Clinical implementation of MOSFETs for entrance dose in-vitro dosimetry with high energy photons for external beam radiation therapy

Abstract

In external beam radiotherapy quality assurance is carried out on the individual components of the treatment chain. The patient simulating device, planning system and linear accelerators are tested regularly according to set protocols developed by national and international organizations. Even though these individual systems are tested errors can be made in the transfer between systems. The best quality assurance for the system is at the end of the treatment planning chain. In-vivo dosimetry measures the dose to the target volume through indirect measures at the end of the treatment planning chain and is therefore the most likely method for picking up errors which might occur earlier in the chain. Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) have been shown to have a similar error in estimating entrance dose for in-vivo dosimetry to diodes, but no studies have been done clinically with entrance dose in-vivo dosimetry with MOSFETs. The time savings for using MOSFETs makes them preferable to TLD's. Due to their small size and versatility in other applications they are useful as more than dedicated in-vivo dosimetry systems using diodes. Clinical implementation of external beam in-vivo dosimetry would add another use to the MOSFETs without purchasing more specialized equipment. My studies have shown that MOSFETs can be used clinically for external beam in-vivo dosimetry using entrance dose measurements. After the MOSFET measurement system was implemented using a custom built aluminium build up cap clinical measurements were performed. A total of 23 patients and 54 fields were studied. The mean for all clinical measurements was 1.3 %, with a standard deviation of 2.6 %. Results were normally distributed around a mean with skewness and kurtosis as -0.39 and 0.34 respectively. For breasts the mean was 1.8 %, with a standard deviation of 2.7 %. For prostates and hips the mean was 1.3 % with a standard deviation of 2.9 %. These results are similar to studies conducted with diodes and TLD's. From these results one can conclude that MOSFETs can be used for entrance dose in-vivo dosimetry and are no worse than diodes or TLD's in terms of their measurement accuracy.