A novel real-time shapeable soft rubber bolus for clinical use in electron radiotherapy

Abstract

We have developed soft rubber (SR) bolus that can be shaped in real-time by heating flexibly and repeatedly. This study investigated whether the SR bolus could be used as an ideal bolus, such as not changing of the beam characteristics and homogeneity through the bolus and high plasticity to adhere a patient in addition to real-time shapeable and reusability, in electron radiotherapy. Percentage depth doses (PDDs) and lateral dose profiles (LDPs) were obtained for 4, 6, and 9 MeV electron beams and were compared between the SR and conventional gel boluses. For the LDP at depth of 90% dose, the penumbra as lateral distance between the 80% and 20% isodose lines (P 80−20) and the width of 90% dose level (r 90) were compared. To evaluate adhesion, the air gap volume between the boluses and nose of a head phantom was evaluated on CT image. The dose profiles along the center axis for the 6 MeV electron beam with SR, gel, and virtual boluses (thickness = 5 mm) on the head phantom were also calculated for the irradiation of 200 monitor unit with a treatment planning system and the depth of the maximum dose (d max) and maximum dose (D max) were compared. The PDDs, P 80−20, and r 90 between the SR and gel boluses corresponded well (within 2%, 0.4 mm, and 0.7 mm, respectively). The air gap volumes of the SR and gel boluses were 3.14 and 50.35 cm3, respectively. The d max with SR, gel and virtual boluses were 8.0, 6.0, and 7.0 mm (no bolus: 12.0 mm), and the D max values were 186.4, 170.6, and 186.8 cGy, respectively. The SR bolus had the equivalent electron beam characteristics and homogeneity to the gel bolus and achieved excellent adhesion to a body surface, which can be used in electron radiotherapy as an ideal bolus.

The development and characterization of an all-purpose bolus for radiotherapy

“…In a previous study, Wakabayashi et al developed the SR bolus that can be molded in real time at temperatures greater than 50 °C and can maintain their shape at room or body temperature (Wakabayashi et al 2021); however, the SR bolus was a non-transparent material, and they experienced cases in which the adhesiveness of the CT simulation could not be reproduced at each treatment. The free shaping temperature of the SR bolus was 53.0 °C, which is greater than that of the HM bolus, and even a short reheat did not sufficiently soften the material.…”

“…The following three parameters are used to define the elastic properties of the HM bolus: the storage modulus (E') is related to elasticity, loss modulus (E") is related to viscosity, and the ratio of the moduli (E"/E') is defined as tanδ. A high tanδ value means high viscosity, while a low tanδ means high elasticity (Geethamma et al 1995, Monzen et al 2019, Wakabayashi et al 2021.…”

“…For both HM and Gel boluses, the size was 30 cm × 30 cm and the diameter was 5 mm. The source to surface distance (SSD) was 100 cm, the field size was 10 cm × 10 cm, and irradiation was performed at 200 monitor units (MU) (figure 3) (Adamson et al 2017, Wakabayashi et al 2021. The dose difference between the Gel bolus and HM bolus was calculated at depths of 0, 3, 15, and 30 mm for the electron beam and 0, 15, 50, and 100 mm for the photon beam using equation (1).…”

“…of the adhesion and reproducibility of the bolus at three points A pelvic phantom (BrainLab AG, Feldkirchen, Germany) was used to evaluate the adhesion and reproducibility of the Gel bolus, soft rubber bolus (SR bolus)(Wakabayashi et al 2021), and HM bolus on uneven body surfaces.…”

Initial Clinical Experience of a Novel Shapeable Bolus for Radiotherapy in a Patient With a Facial Cutaneous Squamous Cell Carcinoma: A Case Report

“…The ideal bolus should adhere closely to the patient's skin without air gaps. The HM bolus (Hayakawa Rubber Co., Ltd., Hiroshima, Japan) was recently developed by Wakabayashi et al and Nakamura et al [8,9]. The HM bolus has unique features, including tissue equivalence, transparency, and reusability, and it can be easily shaped by hand after warming it to approximately 40 °C, allowing it to adhere closely to the skin.…”