Two-dimensional solid-state array detectors: A technique for in vivo dose verification in a variable effective area

Purpose: We introduce a technique that employs a 2D detector in transmission mode (TM) to verify dose maps at a depth of d_max in Solid Water. TM measurements, when taken at a different surface-to-detector distance (SDD), allow for the area at d_max (in which the dose map is calculated) to be adjusted. Methods: We considered the detector prototype “MP512” (an array of 512 diode-sensitive volumes, 2 mm spatial resolution). Measurements in transmission mode were taken at SDDs in the range from 0.3 to 24 cm. Dose mode (DM) measurements were made at d_max in Solid Water. We considered radiation fields in the range from 2 × 2 cm² to 10 × 10 cm², produced by 6 MV flattened photon beams; we derived a relationship between DM and TM measurements as a function of SDD and field size. The relationship was used to calculate, from TM measurements at 4 and 24 cm SDD, dose maps at d_max in fields of 1 × 1 cm² and 4 × 4 cm², and in IMRT fields. Calculations were cross-checked (gamma analysis) with the treatment planning system and with measurements (MP512, films, ionization chamber). Results: In the square fields, calculations agreed with measurements to within ±2.36%. In the IMRT fields, using acceptance criteria of 3%/3 mm, 2%/2 mm, 1%/1 mm, calculations had respective gamma passing rates greater than 96.89%, 90.50%, 62.20% (for a 4 cm SSD); and greater than 97.22%, 93.80%, 59.00% (for a 24 cm SSD). Lower rates (1%/1 mm criterion) can be explained by submillimeter misalignments, dose averaging in calculations, noise artifacts in film dosimetry. Conclusions: It is possible to perform TM measurements at the SSD which produces the best fit between the area at d_max in which the dose map is calculated and the size of the monitored target.