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Abstract
As radiation therapy is one of three major cancer treatment methods, many cancer patients get radiation therapy. To exposure as much radiation to cancer while normal tissues near tumor get little radiation, medical physicists make a radiotherapy plan treatment and perform quality assurance before patient treatment. Despite these efforts, unintended medical accidents can occur by some errors. In order to solve the problem, patient internal dose reconstruction methods by measuring transit dose are suggested. As feasibility study for development of patient dose verification system, inverse square law, percentage depth dose and scatter factor are used to calculate dose in the water-equivalent homogeneous phantom. As a calibration results of ionization chamber and glass dosimeter to transit radiation, signals of glass dosimeter are 0.824 times at 6 MV and 0.736 times at 10 MV compared to dose measured by ionization chamber. Average scatter factor is 1.4 and Mayneord F factor was used to apply percentage depth dose data. When we verified the algorithm using the water-equivalent homogeneous phantom, maximum error was 1.65%.
References
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Fig. 1.
(a) The outline of experimental setting for dose calculation in the homogeneous phantom using the transit dose and (b) beam quality correction method of ionization chamber using IAEA TRS-398. Each Ks, Ts, f and d represent an amount of scattered dose to the bottom point, an amount of scattered dose to the transit dose measurement point, source to surface distance (SSD) and phantom thickness. Phantom dose can be calculated by measured transit dose multiplied by inverse square law factor and percentage depth dose data.
Fig. 2.
Measurement of transit radiation dose according to variation of radiation energy, field size and monitor unit using the correction completed ionization chamber to transit radiation. (a) MU dependency at 10 cm phantom (b) MU dependency at 20 cm phantom (c) MU dependency at 30 cm phantom (d) Field size dependency at 10 cm (e) Field size dependency at 20 cm (f) Field size dependency at 30 cm.
Fig. 3.
(a) 6 MV transit dose measurement results and (b) 10 MV transit dose measurement results measured by glass dosimeter of field size 4 cm×4 cm (◆), 10 cm×10 cm (■), 20 cm×20 cm (▲).
Fig. 4.
Scatter factor measurement results of (a) 6 MV photon energy and (b) 10 MV photon energy when phantom thicknesses are 10 cm (◆), 20 cm (■), 30 cm (▲).
Fig. 5.
Dose variation due to positional change of phantom center to isocenter. Relative dose change when the phantom center is above the isocenter (positive side of horizontal axis), below the isocenter (negative side of horizaontal axis) and on the isocenter (0). The results of (a) 6 MV 10 cm, (b) 6 MV 20 cm, (c) 6 MV 30 cm, (d) 10 MV 10 cm, (e) 10 MV 20 cm, (f) 10 MV 30 cm when field sizes are 4 cm×4 cm (◆), 10 cm×10 cm (■), 20 cm×20 cm (▲).
Table 1.
The comparison of ionization chamber correction between transit dose result and original TRS-398 result.
Table 2.
The comparison between real measurement results at the center and bottom of 10 cm, 20 cm, 30 cm phantom and calculation results using the Mayneord F factor.
Table 3.
Measurement result of transit radiation and comparison of calculation results using the 6 MV photon.
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