An Investigation About The Effect Of Tissue-equivalent Gel On Surface Dosage By Using MOSFET Detector

Objective: Using metal oxide semiconductor field effect transistor(MOSFET) detector to measure surface dosage to explore the effect of tissue-equivalent gel on surface dosage when irradiated with X-ray and electron beams and offer dosimetric basis for clinical radiotherapy.Methods:1 Calibrate MOSFET detector and calculate the corresponding calibration factor .2 Five experiment groups and a control group are involved in the investigations, all are subjected to 6MV and 15MV X-ray irradiation. The 0 group was non-geld, and the 5 geld groups were namedⅠ,Ⅱ,Ⅲ,ⅣandⅤaccording to their increasing thickness. The MOSFET detector was placed on the surface of mannequin, in geld groups the gel was placed on the surface of mannequin tightly and then irradiated, the 0 group was irradiated directly. The source-skin distance(SSD) was 100cm,the field was 10cm×10cm,the output dose of linear accelerator was 100MU with the corresponding dose of 100cGy.Measure the dose of 6 groups individually with MOSFET detector, each for 10 times.3 Use 6MeV,9MeV and 12MeV electron as radiation sources beams,the process was the same with Method 2.Results:1 The measured average CF of the detector was 3.88mV/cGy irradiated with 6 MV X-ray. In this experiment all CF values are considered as 3.88mV/cGy because the same detector had good energy response to X-ray and electron beams.2 The surface dosage after the application of tissue-equivalent gel was remarkably increased when it was irradiated with X-ray, and the highest percentage of the increased dose of 6MV and 15MV X rays were 109.4% and 224.1% respectively. Statistically significant differences were present between the geld groups and the none-geld group(P<0.01).The increased percentage of surface dosage coincides with the increasing thickness of masks. The effects on surface dosage were different from the five kind of gels(P<0.01).3 The surface dosage with the added gel was increased when it was irradiated with electron beams, and the highest percentage of the increased dose of 6MeV ,9MeVand 12MeV electron beams were 32.9%,24.9% and 19.7% respectively. Statistically significant differences were present between the geld groups and the none-geld group(P<0.01).The percentage of surface dosage increased and then decreased with the increasing thickness of glue. The effects on surface dosage were different from the five kind of gels(P<0.01).Conclusions: The surface dosage was increased with the added tissue-equivalent gel when irradiated with photon and electron beams; the surface dosage was related with glue thickness. It was suggested that by using tissue-equivalent gel with proper thickness to increase surface dose during the radiation therapy of. breast cancer, Proper attentions should be focused on the radioactive effects of the skin surface in clinical therapy.