Estimation of organ and effective doses of CBCT scans of radiotherapy using size-specific field of view (FOV): a Monte Carlo study.

The kV cone beam computed tomography (CBCT) is one of the most common imaging modalities used for image-guided radiation therapy (IGRT) procedures. Additional doses are delivered to patients, thus assessment and optimization of the imaging doses should be taken into consideration. This study aimed to investigate the influence of using fixed and patient-specific FOVs on the patient dose. Monte Carlo simulations were performed to simulate kV beams of the imaging system integrated into Truebeam linear accelerator using BEAMnrc code. Organ and size-specific effective doses resulting from chest and pelvis scanning protocols were estimated with DOSXYZnrc code using a phantom library developed by the National Cancer Institute (NCI) of the US. The library contains 193 (100 male and 93 female) mesh-type computational human adult phantoms, and it covers a large ratio of patient sizes with heights and weights ranging from 150 to 190 cm and 40 to 125 kg. The imaging doses were assessed using variable FOV of three sizes, small (S), medium (M), and large (L) for each scan region. The results show that the FOV and the patient size played a major role in the scan dose. The average percentage differences (PDs) for doses of organs that were fully inside the different FOVs were relatively low, all within 11% for both protocols. However, doses to organs that were scanned partially or near the FOVs were affected significantly. For the chest protocol, the inclusion of the thyroid in the scan field could give a dose of 1-7 mGy/100 mAs to the thyroid, compared to 0.4-1 mGy/100 mAs when it was excluded. Similarly, on average, testes doses could be 6 mGy/100 mAs for the male pelvis protocol compared to 3 mGy/100 mAs when it did not lie in the field irradiated. These dose differences resulted in an average increase of up to 27% in the size-specific effective dose of the protocols. Since changing the field size is possible for CBCT scans, the results suggest that patient-specific scanning protocols could be applied for each scan area in a manner similar to that used for CT scans. Adjustment of the FOV size should be subject to the clinical needs, and assist in improving the treatment accuracy. The patient's height and weight might be considered as the main factors upon which, the selection of the appropriate patient-specific protocol is based. This approach should optimize the imaging doses used for IGRT procedures by minimizing doses of a large ratio of patients.