SU-E-T-87: Commissioning of a 50-100 KV X-Ray Unit for Skin Cancer Treatment

Abstract

Purpose: To provide dosimetric data for a 50–100 kV X-ray unit for skin cancer treatments. Discrepancy of measurements using two different types of chambers was investigated. Methods: Two superficial treatment units SRT-100 from Sensus were studied. The unit has three energies: 50, 70 and 100 kV x-ray and five cones from 1.5 to 5 cm diameters. Data collected during commission were: (a) HVL, (b) dose rate, cGy/minute at 15 cm target-to-skin distance in air under a 5 cm cone, and (c) cone size factors. A Farmer-type thimble chamber, PTW 23333, and a thin-window parallel plate (PP) ion-chamber, PTW23342 were used. Measured data were compared with vendor provided values. Results: Average HVL values for 50, 70 and 100 kV of the two units were 0.52, 1.15 and 2.2 mm Al, respectively. HVL values were 5–9 % lower if the thimble chamber was used for 70 and 100 kV. Dose rate were found 3–4% lower with the thimble chamber compared with the value from the PP chamber. As expected, the two type chambers measured the same cone size factors, from 0.97 to 1.0 for 1.5 to 5 cm cone. When the same PP chamber was used for another SRT-100 unit with a similar HVL value, the dose rates were found with a 2–9% difference for 50–100 kV between the two units. Dosimetric data from vendor should be used as a reference only. Conclusions: Data obtained from a thimble chamber underestimated the dose by 3–4% compared with measured by a PP chamber. Dose rate from two identical treatment units can differ by 9%. Commissioning of a soft X-ray machine should be done with proper dosimetric methods.