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

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.

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).

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.

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.