Small field electron beam dosimetry using MOSFET detector

Md Nurul Amin,Mohammad K Islam,Bern Norrlinger,Robert Heaton

doi:10.1120/jacmp.v12i1.3267

Abstract

The dosimetry of very small electron fields can be challenging due to relative shifts in percent depth‐dose curves, including the location of dmax, and lack of lateral electronic equilibrium in an ion chamber when placed in the beam. Conventionally a small parallel plate chamber or film is utilized to perform small field electron beam dosimetry. Since modern radiotherapy departments are becoming filmless in favor of electronic imaging, an alternate and readily available clinical dosimeter needs to be explored. We have studied the performance of MOSFET as a relative dosimeter in small field electron beams. The reproducibility, linearity and sensitivity of a high‐sensitivity microMOSFET were investigated for clinical electron beams. In addition, the percent depth doses, output factors and profiles have been measured in a water tank with MOSFET and compared with those measured by an ion chamber for a range of field sizes from 1 cm diameter to 10 cm× 10 cm for 6, 12, 16 and 20 MeV beams. Similar comparative measurements were also performed with MOSFET and films in solid water phantom. The MOSFET sensitivity was found to be practically constant over the range of field sizes investigated. The dose response was found to be linear and reproducible (within ±1% for 100 cGy). An excellent agreement was observed among the central axis depth dose curves measured using MOSFET, film and ion chamber. The output factors measured with MOSFET for small fields agreed to within 3% with those measured by film dosimetry. Overall results indicate that MOSFET can be utilized to perform dosimetry for small field electron beam.PACS number: 87.55.Qr

Highlights

IntroductionShape and location of the lesion, the field aperture, commonly referred to as cutout, is custom-made using cerrobend
Small superficial cancerous lesions are typically treated with electrons
The results obtained by MOSFET, film and ion chamber agreed well for field sizes equal to and higher than 4 cm diameter

Summary

Introduction

Shape and location of the lesion, the field aperture, commonly referred to as cutout, is custom-made using cerrobend. Accurate dosimetry of such small electron fields usually requires dosimetric measurement. Accurate dosimetry for such small fields is challenging due to the loss of lateral electronic equilibrium within the field This can result in a shift of dmax towards the surface and other modifications of the depth dose curve, as well as a change in the beam profile characteristics at depth. The use of such fields in the clinic requires careful and detailed measurements to characterize both the output and coverage achieved

Methods

Results

Discussion

Conclusion