Evaluation of surface and shallow depth dose reductions using a Superflab bolus during conventional and advanced external beam radiotherapy

Abstract The purpose of this study was to evaluate a methodology to reduce scatter and leakage radiations to patients’ surface and shallow depths during conventional and advanced external beam radiotherapy. Superflab boluses of different thicknesses were placed on top of a stack of solid water phantoms, and the bolus effect on surface and shallow depth doses for both open and intensity‐modulated radiotherapy (IMRT) beams was evaluated using thermoluminescent dosimeters and ion chamber measurements. Contralateral breast dose reduction caused by the bolus was evaluated by delivering clinical postmastectomy radiotherapy (PMRT) plans to an anthropomorphic phantom. For the solid water phantom measurements, surface dose reduction caused by the Superflab bolus was achieved only in out‐of‐field area and on the incident side of the beam, and the dose reduction increased with bolus thickness. The dose reduction caused by the bolus was more significant at closer distances from the beam. Most of the dose reductions occurred in the first 2‐cm depth and stopped at 4‐cm depth. For clinical PMRT treatment plans, surface dose reductions using a 1‐cm Superflab bolus were up to 31% and 62% for volumetric‐modulated arc therapy and 4‐field IMRT, respectively, but there was no dose reduction for Tomotherapy. A Superflab bolus can be used to reduce surface and shallow depth doses during external beam radiotherapy when it is placed out of the beam and on the incident side of the beam. Although we only validated this dose reduction strategy for PMRT treatments, it is applicable to any external beam radiotherapy and can potentially reduce patients’ risk of developing radiation‐induced side effects.


| INTRODUCTION
The number of cancer survivors has been increasing. It is estimated that cancer survivors will account for about 5.4% of the US population in 2024, and approximately half of all cancer patients receive radiotherapy as a part of their treatments (www.cancer.gov). There is a growing concern about patient safety because side effects induced by cancer treatments may remain and seriously affect their quality of life for many of these survivors. While the goal of radiotherapy is to deliver a highly conformal dose to the tumor area only, normal tissues outside the target also receive radiation doses including medium to high dose adjacent to the target, scatter and leakage doses from the treatment machine, and scatter dose within the patient. These normal tissue doses can cause a spectrum of acute and chronic radiogenic side effects for the patients. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Epidemiologic studies indicate the majority of second cancers occurred in the low-or intermediate-dose areas, 15,16 and current data support linear-no-threshold dose-risk model indicating a finite risk of developing second cancer even for the lowest radiation dose. 17,18 The advanced radiotherapy techniques like intensity-modulated radiotherapy (IMRT) will increase the low-dose volume because of beam modulations and can increase the risk of developing second cancers. 19,20 Over the years, people have been trying to reduce normal tissue doses during radiotherapy, and external shielding is one of the effective approaches to reduce the scatter and leakage doses from the treatment machine. Multiple studies [21][22][23][24] reported lead blocks can be used to limit fetal dose during radiotherapy for pregnant patients; lead sheets had also been shown to reduce scatter radiation to the contralateral breast or heart during breast cancer radiotherapy. [25][26][27][28][29] Most of the previous studies chose a high-density metal as the shielding material, and holders usually had to be used to support the heavy shielding blocks, which makes the procedure expensive, troublesome, and time consuming. Occasionally, the shielding block may be dropped by accident and could hurt the patient or staff. Although some specially designed shielding device made of a very thin lead sheet can overcome some of these problems, it has the limited availability, does not match the body surface, and additional material has to be used to fill the gap between the body and the shielding device to reduce the lateral scattering dose. 30 One study reported that placing a Superflab bolus on the surface of the contralateral breast during external radiotherapy could reduce the surface dose under the bolus. 31 This is an attractive alternative because the Superflab bolus is widely available, has a much lower cost compared with lead sheets, and can be put directly on the patient and conform to the body shape very well. The effect of the bolus was only evaluated for the conventional tangential breast radiotherapy in that study and only 1-cm Superflab bolus was used. 31 In this study, we investigated this bolus effect in more details: the bolus was placed in the field and out of the field, on the incident and exit sides of the radiation beam; various bolus thicknesses were used; both surface and shallow depth doses at different distances from beam axis were investigated; both static and IMRT beams were tested; and dose reductions for advanced radiotherapy techniques in clinically realistic situations were also evaluated.

2.A | Surface dose evaluation
A stack of solid water phantoms was used to evaluate the skin dose with and without a Superflab bolus (Radiation Products Design Inc., Albertville, MN, USA) placed in the field or out of the field (Fig. 1). The Superflab boluses used in this study are commercial products and made of synthetic gel which is water equivalent. They have the same size (30 9 30 cm 2 ) but the thickness varies. The bolus can be cut to any shape to fit the patient's contour for any radiotherapy when necessary, although we did not cut them in this study. Thermoluminescent dosimeters (TLDs) were placed at the measurement points shown in Fig. 1. Measurement points 1 and 2 were on the beam axis and in the field, and points 3 and 4 were 10 cm away from the beam axis and out of the field. For in-field measurements, 6 and 10 MV open beams with 10 9 10 cm 2 field size at the isocenter and IMRT beams with a maximum multileaf collimator (MLC) opening less than 10 9 10 cm 2 at the isocenter were used, and 100 monitor units (MUs) were delivered using Elekta Infinity linac (Elekta Corporation, Stockholm, Sweden). The source-to-surface (SSD) was 90 cm and the beam isocenter was located at 10 cm depth and 10 cm away from the edge of the phantom (Fig. 1), and only 1-cm Superflab bolus was used in the in-field measurements and was placed above point 1 or 2. Beam axis F I G . 1. A solid water phantom used for bolus effect evaluation. Surface points 1 and 2 were on the beam axis and in the field, and points 3 and 4 were 10 cm away from the beam axis and out of the field. For the radiation beam shown in the figure, points 1 and 3 were on the incident side, and points 2 and 4 were on the exit side of the beam. We also measured doses 7, 10, and 15 cm away from the beam axis and at various depths.

| RESULTS AND DISCUSSION
The effects of Superflab bolus on surface dose are shown in Table 1.
It is found that surface dose was reduced only when the point of 2. An Atom phantom (CIRS) with a breast attachment. Surface doses were measured on the contralateral breast attachment (points 1-5 in the left figure). A Superflab bolus was placed on the ipsilateral side to improve skin coverage. Another 1-cm Superflab bolus was placed on the contralateral breast during radiation delivery to reduce surface dose but was not shown here for visual clarity.
interest is out of the beam and on the incident side of the beam (point 3 in Fig. 1 and Table 1  When the Superflab bolus was used for clinical PMRT beams, surface doses were reduced in most cases except Tomotherapy, as shown in Table 3. For VMAT plan, the surface dose reduced at most points and the reductions were between 13% and 31%, while the Reduction of undesirable doses from radiotherapy treatments represents an important topic due to the higher life expectancy after the treatments as a consequence of the high healing rate, increasing cancer incidence in the general population, and the increase in peripheral dose from new radiotherapy techniques. 34 Literature demonstrates low radiation dose can induce severe side effects: it has been reported that atomic bomb survivors in the dose range from 5 to 100 mSv show a significantly increased incidence of solid cancer compared with the population who were exposed to less than 5 mSv, 35 a significant risk for acute leukemia was seen in young T A B L E 1 Measured surface doses (mean AE SD of the mean) when Superflab boluses of various thicknesses were placed on a solid water phantom, and 6, 10 MV open (2000 MUs), and IMRT (1982 MUs) photon beams were delivered to the phantom. The setup and measurement point locations are shown in Fig. 1. individuals who were exposed to fallout from nuclear test site and received bone-marrow doses from 6 to 30 mSv, 36 thyroid and breast cancers occurred in children when radiation doses were as low as 100 mGy, 37 and lung cancers happened for doses of 500 mGy in adults. 38 The method presented in this study to reduce peripheral dose is a simple and important tool that could be used in clinical routine to reduce patients' risk of developing radiation-induced side effects and increase patients' safety.

CONFLI CT OF INTEREST
The authors declare no conflict of interest.
T A B L E 3 Measured surface dose (mean AE SD of the mean) when 1-cm Superflab bolus was placed on the contralateral breast of an anthropomorphic phantom and PMRT plans (VMAT, 4-field IMRT and Tomotherapy) were delivered to the phantom. Doses, expressed in cGy, correspond to a total prescription dose of 50 Gy. The measurement locations are shown in Fig. 2.