On the measurement of the effective linear attenuation coefficient mueff in 60Co wedge fields
Zaránd, P.; Polgár, I.
Strahlentherapie und Onkologie Organ der Deutschen Rontgengesellschaft . 172(4): 211-217
1996
ISSN/ISBN: 0179-7158 PMID: 8623084 Document Number: 460884
Purpose: It is difficult to obtain a correct measurement of mu-eff. Moreover, wedge calculations using a single mu-eff value are not correct. We present a program for the analysis of wedge measurements. Materials and Methods: The program employs parameters of an open field to correct the dosimetry of a wedge field. For this the ASCII export files of an MP3 (PTW) system are automatically imported into our program. The principle is illustrated for ionization measurements from a Chisobalt 2B75 cobalt unit. The wedge field dose profiles are corrected by the off-centre ratio of the open field and mu-eff is analysed as well as its dependents on geometry. Results: We found that the dose attenuation by a wedge can be described by the product of wedge thickness and the effective linear attenuation coefficient mu-eff. However, mu-eff values for a given field size decrease with depth. This is interpreted as an effect of beam hardening. For the usual fields in radiotherapy this effect is independent of field size and can be described as an exponential function of wedge thickness. Conclusion: The effective linear attenuation coefficient mu-eff is depth-dependent. A correction could be added to planning programmes, i. e.: exp. (DELTA-mu cntdot t-w cntdot (d-d-max)).