Radiation physics and dosimetry of 6 MeV linear accelerator beam for swine cutaneous irradiation
Saikanth Mahendra,1 Karla D. Thrall2
1Northwest Medical Physics Center, Lynnwood, WA; 2SNBL USA, Ltd., Everett, WA
Radiation Research Society (RRS) Annual Meeting, Puako, HI (2016).
The characteristics of a linear accelerator generated 6MeV radiation beam are well suited for development of radiation-induced skin injury models. Based on literature, swine has been widely accepted as the closest skin model compared to humans. To study the evolution of cutaneous radiation injuries, the lowest electron energy – 6MeV, available in our Varian 21EX linear accelerator was preferred, taking the below parameters into consideration – high surface dose deposition within the first few millimeters, uniformity of dose distribution across the irradiation site and practical range (Rp) of the beam. The dorsal skin surface of three different porcine breeds – Yorkshire, White Sinclair and Göttingen were exposed to a single fraction of 25, 35, 45, and 60Gy to two 4-cm circular fields using a nominal dose rate of 4Gy/min. Ultrasound measurements of swine skin thickness enabled us to employ a 0.8cm superflab bolus material to ensure approximately 95% dose homogeneity from the swine skin surface to a depth of 0.7cm. The depth of maximum dose (dmax) was found to occur around 0.5cm below the swine skin surface. A steep dose fall off beyond the dose deposition depth of 0.8cm was desired to limit the dose to deeper tissues to decrease any normal tissue complications. Relative doses of less than 50% at 1.5cm, 15% at 2cm and less than 5% at 2.2cm below the swine skin surface were observed with virtual water phantom measurements which mimic tissue. The percentage depth-dose dosimetry was characterized using a small-volume parallel-plate ionization chamber which serves as an ideal choice in measuring doses near to the skin surface. The output factors and uniformity profiles were measured using Electron diodes for desired applicators. A 20×20 applicator cone was used along with 3.2mm Lead (Pb) surface cutouts to minimize the radiation penumbral effects on the skin ensuring a more defined irradiation boundary. The variability of beam uniformity – both flatness and symmetry across the 4cm circular irradiated field was observed to be less than 1%. Optically Stimulated Luminescence (OSL) dosimeters were placed on the dorsal surface to evaluate dose delivery and below the lead cutouts to evaluate shielding efficacy and average measured differences were less than 2% and 0.25% respectively.