The objective of this proposal is the development of a "Kinaesthetic Charge Detection (KCD)" system for megavoltage portal imaging. A critical aspect of radiotherapy is the verification of the localized treatment volume with respect to treatment fields. This verification is performed by comparing the portal images with simulation images. The former are generated by megavoltage x-rays, while the latter, used for target localization, are produced by kilovoltage x-rays. Portal images suffer from decreased resolution and poor contrast as compared with simulation images. It is this loss in contrast that leads to poor visualization, hence limiting the accuracy of the verification. KCD imaging offers the potential of increased contrast and quantum detection efficiency (QDE). To date, this technology has been successfully applied to diagnostic imaging. Preliminary results from a small field of view (FOV) research prototype support the hypothesis that, as compared to currently available commercial portal imaging systems: conventional portal imaging and electronic portal imaging devices (EPIDs), the megavoltage KCD system provides images superior in contrast and comparable in resolution, while delivering a small fraction of the dose to the patient. The proposed large FOV prototype KCD system will incorporate 512 detector channels on two separate imaging boards with 0.5 mm and 0.25 mm collector electrode spacing respectively, utilizing a new cylindrical chamber geometry. It will have resolution >1.0 cy/mm, QDE >30%, and increased image contrast over the small FOV prototype. The system's image quality will be modeled and measured using Monte Carlo simulations and test patterns. Preclinical evaluation of image quality will be made using anthropomorphic phantoms. An intercomparison will then be made among the KCD system, conventional portal films and two commercially based EPIDs (Philips and Siemens). A clinical comparison of the megavoltage KCD system and port filming will be done on three anatomical sites of interest: head and neck, lung, and prostate. The major impact of significantly improved digital portal imaging on health care will be improved target dose delivery through better visual verification of the adequacy and reproducibility of radiotherapy techniques, including patient positioning, immobilization and linac parameter settings. This could facilitate the rapid introduction of conformal therapy into clinical practice by allowing for improved identification of anatomical structures compared to current imaging modalities.