Ultrasound transmission gel as a bolus device for skin irradiation of irregular surfaces: technical note

This paper describes an uncommon radiation treatment of the external ear, with ultrasound (US) transmission gel used as bolus device to compensate for the irregularity of the target surface.

Postoperative radiotherapy for cutaneous carcinoma was performed with a single high-energy electron beam directed over the ear auricle. Due to the irregular surface of the target, a "missing tissue" compensator was employed. Daily, after patient positioning, the concha was filled and flattened with US gel, and a dose of 54 Gy in 27 fractions was delivered.

Water equivalence of the gel was verified by comparing the gel's computed tomography (CT) number [Hounsfield units (HU)] and density with the corresponding values for water and another commercial bolus device. Whereas ultrasound gel and water had comparable values (HU: 0; density 1 g/cm(3) for both), the corresponding values for the commercial device were slightly higher (HU: 80; density 1.02 g/cm(3)).

Ultrasound gel proved to be an easy, fast and cheap compensating tool. Its water equivalence allows it to be used as an alternative to water, though easier to position and with lower risk of displacement. Thus, it is recommendable as a practical tool for most irregular sites. Further investigations are warranted to validate this solution in more complex irradiation techniques.