Patient dose estimation in TBI using portal film transit dosimetry in combination with semiconductor detectors

The ability of portal films in combination with diode detectors in estimating in-vivo midline dose distributions during Total Body Irradiation (TBI) has been investigated. A method to estimate midline dose distributions in a lateral field technique for Total Body Irradiation (TBI) by using diodes and portal films has been suggested. The proposed technique has been verified in a phantom simulating quasi-clinical conditions. In order to better assess the reliability of the method in clinical conditions, we tested an algorithm for estimating midline doses in TBI in a number of situations with cork inhomogeneities both symmetrically and asymmetrically positioned with respect to the phantom midline, by on-axis diodes measurements. A good agreement between estimated and measured off-axis midline doses has been found in correspondence of cork inhomogeneities on a phantom simulating the head-thorax-abdomen region; when using high-Z lungs attenuators, the agreement is still acceptable. Regarding on-axis measurements, this agreement is within 1% (considering the sum of opposed fields for nonsymmetrical inhomogeneities), when the longitudinal dimension of the inhomogeneities is large (18 cm). For the critical case of inhomogeneities with a very small longitudinal dimension (6 cm) the agreement is worse but still acceptable for Quality Assurance aims (4.5 % in the worst situation of "large" distance between the inhomogeneities, better than 2 % in the other situations). The reproducibility of the exit dose measurements by portal films has been found within 2 % (1 S.D.), corresponding to about 1 % at the midline level. Results indicate the possibility of application of portal films in combination with semiconductors in the in-vivo estimation of midline dose distributions in lateral opposed fields TBI techniques.