SU‐E‐T‐238: OnedosePlus MOSFET Dosimeter Performance in Tangential Field in Vivo Dosimtery

Abstract

Purpose: To investigate the incident angle dependence of the OnedosePlus MOSFET dosimeter and to evaluate the dosimeterˈs performance in vivo dosimetry for tangential fields treatments. Methods: Two types of external beam radiation plan were generated based on a solid water phantom: one with a group of oblique beams directed toward the anterior surface of the phantom and other with two opposing half blocked beams directed parallel to the anterior surface. The second plan represented an extreme case of tangential field treatments. In the deliveries of the plans, OnedosePlus dosimeters were placed at various locations on the phantom anterior surface. Calibrated Radiochromic EBT2 film dosimeters were placed at the same locations of the phantom anterior surface and at the corresponding locations 1cm away from the anterior surface. The measured doses from the OnedosePlus dosimeters were compared against the corresponding doses in the plans and the film measurements. Results: As the incident beam angle increased from 0 to 90 degree to the CAX, the OnedosePlus reading increased 15%. The doses measured using the OnedosePlus dosimeters in the extreme tangential field plan did not have a clear correspondence to either the surface dose or the doses at 1 cm depth. The film readings were in close agreement with the TPS calculated doses with an average dose difference less than 5%. The OnedosePlus measured doses were higher than the TPS calculated doses at the 1cm depth by a constant (average: 31.45+/− 2.71cGy), and much higher than the surface film measurements by an average of 246%. Conclusions: OnedosePlus dosimeter readings have a strong dependence on the incident beam angle, especially when the beam angle is larger than 45 degree. The point of measurement for OnedosePlus dosimeter is unclear for the treatments involving tangential beams and it overestimates doses for the assumed point of measurement (1cm depth).