In vivo dose estimations through transit signal measured with thimble chamber positioned along the central axis at epid level in medical linear accelerator in carcinoma of middle third esophagus patients undergoing 3DCRT

This study presents a method to estimate mid plane dose (D_iso,transit) in vivo from transit signal (S_t) measured with thimble ionization chamber in cancer of middle third esophagus patients treated with three dimensional radiotherapy. This detector is positioned at the level of electronic portal imaging device in the gantry of a medical linear accelerator. Efficacy in homogeneity corrections of three dose calculation algorithms available in XiO treatment planning system (TPS) for planned dose (for open fields) (D_iso,TPS) was studied with three heterogeneous phantoms. (D_iso,transit) represents measured signal at transit point far away correlating to dose at isocentre. A locally fabricated thorax phantom was used to measure the in vivo mid plane dose (D_iso,mid) which was also estimated through St. Fifteen patients with carcinoma of middle third esophagus treated with 3DCRT were under taken. Transit signals were recorded (three times, with a gap of 5 - 6 fractions during the course of treatment) to estimate D_iso,transit, which was compared with the doses calculated by TPS. The dose predictions by superposition algorithm was superior compared to the other algorithms. Percentage deviation of D_iso,transit, D_iso,mid with D_iso,TPS combined all fields was 2.7 and -2.6% respectively with the thorax phantom. The mean percentage deviation with standard deviation of estimated D_iso,transit with D_iso,TPS observed in patients is -0.73% ± 2.09 (n=39). Mid plane dose estimates in vivo using this method provides accurate determination of planned delivered dose in middle third esophagus treatments. This method could be useful in similar clinical circumstances for dose confirmation and documentation. (author)