scalp surface and internal deformities caused by the destructive nature of the disease.
For this patient we used an oblique electron beam (at 50° incidence angle) to treat a curved surface. Increase in surface dose with increasing incidence angle, with greater effect at lower electron energies and higher beam incidence angles, and the inability of many electron dose calculation models to accurately incorporate these changes are reported in literature [16]. However, pencil-beam-based electron dose calculation algorithms, such as the one used by our RTP system, have been shown to effectively take into consideration this effect for values up to 60° incidence angles [16] (KR Hogstrom, Personal Communication). Currently, Monte Carlo based dose calculation algorithms are proposed as a tool to effectively quantify the effects of electron beam shielding and beam obliquity [17]. The X-ray contamination value for the centrally shielded 16 MeV electron beam was found to be 2.3%, which was lower than our open beam 16 MeV electron beam X-ray contamination value of 2.5%. Similar results are reported elsewhere [14].
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Conclusion
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In summary, a practical treatment technique for scalp irradiation is proposed. This technique uses a standard linac with high energy electron capability and a single set-up point, and achieves improved dosimetry and rapid treatment delivery. The main concern when using multiple abutting electron fields is the daily patient set-up reproducibility and treatment delivery accuracy. Use of a single set-up point with our technique alleviates this problem. Although for medical reasons we used a supine patient position in this study, a prone patient position is optimal in terms of therapy unit and treatment delivery parameters. In addition, the proposed treatment technique offers a simple alternative for achieving results equivalent to intensity modulated radiation therapy. We introduced this treatment technique in our institution to treat superficial cancers of the scalp and other irregular surfaces.
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Acknowledgments
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We gratefully acknowledge Dr J Avadhani, PhD, for his assistance in data plotting.
Received for publication November 21, 2001.
Revision received March 25, 2002.
Accepted for publication April 2, 2002.
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Abstract
Introduction
