Clinical implementation of MOSFET detectors for dosimetry in electron beams

Background and purposeTo determine the factors converting the reading of a MOSFET detector placed on the patient’s skin without additional build-up to the dose at the depth of dose maximum (Dmax) and investigate their feasibility for in vivo dose measurements in electron beams.Materials and methodsFactors were determined to relate the reading of a MOSFET detector to Dmax for 4–15 MeV electron beams in reference conditions. The influence of variation in field size, SSD, angle and field shape on the MOSFET reading, obtained without additional build-up, was evaluated using 4, 8 and 15 MeV beams and compared to ionisation chamber data at the depth of dose maximum (zmax). Patient entrance in vivo measurements included 40 patients, mostly treated for breast tumours. The MOSFET reading, converted to Dmax, was compared to the dose prescribed at this depth.ResultsThe factors to convert MOSFET reading to Dmax vary between 1.33 and 1.20 for the 4 and 15 MeV beams, respectively. The SSD correction factor is approximately 8% for a change in SSD from 95 to 100 cm, and 2% for each 5-cm increment above 100 cm SSD. A correction for fields having sides smaller than 6 cm and for irregular field shape is also recommended. For fields up to 20 × 20 cm2 and for oblique incidence up to 45°, a correction is not necessary. Patient measurements demonstrated deviations from the prescribed dose with a mean difference of −0.7% and a standard deviation of 2.9%.ConclusionPerforming dose measurements with MOSFET detectors placed on the patient’s skin without additional build-up is a well suited technique for routine dose verification in electron beams, when applying the appropriate conversion and correction factors.