Initial patient imaging with an optimised radiotherapy beam for portal imaging

Background and purpose To investigate the feasibility and the advantages of a portal-imaging mode on a medical accelerator, consisting of a thin low-Z bremsstrahlung target and a thin Gd 2 O 2 S/film detector, for patient imaging. Patients and methods The international code of practice for high-energy photon dosimetry was used to calibrate dosimetry instruments for the imaging beam produced by 4.75MeV electrons hitting a 6mm thick aluminium target. Images of the head and neck of a humanoid phantom were taken with a mammography film system and the dose in the phantom was measured with TLDs calibrated for this beam. The first head and neck patient images are compared with conventional images (taken with the treatment beam on a film radiotherapy verification detector). Visibility of structures for six patients was evaluated. Results Images of the head and neck of a humanoid phantom, taken with both imaging systems showed that the contrast increased dramatically for the new system while the dose required to form an image was less than 10 −2 Gy. The patient images taken with the new and the conventional systems showed that air–tissue interfaces were better defined in the new system image. Anatomical structures, visible on both films, are clearer with the new system. Additionally, bony structures, such as vertebrae, were clearly visible only with the new system. The system under evaluation was significantly better for all features in lateral images and most features in anterior images. Conclusions This pilot study of the new portal imaging system showed the image quality is significantly improved.