Determination of build-up region over-response corrections for a Markus-type chamber.

Over-response corrections for a widely used parallel plate ionization chamber were determined using contemporaneous measurement of build up for 4, 6, 10 and 18-MV photon beams utilizing a commercially available extrapolation chamber (PTW model 23392). The resultant over-response corrections were essentially independent of field size (5 X 5 to 30 X 30 cm) and were less for increased depth (from the surface) and higher x-ray energy. The over-response of the parallel plate chamber (Markus-type, PTW model 329) was 13.8, 10.7, 6.2 and 4.7 percent (absolute) at the surface for 4, 6, 10 and 18-MV x-ray energies. At only 2 mm depth, the over-response of the Markus chamber under investigation decreased to 50% of surface value over-response. These corrections, so derived, are applied to Markus parallel plate chamber build-up measurements by simple subtraction of the derived corrections. For example, a 6-MV percentage depth dose of 37.5% measured at the surface with the Markus chamber is reduced by 10.7% so to agree with the surface dose indicated by the PTW extrapolation chamber for this x-ray field, that is, 26.8%.