A dosimetry technique for measuring kilovoltage cone‐beam CT dose on a linear accelerator using radiotherapy equipment

Abstract

This work develops a technique for kilovoltage cone‐beam CT (CBCT) dosimetry that incorporates both point dose and integral dose in the form of dose length product, and uses readily available radiotherapy equipment. The dose from imaging protocols for a range of imaging parameters and treatment sites was evaluated. Conventional CT dosimetry using 100 mm long pencil chambers has been shown to be inadequate for the large fields in CBCT and has been replaced in this work by a combination of point dose and integral dose. Absolute dose measurements were made with a small volume ion chamber at the central slice of a radiotherapy phantom. Beam profiles were measured using a linear diode array large enough to capture the entire imaging field. These profiles were normalized to absolute dose to form dose line integrals, which were then weighted with radial depth to form the DLPCBCT. This metric is analogous to the standard dose length product (DLP), but derived differently to suit the unique properties of CBCT. Imaging protocols for head and neck, chest, and prostate sites delivered absolute doses of 0.9, 2.2, and 2.9 cGy to the center of the phantom, and DLPCBCT of 28.2, 665.1, and 565.3 mGy.cm, respectively. Results are displayed as dose per 100 mAs and as a function of key imaging parameters such as kVp, mAs, and collimator selection in a summary table. DLPCBCT was found to correlate closely with the dimension of the imaging region and provided a good indication of integral dose. It is important to assess integral dose when determining radiation doses to patients using CBCT. By incorporating measured beam profiles and DLP, this technique provides a CBCT dosimetry in radiotherapy phantoms and allows the prediction of imaging dose for new CBCT protocols.PACS number: 87.57.uq

AAPM Task Group 198 Report: An implementation guide for TG 142 quality assurance of medical accelerators

“…It is recommended that users measure the dose for at least the minimum and maximum dose CBCT protocols for a centrally located ion chamber annually to determine if the range of CBCT doses has remained constant. Once the baseline dose has been measured, an additional method for kV‐CBCT dose constancy would be to use an ionization chamber in air 136,137 …”

“…Once the baseline dose has been measured, an additional method for kV-CBCT dose constancy would be to use an ionization chamber in air. 136,137 The times for each task are estimated based on the assumption that each task is carried out independently. As tools are commercially available to address multiple tasks with a single image acquisition, the overall time required to accomplish several tasks in the same category, such as planar kV imaging, can substantially speed up the QA process.…”

Investigation of the radiation dose from cone‐beam CT for image‐guided radiotherapy: A comparison of methodologies

“…Currently, imaging dose is often omitted from treatment plans since, being typically less than 1 Gy for an entire treatment schedule or 1–10 cGy for a single scan, it is two orders of magnitude smaller than the therapeutic doses . However, during an imaging procedure, large portions of the body are irradiated, including radiosensitive structures such as lung, breast, thyroid, and reproductive organs.…”

Evaluation of optimal kilovoltage-cone beam technique on image quality, registration accuracy, time of imaging and relative dose for head radiotherapy: A phantom study

“…However, the use of kV-CBCT imaging often results in additional patient radiation exposure to radiosensitive organs. Scandurra et al 2 reported that an absolute dose of the head protocol was 0.9 cGy, while previous research indicated that image-guided kV-CBCT was effective for the evaluation of set-up accuracy. [3][4][5][6] Delishaj et al 7 suggested that kV-CBCT should first be administered as three fractions and then followed by weekly doses to significantly reduce set-up errors in head and neck cancer treatment using the IMRT technique.…”