Kavanaugh, J A

Abstract 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.

AbstractIntroductionUsing diagnostic CT for radiotherapy (RT) planning can bypass traditional CT simulation but introduces challenges in patient positioning and Hounsfield unit (HU) fidelity, affecting dose accuracy. Here a Varian Ethos adaptive‐driven CT direct‐to‐treatment (DtT) multi‐fraction stereotactic body radiation therapy (SBRT) workflow is presented.MethodsThis study employed institutional diagnostic PET‐CT images to generate an initial reference Ethos treatment plan. HU and dosimetric accuracy were validated for PET‐CT, Ethos CBCT images (with and without Hypersight (HS), and the gold‐standard helical CT simulators). Following the SBRT reference plan creation on the low dose diagnostic PET‐CT, the first fraction was delivered with a newly generated adaptive plan calculated on the HS CBCT (Ethos) images. For multi‐fraction treatments, the first day CBCT images and adaptive plan become the reference for subsequent IGRT treatments. This study includes workflow validation and initial three patient experience.ResultsThe DtT adaptive SBRT workflow was successfully implemented, with initial end‐to‐end testing demonstrating feasibility. In‐house solutions were introduced to facilitate the adaptive to IGRT plan conversion. The Ethos system, especially with HS, maintained HU fidelity and dose calculation accuracy comparable to helical CTs. On‐table adaptive sessions were within 37–51 min, aligning with single‐fraction palliative studies. Subsequent non‐adaptive IGRT fractions were efficiently completed within 7–27 min.ConclusionsThis study demonstrates the feasibility of DtT adaptive‐driven multifraction SBRT using diagnostic PET‐CT. This approach reduces the need for on‐site patient presence prior to treatment, expands planning lead times, and improves overall efficiency in radiotherapy for well selected patients.