Dosimetric characteristics of a single use MOSFET dosimeter for in vivo dosimetry in proton therapy

Entrance dose (or skin dose) is an important part of patient quality assurance in external beam radiation therapy. However, entrance dose verification in proton beam is not routinely performed. In this study, the OneDose single use MOSFET detector system for in vivo dosimetry measurement in proton therapy is investigated.

Using a solid water phantom, several fundamental dosimetric characteristics of the OneDose system are studied with a proton beam: The reproducibility (consistency) of the dosimeter, the linearity with dose and dose rate, energy dependence, directional dependence, LET dependence, and fading (delay readout with time) is studied.

OneDose detectors show dose and dose rate linearity but exhibit pronounced energy dependence at depth and a large variation in dose response with LET. On the other hand, the detector response remain relatively constant (within 3%) at surface over a wide range of energies. There is also a slight angular dependence (about 2%) up to 60 degrees angle of incidence. However, detector orientation such that incidence along the long axis of the detector should be avoided as the proton beam will have to traverse a large amount of the copper backing. Since most in vivo dosimetry involves entrance dose measurement, the OneDose at surface appears to be well suited for such application. OneDose exhibits small intrabatch variation (< or = 2% at one SD) indicating that it is only necessary to calibration a few detectors from each batch. The interbatch variation is generally within 3%.

The small detector size and its relatively flexible design of OneDose allow dose measurement to be performed on a curved surface or in small cavities that is otherwise difficult with the conventional diode detectors. The slight drawback in its angular dependence can be easily handled by angular dependence table. However, since OneDose is a single use detector, the intra-batch consistency must be verified before the remaining detectors from the same batch could be used for in vivo dosimetry. It is advisable that the detectors from the same batch be taken for the same application to reduce the dosimetric uncertainty. For detectors from different batches, inter-batch consistency should also be verified to obtain reliable results. OneDose provides an opportunity to measure in vivo dose with proton beam within acceptable clinical criterion of +/- (5.0%-6.5%).