Session Item

We conducted an independent dosimetry audits to verify output dose and symmetry at the national level for all 27 Tomotherapy machines in Korea.

We were established an external dosimetry audit system using the radiophotoluminescent glass dosimeters (RPLD). A specific phantom was designed and fabricated according to the recommendations of AAPM TG-148 and IAEA TRS-398 using the water equivalents of plastic materials to measure output dose with RGDs (GD-302M, Asahi Techno Glass, Japan) [Figure 1]. Using this phantom, an independent dosimetry audit conducted for a total of 27 Tomotherapy machines operating in Korea for static output and symmetry at gantry 0 degree, and rotational output dose using the method of the end-to-end test. The output doses were analyzed by taking readings through a glass dosimeter reader (FGD-1000 SE, AGC Technology Solutions Co., Ltd., Japan) and applying several correction factors; scaling effect, energy dependence, and radio sensitivity. All dosimetry audits were performed according to the specific procedures. The output dose evaluation criteria were evaluated as an optimal level within ±3% (RGD uncertainty at 1σ) and a tolerance level within ±5% (ICRU recommendation). In addition, in the case of symmetry, within 2% (manufacturer acceptance criteria) as an optimal level and within 3% (RGD uncertainty at 1σ) as a tolerance level were evaluated.

Figure 1 Fabrication of phantom to measure using a RGDs; static output (left) and rotational output (right)

The static and rotational output dose were evaluated to be within the tolerance level (within ± 5%) for all machines. Additionally, the symmetry was also evaluated to be within the tolerance level (within 3%) for all machines. As a result of comparing PDD data collected from each institution for the independent dosimetry audits, it was confirmed that PDD_20,10 of beam quality was between 0.52 and 0.53 in most machines except for one machine [Figure 2].

Figure 2 The results of dosimetry audits for Tomotherapy machines

In this study, independent dosimetry audits were performed on the output dose and symmetry for 27 Tomotherapy machines. As a result of performing corrective actions and repeated processes, it was confirmed that all institutions met the tolerance level. These results indicate the quality control status of Tomotherapy machines used for radiation therapy in Korea. According to the proposed the independent external quality audit procedures, we can help establish a national-level quality management system and the SSDL (Secondary Standard Dosimetry Laboratory) based independent quality audits allow Tomotherapy high-energy beam to be safely used in radiation therapy.