Feasibility Study and First Results of a Real Time Imaging Dosimeter for External Beam Radiation Therapy

The purpose of this study was to determine the feasibility of building an in vivo dosimeter prototype to perform real time dosimetry to detect treatment errors in intensity modulated radiation therapy (IMRT). The dosimeter will enhance patient safety through real time verification of treatment dose when real time beam measurements are compared against treatment plan pre-computed doses. The high speed dosimeter is based on scintillating fiber arrays embedded in a water equivalent plastic support. The fibers are aligned to match each multileaf collimator (MLC) leaf pair and placed in the accessory tray of the LINAC. Each fiber is coupled to a pair of silicon photomultiplier and high speed front end amplifiers to read the scintillator output. The dosimeter response was tested with a broad selection of clinical beam energies, dose rates, and MLC positions and the scintillating fiber sensor output correlated linearly with these parameters. The front end electronics have a response time of 0.2 mu sec, thus every beam radiation pulse can be recorded to verify the beam fluence with high precision. The plastic support and fibers provide a beam attenuation of 2.65%, minimal enough to utilize this technology as an in vivo transmission detector at the head of the LINAC. The linear response of output versus MLC beam let opening provides a measurement of each leaf position with a precision of 1.5 mm. The designed real time imaging dosimeter will use five of these scintillating fiber arrays to reconstruct delivered beam fluences in vivo and compare them in real time against the treatment plan. Should an error occur, the software will notify the therapist or halt the treatment. The reported work shows a proof and initial results of the feasibility of constructing a scintillating fiber array for in vivo real time dosimetry to enhance patient safety and treatment verification accuracy in external beam radiotherapy.