Bolus: Tissue Equivalent, But are They Equivalent?

Purpose/Aim

Bolus is a crucial tool in manipulating radiation dose for disease sites with irregular contours and superficial disease, such as the head and neck disease site. Bolus is made with varied materials across different cancer centers and disease sites. They are determined as “tissue-equivalent” materials based on their density and ability to pull dose superficially. Radiation beams reach their “maximum dose” at a certain depth, depending on energy, because they require a build-up region before depositing their maximum dose. Placing bolus on top of the skin creates a “false” build-up region, meaning maximum dose will be deposited at the skin surface. The purpose of this study is to compare whether PLA (polylactic acid filament) and Vaseline/petroleum gauze bolus are true equivalents considering their difference in positioning, material factors and density.

Methods/Process

This study compares PLA (polylactic acid filament) 3D-printed bolus and Vaseline/petroleum gauze bolus on head and neck patients through measuring their accuracy to the planned dose and their inter-fraction variation. Dose to skin was measured in cGy on 3-4 consecutive days of the participating patients’ treatments using a TLDs (thermolumiscent detectors) placed underneath the bolus. The measurements of each patient were analyzed each cohort was compared.

Results or Benefits/Challenges

Both boluses had, on average, equivalent accuracy to the planned dose but upon observing CBCTs and bolus positioning with each patient, there were challenges unique to each material. Vaseline bolus’ reproducibility was better than that of 3D, possibly due to the cast being molded over the bolus and keeping it in place. 3D-printed had more uniform thickness than Vaseline but the positioning over the thermoplastic cast presented air gaps and positioning errors that affected its reproducibility.

Conclusions/Impact

Overall, both materials were acceptable for treatment but strides can be taken to improve 3D bolus positioning so it will truly be customized to the patient and what contours are most appropriate for each head and neck site.