In vivo dose estimations through transit signal measured with thimble chamber positioned along the central axis at electronic portal imaging device level in medical linear accelerator in carcinoma of the middle-third esophagus patients undergoing three-dimensional conformal radiotherapy

Objective: This study presents a method to estimate midplane dose (D_{iso, transit}) in vivo from transit signal (S_t) measured with thimble ionization chamber in cancer of the middle-third esophagus patients treated with three-dimensional radiotherapy (RT). This detector is positioned at the level of electronic portal imaging device in the gantry of a medical linear accelerator. Materials and Methods: Efficacy of inhomogeneity 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 (S_t) far away correlating to dose at isocenter. A locally fabricated thorax phantom was used to measure the in vivo midplane dose (D_{iso, mid}) which was also estimated through S_t. Thirteen patients with carcinoma of the middle-third esophagus treated with three-dimensional conformal RT were studied. S_t was recorded (three times, with a gap of 5-6 fractions during the treatment) to estimate D_{iso, transit}, which was compared with the doses calculated by TPS. Results: The dose predictions by superposition algorithm were 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 was within standard deviation -0.73% ±2.09% (n = 39). Conclusions: Midplane dose estimates in vivo using this method provide accurate determination of delivered dose in the middle-third esophagus RT treatments. This method could be useful in similar clinical circumstances for dose confirmation and documentation.