Metal Oxide Semiconductor Field Effect Transistor (MOSFET) radiation dosimeters have found recent application in providing real-time measurement in diagnostic radiology as well as in radiotherapy. Due to the design of the MOSFET dosimeter, the response is dependent on both energy and angulation with respect to the direction of primary radiation. The axial angular dependence has been characterized for both free-in-air and for tissue-equivalent phantoms. However, neither the angular dependence normal (90-degree) to the axial rotation, nor the effects of various tissue compositions on angular dependence, have been investigated for radiation energies in the diagnostic range. To characterize the angular dependence normal to the axial rotation, we exposed three "high sensitivity" MOSFET dosimeters simultaneously to x-rays from a medical diagnostic x-ray unit over a 360-degree rotation, at 22.5-degree increments, for both free-in-air and in lung, skeletal, and soft tissue-equivalent phantoms. The MOSFET dosimeters clearly showed an angular dependence in the orientation normal-to-axial as well as in the axial rotation, both for free-in-air and in tissue-equivalent phantoms. Significant variations in response occurred when the MOSFETs were exposed at incident angles between 90 degrees and 180 degrees normal-to-axial, as compared to the normal position (i.e., the zero-degree position with the bubble-side of the MOSFETs facing the radiation source). A maximum decrease in response to 32% of normal was observed when the distal ends (end opposite the wire lead) of the dosimeters were pointing directly away from the x-ray source (270-degree position). To avoid significant errors in MOSFET dosimeter readings, placement of the dosimeters should be consistent, and care should be taken to avoid orienting the dosimeter with its sensitive region (bubble side) facing away from the source of primary radiation at particular angles.

Pediatric radiographic examinations yield medical benefits and/or diagnostic information that must be balanced against potential risk from patient radiation exposure. Consequently, clinical tools for measuring internal organ dose are needed for medical risk assessment. In this study, a physical phantom and Monte Carlo simulation model of the newborn patient were developed based upon their stylized mathematical expressions (ORNL and MIRD model series). The physical phantom was constructed using tissue equivalent substitutes for soft tissue, lung, and skeleton. Twenty metal-oxide-semiconductor field effect transistor (MOSFET) dosimeters were then inserted at three-dimensional positions representing the centroids of organs assigned in the ICRP's definition of the effective dose. MOSFET-derived point estimates of organ dose were shown to be in reasonable agreement with Monte Carlo estimates for representative newborn head, chest, and abdomen radiographic exams. Ratios of average organ dose assessed via MCNP simulations to the MOSFET-derived point doses (point-to-organ dose scaling factors, SF(POD)) are tabulated for subsequent use in clinical irradiations of the newborn phantom/MOSFET system. Values of SF(POD) indicate that MOSFET measurements of point dose for in-field exposures need to be adjusted only to within 10% to report volume-averaged organ dose. Larger adjustments to point doses are noted for organs out-of-field. For walled organs, point estimates of organ dose at the content centroid are shown to underestimate the average wall dose when the organ is within the primary field: SF(POD) of 1.19 for the stomach (AP chest exam), and SF(POD) of 1.15 for the urinary bladder (AP abdomen exam).