Author: Maria F. Chan, Seng Boh Gary Lim, Shih-Chi Lin, Dustin W. Lynch, Usman Mahmood, Jeonghoon Park 👨🔬
Affiliation: Department of Medical Physics, Memorial Sloan Kettering Cancer Center 🌍
Purpose: To explore the feasibility of using GafChromic LD-V1 film for annual imaging dose assessment across kV radiography, fluoroscopy, and cone beam computed tomography (CBCT) imaging protocols, as an alternative to dedicated diagnostic X-ray dose evaluation devices in radiotherapy clinics.
Methods: Calibration doses (0-10 cGy) for the LD-V1 film were delivered in fluoroscopy mode (120 kVp, 18 mA, 80 pulse mA, 15 frame rate). The reference dose rate was established as 0.27 cGy/sec based on readings from a RaySafe X2 CT sensor. Calibration doses for the film were calculated by multiplying the exposure times by 0.27 cGy/sec. The film's sensitivity to low doses (< 5 cGy) and its energy dependency across the kV range were also assessed. Films were scanned using an Epson V600 flatbed scanner at a resolution of 96 dpi. A calibrated RaySafe X2 CT chamber was used as the reference for dose comparison, with both dosimeters measuring surface and center doses in a 32 cm CTDI phantom.
Results: Dose response data for the LD-V1 film were collected across various kV settings. For a full pelvis scan, surface dose ranged from 2.10 cGy at the skin to 1.44 cGy at the center. Spotlight mode produced comparable dose distributions, while the head and neck mode resulted in doses below 1 cGy. Both the LD-V1 film and the CT chamber yielded similar dose estimates with uncertainties, with discrepancies of up to 30% anticipated due to energy dependency at lower kV and in the low-dose range.
Conclusion: The dose-response of LD-V1 film was evaluated across different kV settings and CBCT protocols, yielding results comparable to the CT chamber with corrections. When properly calibrated, the film offers a practical, time-saving alternative for measuring kV IGRT doses during annual QA, eliminating the need for repeated CBCT scans for chamber repositioning.