SU‐FF‐T‐339: Small Field Electron Beam Dosimetry Using MOSFET Detector

Abstract

Purpose: 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. With the advent of electronic imaging, modern radiotherapy departments are moving to a filmless environment and an alternate clinical dosimeter is required for routine clinical dosimetry. In this work, we have studied the performance of MOSFET as a relative dosimeter for small field electron beams. Methods and materials: The reproducibility, linearity and sensitivity of a high sensitivity micro MOSFET 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 and film for a range of field sizes from 1 cm diameter to 10×10 cm2 for 6, 12, 16 and 20 MeV beams. Similar comparative measurements were also performed with MOSFET as well as with films in solid water phantom. Results: The MOSFET sensitivity was found to be stable over the range of field sizes investigated. The dose response was found to be linear and reproducible (within ±1% for 100 cGy). For 1, 1.5, 2, 2.5, 3, 4, 5, 6 cm circular and 10 cm square field, excellent agreement (within ±2% and ±2mm) 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 2% with those measured by film dosimetry. Conclusion: Overall results indicate that MOSFET can be utilized to perform dosimetry of small electron fields for routine clinical use.