Comparison of peripheral dose measurements using Ionization chamber and MOSFET detector

Abstract

Background

In radiation therapy, the peripheral dose (PD) – the dose outside the geometric boundaries of the radiation field – is of clinical importance. A metal oxide semiconductor field effect transistor (MOSFET) detector is used to estimate the peripheral dose.

Aim

The aim of this study is to investigate the ability of a MOSFET dosimetry system to accurately measure doses in peripheral regions of high energy X-ray beams.

Materials & Methods

The accuracy of the MOSFET system is evaluated by comparing peripheral region dose measurement with the results of standard ionization chamber measurements. Furthermore, the measurement of PD using a MOSFET detector helps us to keep the tolerance dose of any critical organ closer to the treatment field within the acceptable limits. The measurements were carried out using a 0.6 cc Farmer type ionization chamber and MOSFET 20 dosimetry system for field sizes ranging from 5 × 5 cm² to 20 × 20 cm² at three depths of 1.5 cm, 5 cm and 10 cm in a blue water phantom. PD were measured at distances varying from 1 cm to 30 cm from the field edges along the x axis for the open fields, with collimator rotation and with beam modifiers like 15 degree, 30 degree and 45 degree wedges.

Results

The results show a good agreement of measured dose by both methods for various field sizes, collimator rotation and wedges.

Conclusion

The MOSFET detector has a compact construction, provides instant readout, is of minimal weight and can be used on any surface.