Validation of the dosimetry for a Lay-down Total Skin Irradiation techniques by Monte Carlo Simulation

Total skin irradiation (TSI) with electron beam has been very effective for patient with Mycosis fungoides. We recently developed and implemented a technique of laying down position for patients who are too frail for the standard standing position. In this study, we validated these measurements with Monte Carlo (MC) simulation which can provide more information on dose distributions and guidance on further optimization of the technique. The laydown technique consists of 6 equi-spaced beam directions relative to the patient cranial-caudal axis, similar to the standup technique. For the AP/PA directions (vertex fields), patient is placed directly under the gantry at 195cm source-to-skin distance (SSD) and 3 overlapping fields with gantry angles 60˚ apart are used. For the four oblique directions, patient is repositioned on the floor parallel to the gantry rotation axis at SSD of 305 cm with gantry at 300˚. A customized 0.25 mm Cu filter was placed in the linac interface mount to further broaden the beam. Each treatment fraction consists of 10 fields and 3 of them are unique. The Monte Carlo simulation was performed within the EGSnrc environment, using the phase space file provided by the linac vendor. The following quantities were studied and compared with the measurements: for each field/direction at the treatment SSDs, the percent depth dose (PDD), the profiles at the depth of maximum, and the absolute dosimetric output on the flat water phantom; the composite dose distribution on a cylindrical phantom of 30 cm diameter. Cu filter increases the beam FWHM by 44% but also reduces the output by 60%. The central regions within ±10% of the prescription dose were 170×70 cm2 for vertex fields and 140×80 cm2 for oblique fields. Profiles and output factors for both vertex fields and oblique fields agreed within 3% between MC and measurements. Vertex fields has dmax at (0.55: MC; 0.67: measurement)cm and R80 at (1.15; 1.40)cm, oblique field has dmax at (1.05; 0.86)cm and R80 at (1.55; 1.40)cm. When all fields are combined on the cylindrical phantom, the dmax shifted toward surface region. The composite dose distribution has the surface dose at (99.0; 95.2) %, dmax at (0.15; 0.15)cm, and R80 at (0.55; 0.75)cm. The maximum X-ray contamination at the central axis was (2.2; 2.1)%, and reduced to 0.2% at 40 cm off the central axis. Cylindrical phantom of 20 cm and 40 cm diameters for patient size simulation shows the surface dose of 93% and 103%, compared to 30 cm diameter. The Monte Carlo results in general agree well with the measurement, which provides secondary support in our commissioning procedure. In addition to those measurable quantities, the Monte Carlo simulation can provide further information such as the full dose distribution of the patient phantom, and the ability to investigate and optimize techniques such as different filter design, SSD and field size variations.