Energy responses and visibility for thin film-thermoluminescent dosimeters in mammography

Purpose: To evaluate energy response and visibility for thin-film TLDs in mammography. Materials and methods: The energy responses of GR-200F thin-film TLDs were investigated at mammographic energies from 24 to 32 kVp for the Mo/Mo combination and from 26 to 34 kVp for the Mo/Rh combination. A simple model to describe the variation of TLD response R with energy E was derived. Thin-film TLDs on 2- to 7-cm breast thicknesses of BR-12 phantoms and an ACR accredited mammographic phantom were examined under automatic exposure control. Visibility of thin-film TLDs in digital mammographic images was assessed through subjective and objective methods. Entrance surface doses (ESDs) were measured and mean glandular doses (MGDs) were estimated. Results: The energy responses of GR-200F TLDs relative to ¹³⁷Cs are from 0.99 to 1.14, and increase slightly with the increase of mean energy. The TLD response R with energy E (keV) in this study is R (E) = { 1 - exp [- 6.03 × 10^{- 3} × (E + 1.17)] } [1 + 7.83 × 10⁴ × (E + 76.7)^{- 3}]. The digital images with the thin-film TLDs were only just visible on the phantom thickness less than 4 cm, while the thin-film TLD was invisible on a 6 cm thick phantom. The tube current-time products, ESDs and MGDs with and without thin-film TLDs under automatic exposure control were the same (P > 0.05). Conclusions: The energy responses increase slightly with the increase of mammographic energy. Thin-film TLDs are nearly invisible in digital mammograms. Furthermore, they neither affect the technical factor selection by aligning the equation on the text line automatic exposure control, nor change ESDs and MGDs. Hence, thin-film TLDs are suitable in vivo dosimetric measurements of mammography procedures.