MOSFET dosimetry with high spatial resolution in intense synchrotron-generated xray microbeams
Various dosimeters have been tested for assessing absorbed doseswith microscopic spatial resolution in targets irradiated by high-flux, synchrotron-generated,low-energy (~30–300 keV) x-ray microbeams. A MOSFET detector has been usedfor this study since its radio sensitive element, which isextraordinarily narrow (~1 µm), suits the main applications of interest, microbeamradiation biology and microbeam radiation therapy (MRT). In MRT, micrometer-wide,centimeter-high, and vertically oriented swaths of tissue are irradiated byarrays of rectangular x-ray microbeams produced by a multislit collimator(MSC). We used MOSFETs to measure the dose distribution, producedby arrays of x-ray microbeams shaped by two different MSCs,in a tissue-equivalent phantom. Doses were measured near the centerof the arrays and maximum/minimum (peak/valley) dose ratios (PVDRs) werecalculated to determine how variations in heights and in widthsof the microbeams influenced this for the therapy, potentially importantparameter. Monte Carlo (MC) simulations of the absorbed dose distributionin the phantom were also performed. The results show thatwhen the heights of the irradiated swaths were below thoseapplicable to clinical therapy (<1 >mm) the MC simulations produce estimatesof PVDRs that are up to a factor of 3higher than the measured values. For arrays of higher microbeams(i.e., 25 µm×1 cm instead of 25×500 µm2), this difference between measured andsimulated PVDRs becomes less than 50%. Closer agreement was observedbetween the measured and simulated PVDRs for the Tecomet® MSC(current collimator design) than for the Archer MSC. Sources ofdiscrepancies between measured and simulated doses are discussed, of whichthe energy dependent response of the MOSFET was shown tobe among the most important.