Characterization and clinical validation of patient-specific three-dimensional printed tissue-equivalent bolus for radiotherapy of head and neck malignancies involving skin

Purpose: Megavoltage radiotherapy to irregular superficial targets is challenging due to the skin sparing effect. We developed a three-dimensional bolus (3DB) program to assess the clinical impact on dosimetric and patient outcomes. Materials and Methods: Planar commercial bolus (PCB) and 3DB density, clarity, and net bolus effect were rigorously evaluated prior to clinical implementation. After IRB approval, patients with cutaneous or locally advanced malignancies deemed to require bolus for radiotherapy treatment were treated with custom 3DB. Results: The mean density of 3DB and PCB was of 1.07 g/cm 3 and 1.12 g/cm3, respectively. 3DB optic clarity was superior versus PCB at any material thickness. Phantom measurements of superficial dose with 3DB and PCB showed excellent bolus effect for both materials. 3DB reduced air gaps compared with PCB - particularly in irregular areas such as the ear, nose, and orbit. A dosimetric comparison of 3DB and PCB plans showed equivalent superficial homogeneity for 3DB and PCB (3DB median HI 1.249, range 1.111-1.300 and PCB median HI 1.165, range 1.094-1.279), but better conformity with 3DB (3DB median CI 0.993, range 0.962-0.993) versus PCB (PCB median CI 0.977, range 0.601-0.991). Patient dose measurements using 3DB confirm the delivered superficial dose was within 1% of the intended prescription (95% CI 97-102%; P = 0.11). Conclusions: 3DB improves radiotherapy plan conformity, reduces air gap volume in irregular superficial areas which could affect superficial dose delivery, and provides excellent dose coverage to irregular superficial targets.