Cone-Beam CT Dosimetry in Radiation Therapy

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Cone-beam computed tomography (CBCT) is a 3D x-ray imaging technique where a x-rays are used in a wide-beam geometry to image an object with 3D reconstruction. It's a faster acquisition than classic fan-beam computed tomography (CT) and has a much wider coverage length per scan making it an ideal choice for imaging radiotherapy (RT) patients before treatment. The availability of commercial CBCT systems has dramatically increased and most new clinical linear accelerator purchases include a CBCT on the same platform as the accelerator.

Many patients are imaged with CBCT systems just before treatment to align the target structures or to ensure critical normal tissue targets are properly out of the field. CBCT is typically reserved for more complicated delivery techniques where planning margins have been reduced. The frequency of CBCT imaging for RT treatments varies among clinics, clinicians, treatment modality, and other technologies available. It is not uncommon for RT patients to be imaged daily before treatment for several weeks over the course of their treatment regimen.

The dose received by the patient is largely unaccounted for in the treatment plan (with the exception of megavoltage CBCT in use by some manufacturers). Most treatment planning systems (TPS) are designed to calculate doses from megavoltage beams of radiation and are not designed to predict doses from the x-ray generators used in CBCT. Therefore, the dose to the patient is estimated based on a number of different methods, but not necessarily specific to the patient.

The doses from CBCT are low, but not negligible. There are known risks of secondary cancers from low-doses of radiation to normal tissues and these need to be considered. There is a need for patient-specific dosimetry from CBCT to be able to balance the risks of delivering low doses of radiation with the benefit of being able to align the internal anatomy of a patient with higher precision before treatment. Physicians should be equipped with more knowledge of these doses to make more educated decisions on the frequency in which CBCT should be used.

Patient-specific CBCT dosimetry starts with accurate dosimetry using dosimeters with known responses on phantoms. Accurate knowledge of the energy-dependent nature of the dosimeter is critical, since many dosimeters are sensitive to energy changes in the range of CBCT x-ray generators. Dosimeters such as TLDs, diodes, ionization chambers, MOSFETs, OSLDs, and gel dosimeters are options for CBCT dose verification. Comparisons with calculations should accompany these measurements and subsequent calculation platforms should be developed to accurately predict doses to the patient.