The dependency of wedge factors on field size and depth in megavoltage beams.

AIM

To investigate the variation of wedge factors on field size, depth on 2 types of telecobalt units and 3 types of medical linear accelerators and to make a simplified approach for day to day calculation procedure.

MATERIALS AND METHODS

A 0.125 cm3 ion chamber was used to determine the wedge factors which is connected to the computer controlled radiation field analyser. The wedge factors were determined for field sizes varying from 5 x 5 cm to the maximum square field size available for each wedge angles and at multiple depths upto 25 cm of respective teletherapy units. The results obtained are fitted to a second degree polynomial function.

RESULTS

There is no significant variation of wedge factor on field size for all the 3 linear accelerators. The wedge factors are found to vary mainly at larger depths and wedge angles. The variation of wedge factors for 2 types of cobalt units were similar and increase of 3%, 4% and 5.5% is observed for wedge angles 30 degrees, 45 degrees and 60 degrees respectively with respect of depth of maximum build-up. The trend was similar for linear accelerators with maximum increase in wedge factor up to 7.5% for 60 degrees wedge angle at 25 cm depth for 6 MV photon beams.

DISCUSSION AND CONCLUSIONS

The determination of wedge factors at various field sizes and depths is essential to ensure accurate dose delivery. With experimental wedge data fitted with second degree polynomial function is a simplified and alternative method which can be adopted for routine dosimetric calculations.