Patient dosimetry for total body irradiation using single-use MOSFET detectors

Abstract

We studied the usefulness of a new type of solid-state detector, the OneDose single-use MOSFET (metal oxide semiconductor field effect transistor) dosimeter, for entrance dose measurements for total body irradiation (TBI). The factory calibration factors supplied by the manufacturer are applicable to conventional radiotherapy beam arrangements and therefore may not be expected to be valid for TBI dosimetry because of the large field sizes and extended source-to-axis distances used. OneDose detectors were placed under a 1-cm thick bolus at the head, neck, and umbilicus of 9 patients undergoing TBI procedures. Thermoluminescent dosimeters (TLDs) were placed beside the detectors. We found that the OneDose readings differed from the TLD readings by 4.6% at the head, 1.7% at the neck, and 3.9% at the umbilicus, with corresponding standard deviations of 3.9%, 2.2%, and 2.7%. For all patient measurements, 95% of the OneDose readings fell within 3.3% +/- 6.0% of the TLD readings. Anthropomorphic phantom measurements showed differences of -0.1% at the neck and -1.2% midway between the phantom's carina and umbilicus. Our results suggest that these detectors could be used for TBI quality assurance monitoring, although TLDs should remain the standard when critical dose measurements are performed. If OneDose detectors are to be used for TBI, the use of more than one at each location is strongly recommended. Because the detectors are designed for single use, they cannot be individually calibrated. However, to obtain institution-specific correction factors for better applicability to TBI dosimetry, measurements of several detectors taken from a particular lot could also be obtained in phantom with the TBI geometry configurations used for patient treatment.

Figure 1

The most common total body irradiation configuration used at our institution: a couch unit in which the patient lies on his or her side at a source‐to‐axis distance (SAD) of 380 cm. An 18‐MV beam is used, and the collimator is rotated by 45 degrees and set to $40\times 40\hspace{0.17em}\text{cm}$.

Figure 2

Percent deviation of the OneDose detectors from the thermoluminescent dosimeter standard (STD) for the 9 calibration measurements taken at conventional geometry. OneDose detectors taken from lot 1 are shown in gray, those from lot 2 in black, and those from lot 3 in white.

Figure 3

Percent deviation from thermoluminescent dosimeter (TLD) measurements for the 9 total body irradiation patients. The detectors were placed at the head, neck, and umbilicus of each patient. The prescribed dose to the patient's midsection was 75 cGy for patient 1, 87.5 cGy for patients 2 – 4, and 150 cGy for patients 5 – 9. Patients 1 – 5 (filled symbols) received detectors from lot 1, patients 6 and 7 from lot 2 (open symbols), and patients 8 and 9 (crosses) received detectors from lot 3.