Determination of KQ Factor for PTW Bragg Peak Chamber Used in Proton Pencil Beam Dose Calibration
B Arjomandy1*, W Hsi1, V Nazaryan2, M Pankuch3, V Kozlyuk1, N Sahoo4, (1) McLaren Cancer Institute, Flint, MI, (2) ProTom International, Inc., Davison, MI, (3) ProCure Treatment Centers, Warrenville, IL, (4) MD Anderson Cancer Center, Houston, TXSU-E-T-77 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: Dose calibration protocols such as AAPM TG51 for photons and IAEA TR389 for protons recommend using specific KQ quality factor for different ionization chambers for absolute dosimtery in radiation therapy beams. However, there is no quality factor for Bragg Peak chambers (BPC) which are used for proton pencil beams. We have investigated a method to determine the KQ value for this chamber for proton pencil beam dose calibration.
Method and Material: A uniform scanning proton beam from a cyclotron accelerator was used to irradiate a PTW-BPC and an ADCL calibrated IBA-PPC05 parallel plate chamber (PPC) to obtain the Nw x KQ factor for BPC. Distinctly, measurements of number of protons and the dose were measured simultaneously using the BPC, a Faraday cup, and EBT3 film dosimetry using a single proton pencil beam with a synchrotron accelerator. The calibration of film absolute dosimetry was assessed in a calibrated proton beam with a SOBP width of 10-cm and range of 16-cm. The Nw for BPC was determined using TG21 protocol against a PTW-PPC in a high energy electron beam.
Results: Our analysis indicates that KQ for BPC is equal to 1.08 with an uncertainty of ±1%. Independent measurements of the same chamber at different cyclotron proton center produced an agreement of 0.4% for Nw x KQ factor. However, the determination of Nw. KQ of different chamber using a synchrotron proton therapy center disagreed by 6%. The Nw obtained from TG21 protocol was in agreement with manufacturer value to within 0.2%.
Conclusion: This investigation produced some results that suggest that there is a possible dose rate dependency of BPC in proton beams. This result is currently under investigation, but it is believed that the KQ factor is not affected by this dependency.