Implementation of Total Skin Electron Irradiation Technique

Purpose : To establish the technique of Total Skin Electron Irradiation (TSEI) and patient dose verifi cation for Chulabhorn Hospital. Methods and materials : Methods and materials : The high dose rate total skin electron irradiation mode (High Dose Rate TS e-) of 6 MeV using a dual-fi eld Stanford technique (AP, RPO, LPO, PA, RAO and LAO) with a 1 cm lucite scatter plate was used in this study. To determine the appropriate irradiation gantry angle,beam profi les in vertical and horizontal directions were measured by using DPD-12pc system of Scanditronix Wellhofer with a styrofoam sheet (100W × 200H cm), varying the gantry angles between ±17o to ±19.5o.. The percentage depth dose and x-ray contamination of single dual fields were measured by using the Gafchromic EBT film,TLD-100 chips and Wellhofer PP-40 chamber in square solid water phantom (30 × 30 × 30 cm) and six dual fi elds by using the Gafchromic EBT film in a Rando phantom. A round phantom with 30 cm diameter was used to measure the point dose (single dual fi elds) and surface dose (six dual fi elds) using both the TLD and fi lm. Then the overlapping factor (the ratio between surface dose and point dose) and the monitor units for 2 Gy/cycle were calculated. Before treating the patient, the surface dose distribution was verifi ed by using TLDs and Rando phantom. During patient irradiation with six dual fi elds, in vivo measurement of the skin dose was done using the DPD-12pc detectors to verify the dose distribution and patient positioning. Results : The optimal gantry angle for this study was ± 18o with fl atness of ± 8% along 190 cm of patient’s length and ± 6% along 60 cm of patient’s width. The x-ray contamination was 0.5%, 0.3 % and 2.3 % for the chamber, TLD and EBT film measurements respectively. The overlapping factor was 3.09 with the calculated MU of 526 MU/angle for 2 Gy/cycle. The uniformity of the dose on the skin of two patients was within ±10% except the localized regions of extreme non-uniformity of the patient’s skin dose. Discussion and conclusion : TLD is an important part of the beam data collection and quality assurance programs for TSEI. EBT film is not convenient for routine measurement because it needs least one 24 hours delay before reading and cannot measure very low dose as the x-ray contamination.