Feasibility of kV dose measurement in IGRT using MV-calibrated ionization chambers

In image-guided radiation therapy (IGRT), kV imaging is routinely used for patient positioning. However, repeated imaging can result in a significant cumulative dose. Ideally, dose measurements should be performed using ionization chambers calibrated for kV energies, but many facilities only have chambers calibrated for MV energies. This study investigated whether ionization chambers calibrated for MV energies could accurately measure kV doses in IGRT. Several cylindrical chambers were evaluated. Cross-calibration was performed using a kV-calibrated reference chamber. Surface doses were measured under clinical conditions and compared with nanoDot dosemeters. MV-calibrated chambers showed good agreement with the kV-calibrated reference. The quality conversion factor for the kV energy range was determined for the Farmer chambers. The obtained value was 0.931 ± 0.008 (N = 90). The surface dose with farmer-type chambers demonstrated dose deviations within ±1% compared nanodot dosemeter. Our results demonstrate the magnitude of uncertainty that arises when a Farmer-type ionization chamber, calibrated based on the water absorbed dose in a 60Co beam, is used for kV energy range surface dose measurements with an onboard imaging system of a linac without applying an energy correction factor. With appropriate cross-calibration, MV-calibrated ionization chambers can be used for kV dose measurement in IGRT. This method allows accurate and practical dosimetry, enhancing clinical efficiency and patient safety.