In Vivo Portal Dosimetry by an Ionization Chamber

As all methods for in-vivo dosimetry require special efforts many physicists are often discouraged in verifying the middle dose in a patient along the beam central axis. This work reports a practical method for the determination of the middle dose value, Dm, on the central beam axis, using a signal St, obtained by a small thimble ion-chamber positioned at the center of the electronic portal imaging device, and irradiated by the x-ray beam transmitted through the patient. The use of a stable ion-chamber reduces many of the disadvantages associated to the use of diodes as their periodic recalibration and time consuming positioning. The method makes use of a set of correlation functions obtained by the St and Dm ratios, determined by irradiating a water-equivalent phantom with 6MV, 10 MV and 15 MV x-ray beams. Several tests were carried out in phantoms with asymmetric inhomogeneities. The method here proposed is based on the determination of the water-equivalent thickness of the patient, along the beam central axis, by the treatment planning system that makes use of the electron densities obtained by a computer tomography scanner, that works with calibrated Hounsfield numbers. This way, it is therefore possible to compare the dose, Dm,TPS, obtained by a treatment planning system, with the in-vivo dose Dm value, both defined at density middle point (identified along the beam central axis, where the thick material, in terms of g cm-2, above and below, is the same). The method has been applied for the in-vivo dosimetry of 30 patients, treated with conformed beams for pelvic tumor, checking: anterior-posterior or posterior-anterior irradiations and lateral-lateral irradiations. For every checked field at least five measurements were carried out. Applying a correct quality assurance program based on the tests of the patient set-up, machine settings and calculations, results showed that the method is able to verify agreements between the dose Dm,TPS and the in-vivo dose value Dm, within 4% for 95% of the 240 measurements carried out in-vivo.