Megavoltage linac photons

Radiotherapy ionisation chambers can now be calibrated directly in megavoltage photon beams from a medical linear accelerator.

The accelerator (see Figure 1) and the beams used for the calibration are specified in the table below.

Megavoltage photon beams available for calibration at ARPANSA
Beam¹	TPR_20,10	Rate²	Pulse rate
6 MV	0.673	400 MU/min	400 Hz
10 MV	0.734	375 MU/min	200 Hz
18 MV	0.777	490 MU/min	200 Hz

¹ Nominal accelerating potential
²Linac output rate. The linac delivers approximately 1 cGy/MU at dmax in an isocentric setup (100 cm source-detector distance) with a 10 cm x 10 cm field.

Figure 1: The Elekta Synergy Linac at ARPANSA

Advantages

The direct megavoltage calibration service is intended to complement the Cobalt-60 calibration service. Instead of using k_Q correction factors calculated from the nominal dimensions of the chamber, the direct calibration allows these factors to be measured and interpolated to the user’s beam qualities.

The direct calibration is slightly more accurate than using Cobalt-60 and does not rely on assumptions about the behaviour of the ionisation chamber. A discussion of the implications of the new services is available in reference [2].

Validation

ARPANSA staff prepare a monitor chamber (left) used to confirm the linac output before measurements with the primary standard graphite calorimeter (right)

Figure 2: ARPANSA staff prepare a monitor chamber (left) used to confirm the linac output before measurements with the primary standard graphite calorimeter (right)

The technical details of the work done to implement the primary standard of absorbed dose on a linear accelerator are described in a recent Technical Report, TR166 [1]. The results of international comparisons of absorbed dose to water for linac beams performed in 2012 and 2013 are available in publications [3] and [4].

Extract from the linac photon calibration report

The linac megavoltage photons are measured in addition to Cobalt-60, and the results are presented as shown in Figure 3 as ratios to the chamber calibration coefficient at Cobalt-60. To enable users to be able to interpolate these results to different beam qualities, a quadratic fit is also given the report. An example is shown in Figure 4.

Figure 3: Excerpt from a calibration certificate for a Farmer-type chamber. To find k_Q for a user beam, a simple quadratic equation is used.

Example calibration results. The calibration coefficient ND,w is given for 60Co. The linear accelerator calibration results are presented as correction factors for the different beams

Figure 4: Example calibration results. The calibration coefficient N_D,w is given for Cobalt-60. The linear accelerator calibration results are presented as correction factors for the different beams

References

[1]. The Australian Primary Standard for absorbed dose to water (graphite calorimeter), Ganesan Ramanathan, Peter Harty, Tracy Wright, Jessica Lye, Duncan Butler, David Webb and Robert Huntley, ARPANSA Technical Report Series No. 166. Published June 2014. Available online at: TR-166.
[2] Direct megavoltage photon calibration service in Australia, D. J. Butler, G. Ramanathan, C. Oliver, A. Cole, J. Lye, and P.D. Harty, T. Wright, D. V. Webb, Australas. Phys. Eng. Sci. Med. (2014) Epub ahead of print 22 Aug 2014.
[3] Key comparison BIPM.RI(I)-K6 of the standards for absorbed dose to water of the ARPANSA, Australia and the BIPM in accelerator photon beams, S. Picard, D. T. Burns, P. Roger, P. D. Harty, G. Ramanathan, J. E. Lye, T. Wright, D. J. Butler, A. Cole, C. Oliver, D. V. Webb, Metrologia 2014 51 Tech. Suppl. 06006
[4] Comparison of the NMIJ and the ARPANSA standards for absorbed dose to water in high-energy photon beams, M. Shimizu; Y. Morishita; M. Kato; T. Tanaka; T. Kurosawa; N. Takata; N. Saito; G. Ramanathan; P. D. Harty; C. Oliver; T. Wright; D. J. Butler, Radiation Protection Dosimetry 2014; doi: 10.1093/rpd/ncu272