A low Z linac and flat panel imager: comparison with the conventional imaging approach

• Published in: Physics in medicine & biology 2008-11, Vol.53 (22), p.6305-6319
• Main Authors: Roberts, D A, Hansen, V N, Niven, A C, Thompson, M G, Seco, J, Evans, P M
• Format: Article
• Language: English
• Subjects: Monte Carlo Method; Phantoms, Imaging; Photons; Radiation Dosage; Radiotherapy Planning, Computer-Assisted
• ISSN: 0031-9155; 1361-6560
• DOI: 10.1088/0031-9155/53/22/003

Experimental and Monte Carlo simulations were conducted for an Elekta Ltd Precise Treatment System linac fitted with a low Z insert of sufficient thickness to remove all primary electrons. A variety of amorphous silicon based panels employing different scintillators were modelled to determine their response to a variety of x-ray spectra and produce an optimized portal imaging system. This study has shown that in a low Z configuration the vast majority of x-rays are produced in the nickel electron window, and with a combination of a carbon insert and caesium iodide based XVI-panel, significant improvement in the object contrast was achieved. For thin, head and neck-type geometries, contrast is 4.62 times greater for 1.6 cm bone in 5.8 cm water than the standard 6 MV/iViewGT system. For thicker, pelvis-type geometries contrast increases by a factor of 1.3 for 1.6 cm of bone in 25.8 cm water. To obtain images with the same signal-to-noise ratio as the 6 MV/iViewGT system, dose reductions of a factor of 15 and 4.2 are possible for 5.8 cm and 25.8 cm phantoms respectively. This design has the advantage of being easily implemented on a standard linac and provides a portal image directly from the therapy beam aperture.