Assessment of LD‐V1 radiochromic film low‐dose performance for mega‐voltage radiotherapy quality assurance

BACKGROUND: Highly modulated radiotherapy delivery demands functional multileaf collimator (MLC) quality assurance (QA) with the ability to measure very low doses during short (<1 s) temporal segments of beam delivery. A radiochromic film with adequate sensitivity in the few-cGy range is therefore needed. LD-V1 film was designed for 2-20 cGy measurements in diagnostic kV energy ranges, but its performance for megavoltage (MV) beam qualities has not been well characterized. PURPOSE: To demonstrate the feasibility of LD-V1 film for low-dose MV measurements and characterize its signal dependence under conditions relevant for clinical QA. METHODS: A 6 MV calibration curve was generated using eight dose levels between 0 and 40 cGy and compared with EBT4 film irradiated under identical conditions. Film characteristics evaluated included dose response, scanner and film signal uniformity, scan repeatability, energy dependence, post-exposure signal evolution, scanning-orientation dependence, and one-dimensional (1-D) profile visualization. RESULTS: LD-V1 demonstrated higher net optical density (OD) and significantly less measurement uncertainty compared to EBT4 at dose levels less than 40 cGy. Significant energy dependence between kV and MV beam qualities was seen. Average net OD differences of 6.3% were seen between landscape and portrait scanning orientations. Post-irradiation signal growth followed a logarithmic trend, stabilizing at ∼24 h. Scan-to-scan variation was <0.1%. Film-signal uniformity was within 3.9%. 1-D profiles showed discernible dose fall-off for doses as low as 10 cGy. CONCLUSIONS: LD-V1 demonstrates suitability for low-dose MV measurements and exhibits improved low-dose sensitivity relative to EBT4, indicating its potential value for MLC QA in highly dynamic treatment deliveries. Nonetheless, its pronounced energy and scanning-orientation dependence underscores the need for rigorous and consistent calibration and handling procedures.