Improving Health Through Medical Physics

AAPM Newsletter — Volume 42 No.5 — September|October 2017

IROC REPORT Paola E. Alvarez, MS, Houston, TX

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IROC Houston's TLD and OSLD Dosimetry Systems for Remote Audits of Radiotherapy External Beam Calibration

The IROC-H QA Center is funded to provide radiation oncology core support and quality audit programs in support of the National Cancer Institute's (NCI's) clinical trials. These programs help to ensure high quality clinical trial data, thereby maximizing clinical trial power and improving clinical outcomes for cancer patients. One of the initial requirements for an institution wishing to participate in NCI- funded clinical trials is the verification of the reference output ofthe beams used for radiotherapy treatment. This program began at the Radiological Physics Center (now IROC-H) in 1968 and has continually grown over the decades, and during 2016 it monitored nearly 2,200 institutions and verified the output of more than 16,700 beams, making this by far the largest program of its size in the world.

IROC-H's remote dosimetry audit program is based on passive detectors that can be mailed, have a simple irradiation geometry, retain their signal after irradiation in a predictable manner, and can be read -out reproducibly. Both TLD and OSLD are commissioned to be used for photon and electron clinical energies. TLD was the primary dosimeter in use up until 2010, and this program was previously reviewed by Kirby et al., (1992); a similar program operated by the IAEA has also been described (Izewska et al., 2003). In 2010, IROC-H began to primarily use OSLD because it had a much shorter readout process, improving throughput and thereby being less costly. At the time, OSLD achieved comparable accuracy and precision as TLD.

The TLD and OSLD beam audit results of more than 20,000 beam output checks are shown in figure 1. This reference output audit program reports the ratio of the dose measured to the dose reported by the institution. The average ratio was 1.000 ± 1.9% for TLD and 0.999 ± 1.7% for OSLD. Of the ~20,000 measurements, ~480 (2.4%) were outside the ±5% tolerance established by IROC-H, prompting follow-up action. For the OSLD program, our analysis found the uncertainty in the reported dose from this dosimetry system was 1.7% at one sigma level, which is slightly larger than the 1.3% stated by Kirby et al. for TLD. This difference is in large part because only 2 dosimeters are used instead of 3 for TLD. This calculated uncertainty is consistent with the observed standard deviation in the OSLD results from multiple experimental readings (Fig. 1). These data indicate that the OSLD program is comparable to the TLD program in terms of accuracy. These results also support the continued use of the ±5% tolerance for output, as, given the standard deviation calculated and observed, only 0.12% of results would be expected to fall outside of this tolerance criteria because of measurement uncertainty (i.e., fewer than 25 results out of 20,000 would be expected to fall outside of ±5%, while 480 were measured to be outside this tolerance).

Fig. 1 Histogram of results of mailed output check verification done with TLD and OSLD
Izewska, J., Andreo, P., et al., 2003. The IAEA/WHO TLD postal dose quality audits for radiotherapy: a perspective of dosimetry practices at hospitals in developing countries. Radiotherapy Oncol. 69 (1), 91e97.
Kirby, T.H., Hanson, W.F., et al., 1992. Uncertainty analysis of absorbed dose calculations from thermoluminescence dosimeters. Med. Phys. 19 (6), 1427e1433.

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