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Determination of build‐up region over‐response corrections for a Markus‐type chamber
Author(s) -
Mellenberg David E.
Publication year - 1990
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.596579
Subject(s) - ionization chamber , extrapolation , physics , photon , optics , ionization , atomic physics , nuclear physics , mathematics , ion , statistics , quantum mechanics
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 (5×5 to 30×30 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%.