A dosimetric study of deep skin electron therapy with overlapping stationary radiation fields – A case report

Background: For radiotherapy of total skin including sub-cutaneous tissue up to a depth of 3 cm on the entire left leg of an adult (Angiosarcoma skin), a complex treatment with multiple stationary electron fields was planned at our clinic. The details of dosimetry, clinical dose measurements are presented. Materials and Methods: The treatment planned with 6 overlapping 9 MeV electron fields in Clinac 2300CD linac. With 25&times;25 cm cone, a cut-out insert provided 56 &times; 30 cm field at FSD 213 cm, while patient lying on the floor. Dose distributions were checked using Kodak V densitometric film in cylindrical plastic can phantom. The calibration was carried out using solid water phantom, water equivalent IMRT phantom and water can leg phantom. A dose of 45 Gy in 23 fractions at 5 fractions/week was prescribed. 6 field overlapping field factor was measured by the method described for total body electron irradiation (AAPM). Skin doses were estimated at random selected points using TLD chips and semiconductor diodes. Results: Measured absorbed doses by three methods were 0.174 cGy/MU, 0.166 cGy/MU and 0.162 cGy/MU agreed well with the calculated value 0.163 cGy/MU. 6 field overlap factor was 2.315. Clinical dose estimates of mean skin dose was 246.0 + 14 cGy (n=18), delivering higher dose by 23%. The gonad dose estimate under shield was <5%. The excess dose to skin delivered in first 14 fractions was adjusted in following 9 fractions. Conclusion: It appears that the excess dose in real situation may be due to either floor backscatter or non uniform overlap of dose from adjacent fields.