A generalized calibration procedure for in vivo transit dosimetry using siemens electronic portal imaging devices

A practical and accurate generalized in vivo\r\ndosimetry procedure has been implemented for Siemens\r\nlinacs supplying 6, 10, and 15 MV photon beams, equipped\r\nwith aSi electronic portal imaging devices (EPIDs). The in\r\nvivo dosimetry method makes use of correlation ratios\r\nbetween EPID transit signal, st\r\n0(TPR,w,L), and phantom\r\nmid-plane dose, D0(TPR,w,L), as functions of phantom\r\nthickness, w, square field dimensions, L, and tissue-phantom\r\nratio TPR20,10. The st\r\n0(TPR,w,L) and D0(TPR,w,L)\r\nvalues were defined to be independent of the EPID sensitivity\r\nand monitor unit calibration, while their dependence\r\non TPR20,10 was investigated to determine a set of generalized\r\ncorrelation ratios to be used for beams with\r\nTPR20,10 falling in the examined range. This way, other\r\nradiotherapy centers can use the method with no need to\r\nlocally perform the whole set of measurements in solid\r\nwater phantoms, required to implement it. Tolerance levels\r\nfor 3D conformal treatments, ranging between ±5 and\r\n±6% according to tumor type and location, were estimated\r\nfor comparison purposes between reconstructed isocenter\r\ndose, Diso, and treatment planning system (TPS) computed\r\ndose Diso,TPS. Finally a dedicated software, interfaceable\r\nwith record and verify (R&V) systems used in the centers,\r\nwas developed to obtain in vivo dosimetry results in less\r\nthan 2 min after beam delivery.