Comparison of peripheral dose from image-guided radiation therapy (IGRT) using kV cone beam CT to intensity-modulated radiation therapy (IMRT)

Purpose: The growing use of IMRT with volumetric kilovoltage cone-beam computed tomography (kV-CBCT) for IGRT has increased concerns over the additional (typically unaccounted) radiation dose associated with the procedures. Published data quantify the in-field dose of IGRT and the peripheral dose from IMRT. This study adds to the data on dose outside the target area by measuring peripheral CBCT dose and comparing it with out-of-field IMRT dose. Materials and methods: Measurements of the CBCT peripheral dose were made in an anthropomorphic phantom with TLDs and were compared to peripheral dose measurements for prostate IMRT, determined with MOSFET detectors. Results: Doses above 1cGy (per scan) were found outside the CBCT imaged volume, with 0.2cGy at 25 cm from the central axis. IMRT peripheral dose was 1cGy at 20 cm and 0.4cGy at 25 cm (per fraction). Conclusions: An appreciable dose can be found beyond the edge of the IGRT field; of similar order of magnitude as peripheral dose from IMRT (mGy), and approximately half the dose delivered to the same point from the IMRT treatment (0.2cGy c.f. 0.4cGy 25 cm from the isocenter). This shows that peripheral dose, as well as the in-field dose from CBCT, needs to be taken into account when considering long term care of radiation oncology patients.