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The aim of this study is to develop a simple method which computes in-vivo doses from transmission doses measured during 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(rp) and phantom thickness(Tp). Secondly, we used the concept of effective field to the cases with 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. The hypothesis is that the presence of 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(rp,eff) and Sp(rp,eff) from the effective field size. After that, when TMR(d, rp,eff) and Sp(rp,eff) are acquired, the tumor dose is as follows. Dcenter = Dt/PTF(dg, Tp) × ( SCD/SAD)² × Sp(rp,eff) × TMR(d, rp,eff) As the field size increased, the errors showed a tendency to increase. Generally, with this method the absolute midline dose was predicted with an accuracy of about 2.53% and 2.37% for 6 ㎹ and 10MV X-rays. A simple method of in-vivo dose reconstruction and verification was developed. The efficacy of this method is limited by the field size. Thus, the method developed in this study should be used with care for relatively large field sizes. Further clinical test awaits this study.
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