INTRODUCTION: Radiotherapy for advanced head and neck malignancies has improved with the use of more conformal techniques, such as intensity modulated radiation therapy. However, obtaining adequate surface dose delivery is problematic with the high energy photons or electrons generated by modern linear accelerators. Sheets of hand cut planar tissue-equivalent bolus (TEB) have traditionally been used to increase surface dose but are difficult to accurately position around irregular surfaces present in the head and neck. We therefore developed a patient-specific three-dimensional (3D) printing technique using a tissue-equivalent, translucent, rubber-like polymer to improve the conformity of bolus to skin and accuracy of delivered radiation dose for patients with head and neck malignancies requiring skin dose coverage.
METHODS: Following computed tomography (CT), a 3D bolus structure file was virtually generated in Philips Pinnacle3 treatment planning software, which allows for detailed customization to irregular surfaces, as well as modulation of thickness for compensator effect. The 3D TEB was printed on a PolyJet 3D printer with a commercially available translucent rubber-like photopolymer. This material was chosen for its flexibility, allowing for better conformity to the skin surface, and translucence aiding in reproducible placement on the patient.
We analyzed the density properties of planar and 3D printed bolus using CT. Surface dose delivery was measured on an anthropomorphic phantom, and then on patients, using metal-oxide semiconductor field-effect transistor (MOSFET) measurements validated on three separate channels. Radiation was delivered on an Elekta Synergy clinical linear accelerator.
RESULTS: The 3D TEB demonstrated a density (1.087 g/cm3) similar to planar bolus (1.125 g/cm3). Measurements taken from across the 3D TEB demonstrated equal densities (range 1.086-1.088 g/cm3, SEM 0.002) similar to planar bolus (range 1.080-1.150 g/cm3, SEM 0.03). Phantom measurements of superficial dose were evaluated without bolus, with commercial planar TEB and with 3D TEB. The average output during MOSFET calibration was 99.7% (+ 2.7% SEM) of the prescribed dose. Without bolus material the measured surface dose was 36% of prescribed dose. 3D TEB and planar bolus were equivalent in providing adequate superficial dose, 104% versus 105% of prescribed dose, respectively. An initial cohort of 5 patients treated with 3D printed bolus were evaluated for surface dose delivery by MOSFET. The average measured difference in dose from prescription was 0.10% (SEM 0.014%) over 52 measurements.
CONCLUSION: We demonstrate that patient-specific 3D printed bolus has equivalent density to commercial planar TEB and uniform density throughout the structure. Additionally, the 3D TEB provides excellent surface dose delivery in both phantom studies and clinically treated patients. The 3D printed bolus has the advantage of conforming to complex surface geometries compared to planar bolus. Furthermore, the 3D bolus’ flexibility and transparency allows for more reproducible positioning compared to commonly used 3D printed plastics which are rigid and opaque. Ongoing studies aim to further optimize the use of this 3D TEB for patients with head and neck malignancies requiring skin dose coverage.