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Comparison of Absorbed-Dose to Water in High-Energy Photon Beams Due to Differences in Beam Quality Conversion Factor

N Kinoshita*, A Takemura, Y Nishimoto, A Kita, A Toi, E Murai, T Adachi, H Shioura, Radiological Center, University of Fukui Hospital, Yoshidagun, Fukui

SU-E-T-108 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: In September 2012, the Japan Society of Medical Physics published a new absorbed-dose to water clinical reference dosimetry method for external beam radiation therapy (JSMP12). In JSMP12, the use of corrections based on ionization chambers according to absorbed-dose to water references was initiated, and the values of beam quality conversion factor were updated. In this study, whether differences among the absorbed-dose to water values of JSMP12 and American Association of Physicists in Medicine Task Group 51 (TG-51) and International Atomic Energy Agency Technical Report Series No. 398 (TRS-398) arise due to differences in the respective beam quality conversion factors was examined.

Methods:The differences in basic data present in JSMP12, TG-51 and TRS-398 were analyzed in detail.
The photon beam absorbed-dose to water dosimetry protocols recommended by the three parties were performed, and the respective absorbed-dose to water was measured. PTW30013 was used as the Farmer-type chamber. A 6 MV photon beam was used as the radiation beam.

Results:The absorbed-dose to water ratio of the 6 MV photon beam was 0.999 in TG-51/JSMP12 and 1.002 in TRS-398/JSMP12. Beam quality conversion factors agreed within 0.2%, despite differences in the basic data included in beam quality index and beam quality conversion factor.

Conclusion:Despite the differences in the ion recombination correction method and in the basic data present in beam quality index and beam quality conversion factor in the beam quality conversion factor, no major differences were seen in absorbed dose. However, energy sources other than a 6 MV photon beam were not assessed, and it was therefore considered necessary to investigate whether differences in absorbed-dose to water arise due to other sources of energy.

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