• 한국의학물리학회지:의학물리
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• 제13권2호
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• pp.74-78
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• 2002

환자 치료시 환자의 위치고정과 매번 치료에서 환자 치료 자세의 재현을 위해 사용하는 고정물(immobilizer)중 Vacuum cushion을 사용시 Vacuum cushion으로 인해 예상되는 표면선량 또는, 투과량의 변화들을 측정하여 Vacuum cushion의 특성을 평가하였다. 광자선 에너지 4 MV (Varian 4/100, 미국), 6MV, 15 MV (Varian CL2100C/D, 미국)에 대해서 조사면의 크기를 5$\times$5, 10$\times$10, 20$\times$20, 30$\times$30, 40$\times$40 $\textrm{cm}^2$로, Vacuum cushion의 두께는 12, 32, 48 mm, 그리고 스티로폼이 없이 진공 봉지만 있는 경우로 변화해가며 Vacuum cushion에 대해 팬톰 표면에서 d$_{max}$까지의 선량을 측정하였다. 그 결과 vacuum cushion 두께에 대한 투과율은 0.9953-1.0043의 분포로 거의 차이가 없었다. 그리고 vacuum cushion의 두께가 두꺼워질수록, 환자가 받는 표면 선량은 증가하였다. 에너지, 조사면 크기에 대해 Vacuum cushion의 두께에 따라 표면 선량의 변화가 있었으나 6 MV와 15 MV에 대해 알려진 aquaplast의 데이타와 가장 두꺼운(48mm) vacuum cushion의 표면선량 증가율을 비교시 aquaplast보다 대략 16, 12% 낮아 임상에 적용하는데 무리가 있을 만큼 심각한 문제가 아니었다.

• 대한방사선기술학회지:방사선기술과학
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• 제25권1호
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• pp.63-71
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• 2002

The aim of this study Is to develop a simple and fast method which computes in-vivo doses from transmission doses measured doting patient treatment using an ionization chamber. Energy fluence and the dose that reach the chamber positioned behind the patient is modified by three factors: patient attenuation, inverse square attenuation. and scattering. We adopted a straightforward empirical approach using a phantom transmission factor (PTF) which accounts for the contribution from all three factors. It was done as follows. First of all, the phantom transmission factor was measured as a simple ratio of the chamber reading measured with and without a homogeneous phantom in the radiation beam according to various field sizes($r_p$), phantom to chamber distance($d_g$) and phantom thickness($T_p$). Secondly, we used the concept of effective field to the cases with inhomogeneous phantom (patients) and irregular fields. The effective field size is calculated by finding the field size that produces the same value of PTF to that for the irregular field and/or inhomogeneous phantom. The hypothesis is that the presence of inhomogeneity and irregular field can be accommodated to a certain extent by altering the field size. Thirdly, the center dose at the prescription depth can be computed using the new TMR($r_{p,eff}$) and Sp($r_{p,eff}$) from the effective field size. After that, when TMR(d, $r_{p,eff}$) and SP($r_{p,eff}$) are acquired. the tumor dose is as follows. $$D_{center}=D_t/PTF(d_g,\;T_p){\times}(\frac{SCD}{SAD})^2{\times}BSF(r_o){\times}S_p(r_{p,eff}){\times}TMR(d,\;r_{p,eff})$$ To make certain the accuracy of this method, we checked the accuracy for the following four cases; in cases of regular or irregular field size, inhomogeneous material included, any errors made and clinical situation. The errors were within 2.3% for regular field size, 3.0% irregular field size, 2.4% when inhomogeneous material was included in the phantom, 3.8% for 6 MV when the error was made purposely, 4.7% for 10 MV and 1.8% for the measurement of a patient in clinic. It is considered that this methode can make the quality control for dose at the time of radiation therapy because it is non-invasive that makes possible to measure the doses whenever a patient is given a therapy as well as eliminates the problem for entrance or exit dose measurement.