Effects on skin dose from unwanted air gaps under bolus in photon beam radiotherapy

The dose under small air gaps located under bolus material of up to 10 mm h ave been measured with an Attix parallel plate ionization chamber and radio chromic film. For a 6 MV x-ray beam with 10 mm bolus, an air gap of 2 mm pr oduced no reduction in skin dose when measured with the Attix chamber. An a ir gap of 4 mm will introduce a reduction of dose to the basal layer of app roximately 0-4% depending on field size, angle of incidence and other patie nt specific parameters and a reduction of up to 10% could be seen at the ba sal cell layer for a 10 mm air gap. The 10% reduction was for a small x-ray field at 60 degrees angle of incidence and was a reduction in dose from 10 0% of D-max down to 90% of D-max. Results at oblique angles of incidence sh ow that larger reductions in dose are seen with increasing angle of inciden ce. Radiochromic film results agree with the Attix chamber results measurin g 2% +/- 2% decrease for a 4 mm gap and 4% +/- 2% for a 10 mm gap at normal incidence. Clinically, results show that small air gaps can reduce skin do se, however, at least 90% of maximum dose is still delivered for air gaps u p to 10 mm. (C) 2000 Elsevier Science Ltd. All rights reserved.