[Examination of phantom scatter factors in irregular fields with enhanced dynamic wedge].

ABSTRACT

Use of an enhanced dynamic wedge (EDW) changes the size of the irradiation field in real time because of the shift of the flexible aperture during irradiation. Therefore, when EDW is used, it is speculated that the scatter factors proposed by Khan et al. may differ from that obtained in an open irradiation field. The aim of the present study was to investigate whether the total scatter factors (S(cp)) can also be separated into the collimator scatter factors (S(c)) and phantom scatter factors (S(p)) when using EDW, and whether S(p) in an irregular irradiation field using EDW and one without the use of EDW can likewise be handled. Results indicated that the values of S(cp) and S(c) in a square or rectangular irradiation field using EDW tended to differ from those in an open irradiation field, whereas S(p) was almost the same, with a slight variation of about 0.5%, indicating that S(cp) could be separated into S(c) and S(p). However, for an irregular irradiation field, the variations in S(p) exceeded 1% in some cases irrespective of the use or non-use of EDW. Depending on the size and geometry of the irradiation field, it was within the range of the degree of uncertainty of 2.5% in the evaluation of output radiation dosage, which was described in AAPM Report 13. Therefore, the theory proposed by Khan et al. was found to be valid also for an irregular irradiation field using EDW, which could be handled in the same manner as in the irregular irradiation field without using EDW.