Commissioning experience of a superficial kV system for guinea pig cutaneous irradiation

Saikanth Mahendra,¹ Adam Schoen,¹ Karla D. Thrall^2
1Northwest Medical Physics Center, Lynnwood, WA; ²SNBL USA, Ltd., Everett, WA

Radiation Research Society (RRS) Annual Meeting, Cancun, Mexico (2017).

Cutaneous radiation injury (CRI) studies in guinea pigs are considered a good small animal model to assess the human dermal response to low-penetrating acute irradiation exposures for prospective medical countermeasures (MCMs). The characteristics of superficial Xstrahl 100 kV irradiator were utilized for development of dose-response models and key dosimetric parameters are presented. In order to mimic the depth of penetration and dose fall off under the skin similar to a pure beta source, we selected a 60kV beam potential, a beam current of 6mA and a custom 0.15mm inherent filter setting. The irradiation area was achieved by a 4x4cm square applicator placed in contact with the skin. All measurements were performed in accordance with the AAPM Task Group 61 (TG-61) protocol. A soft X-ray parallel plate chamber was utilized in air and in a Soft X-ray slab phantom to determine different beam characteristics. The Half Value Layer (HVL) of the beam was measured in air and compared using three different purities of Aluminum (Al) filters. Al purity of 99.0%, 99.99% and 99.999% yielded 0.163, 0.171, 0.161 mm of Al as HVL’s respectively. The Percent Depth Dose (PDD) at 50% was found to be 1.884mm and less than 30% at 4mm in water equivalent plastic slabs. A steep dose fall off was desired to limit the dose to deeper tissues to decrease any normal tissue complications. The absolute beam output was determined to be 6.532 Gy/min at surface, 4.290 Gy/min at 1mm depth and 3.165 Gy/min at 2mm depth. Beam consistency and constancy check measurements were performed with minimal coefficient of variation (0.1%). GafChromic EBT-3 film was used to determine dose homogeneity across the irradiated area and was found to be well within the required +/- 5%.