Midplane dose determination using in vivo dose measurements in combination with portal imaging.

The possibility of using portal films in combination with semiconductor in vivo measurements for midplane dose distribution is investigated. A general algorithm, using measured entrance and exit doses and available beam data of the Linac, is proposed to derive the midplane dose for symmetrical inhomogeneities. Experimental verification of the algorithm with phantom measurements is performed for different kinds of inhomogeneities (Al, air and cork) and phantom thicknesses from 13 cm to 30 cm. When using only the entrance dose and the exit dose, provided by the diodes on the beam axis, the algorithm predicts for the different inhomogeneities midplane doses in all cases within 1% of the midplane doses measured with an ionization chamber. When using the portal film in combination with entrance and exit dose measurements to estimate the midplane dose in an off-axis position, the calculated midplane doses are within 3% of the midplane doses measured with an ionization chamber. The midplane doses calculated with the algorithm are compared to the midplane doses obtained with simplified calculation methods, i.e. the arithmetical mean and the geometrical mean of the measured entrance and exit doses. The geometrical mean especially seems to give acceptable results (within 5%) and can as such be used as an easy rule of thumb to estimate roughly the midplane dose. Finally, critical considerations on the validity and the precision of the proposed algorithm are given. The present results confirm the possibility of using the portal film for midplane dose distribution determination at the patient level.