Development Of The Measurement Methodology For In Vivo Dosimetry With Metal Oxide Semi-conductor Field Effect Transistor (MOSFET) In Radiotherapy

Objective:To study the measurement of the treatment dose on the patient with photon beam using metal oxide semiconductor field effect transistor(MOSFET) detector in radiotherapy.Methods:Using 6MV X-ray beam and solid phantom to development experiments of the reproducibility,dose rate dependence,dose levels dependence,dose response non-linearity,energy and angle of incidence with MOSFET detector.Then,changes the field size,distance from source to skin,wedge, block and tray should be measured,and the corresponding correction factors were calculated.It is carrying out dose validation using Alderson phantom to choose pelvis,head and neck,CT were used by scan with Alderson phantom for 3mm slice thickness,it will design treatment plan with scan image,two treatment plans were designed for pelvis.The first plan all three fields should be 20 cm×20 cm without wedges,doses will be calculated by the TPS to deliver 50 cGy to the isocentre,which must be at the centre of the slices.The second plan is with 10 cm×10 cm fields with 30°wedges on the two lateral fields with the thin end of the wedge.Using two parallel-opposed 6 cm×6 cm fields without wedges for head and neck.The center of the field should be again at the entre of a slice in the region of the junction of the neck and the chin.100 cGy was delivered to the isocentre for each field.These measurements will be done with and without immobilization mask.MOSFET detector was measured place on the surface of Alderson phantom.It is comparison with measurement values and TPS calculation values,the relative deviation values is within±3%.A lot of number pelvis and head and neck on patients were choose in radiotherapy,MOSFET detector would be placed on cancer patients to measurement their treatment dose during radiotherapy. It is comparison with MOSFET measurement values and TPS calculation values,and the relative deviation is within±5%.Results:The dose of pelvis was validated by Alderson phantom,it is comparison with MOSFET and TPS,the relative deviation are within 0.3%——1.7%.The dose of head and neck was validated by Alderson phantom,it is comparison with MOSFET and TPS,the relative deviation are within -0.1%——2.2%.MOSFET detector have measured for 38 pelvis patients and the dose points are for 386,the relative deviation is within±5%for 305 with dose patients,it is make up 79.0%all dose points.The relative deviation is without±5%for 81 patients,it is make up 21%all dose points.MOSFET detector have measured for 41 head and neck on patients and the dose points are for 313,the relative deviation is within±5%for 278 with dose point,it is make up 88.8% all dose points.The relative deviation is without±5%for 35 dose point,it is make up 11.2%all dose points.Conclusions:MOSFET was measured to set up in vivo dosimetry with methodology,techniques and procedures in radiotherapy by the research project.The level is same as in developed country.The measurement methods are accurate and reliable.It is could be checked speedy with all mistake during radiotherapy to avoid occur accident in radiotherapy,at same time it is one of effective way with quality control in radiotherapy.