Room: AAPM ePoster Library
Purpose: Investigate the feasibility of clinical application utilizing 50 kVp X-rays with flat applicators (IntraBeam, Zeiss, Germany) for superficial lesions in the head while sparing normal tissue.
Methods: Dose was measured using calibrated GAFChromic EBT3 film that was cut to follow the contour of a heterogeneous anthropomorphic head phantom (Steeve, CIRS, VA) in the axial plane and placed perpendicular to the surface of the phantom. A CT scan of the phantom was obtained for dose calculations using Radiance (GMV, Spain), a Monte Carlo based treatment planning system (TPS) for IntraBeam. Additionally, the scan was used to overlay the dose measured from film over the phantom structures. Bolus of 3 mm thickness was placed on top of the phantom to allow the entire applicator direct contact to the surface of the phantom. A 3 cm diameter flat applicator was used to deliver dose with prescription of 3.2 Gy at a depth of 8 mm. The beam profile along the central axis measured from film and calculated from the TPS were compared.
Results: The dose measured at 8 mm depth on film was 4.95 Gy which was 14.7% higher than the TPS calculated dose of 4.00 Gy. Both calculated and measured doses showed sharp dose falloffs from the skin. However, the TPS showed a 320% increase of the prescription dose at the bone interface which was not shown in EBT3 film. The dose in film decreased with increasing depth, with highest falloff in skin and brain tissue, and less steep dose falloff at the bone interface.
Conclusion: TPS showed increasing dose at bone interface due to increased photoelectric effect (PE) that the film did not show due to the film's low response to 50 keV photons. Dose measurements will be repeated with film with a high response to low-energy x-rays.