FIRST CLINICAL-TESTS USING A LIQUID-FILLED ELECTRONIC PORTAL IMAGING DEVICE AND A CONVOLUTION MODEL FOR THE VERIFICATION OF THE MIDPLANE DOSE

Background and purpose: Recently, algorithms have been developed ro de rive the patient dose from portal dose measurements using a liquid-fil led electronic portal imaging device. These algorithms have already be en validated for several phantom geometries irradiated under clinical conditions. It is the aim of the present study to investigate the appl icability of a liquid-filled electronic portal imaging device in combi nation with these algorithms for two-dimensional midplane dose verific ation in clinical practice. Measurements and methods: Portal dose imag es were obtained during several patient treatments under routine clini cal conditions. Measurements were performed to verify the midplane dos e during radiotherapy of larynx cancer with 4 MV beams, breast and lun g cancer with 8 MV beams and prostate cancer with both 8 and 18 MV bea ms. Midplane doses, determined from portal dose measurements and analy zed with our algorithms, were compared with midplane doses calculated with our three-dimensional (3D) treatment planning system (TPS). Resul ts: For the larynx treatment the measured 2D midplane dose agreed with in 2.0% with TPS calculations in most parts of the field. Larger diffe rences were found in a small region below the skin due to the absence of electron equilibrium, which is not taken into account in our portal dose analysis. For breast irradiations the measured midplane dose sho wed a homogeneous distribution in the AP direction in the axial plane, while high dose regions were observed in the cranial and caudal part of the breast. Portal dose measurements and TPS calculations agreed wi thin 2.5% for most of the prostate and lung irradiations. For a few of the prostate and lung treatments larger local differences were found due to differences between the actual patient anatomy and the planning CT data, e.g. as a result of variable nas filling in the rectum and a natomical changes in the lung. Conclusions: Portal dose measurements w ith a liquid-filled electronic portal dose for various treatment sites in clinical practice. Portal in vivo dosimetry has proven to be impor tant in detecting changes in the patient's anatomy and its influence a n the dose delivery. It is concluded that portal dosimetry is an excel lent tool for accurate and independent verification of the dose in the entire (2D) midplane during patient treatment. However, a limited num ber of patients were involved in this study and the results are theref ore preliminary. More research is needed to fully assess the clinical value of portal dose measurements. (C) 1998 Elsevier Science Ireland L td. All rights reserved.