Abstract：
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.
摘要：
kV锥形束计算机断层扫描（CBCT）是用于图像引导放射治疗（IGRT）程序的最常见成像方式之一。向患者提供额外的剂量，因此应考虑成像剂量的评估和优化.本研究旨在研究使用固定和患者特异性FOV对患者剂量的影响。使用BEAMnrc代码进行蒙特卡罗模拟以模拟集成到Truebeam线性加速器中的成像系统的kV光束。使用美国国家癌症研究所（NCI）开发的幻像库，通过DOSXYZnrc代码估算了由胸部和骨盆扫描方案产生的器官和特定大小的有效剂量。该库包含193个（100个男性和93个女性）网格类型的计算人类成年体模，它涵盖了一个大比例的病人的大小与身高和体重范围从150到190厘米和40到125公斤。使用三种大小的可变FOV评估成像剂量，小（S），中等(M),和大（L）为每个扫描区域。成果显示，FOV和患者年夜小在扫描剂量中起主要感化。完全在不同FOV内的器官剂量的平均百分比差异（PD）相对较低，两种方案都在11%以内。然而,对部分或靠近FOV扫描的器官的剂量受到显著影响.对于胸部协议，在扫描场中包含甲状腺可以给甲状腺提供1-7mGy/100mAs的剂量，与排除时的0.4-1mGy/100mAs相比。同样，平均而言,男性骨盆方案的睾丸剂量可能为6mGy/100mAs，而不在照射的场地中为3mGy/100mAs。这些剂量差异导致方案的大小特异性有效剂量平均增加高达27%。因为改变场的大小是可能的CBCT扫描，结果表明,患者特异性扫描方案可以以类似于CT扫描的方式应用于每个扫描区域.FOV大小的调整应根据临床需要，并协助提高治疗的准确性。患者的身高和体重可能被认为是主要因素，选择合适的患者特异性方案是基于.这种方法应该通过最小化大比例患者的剂量来优化用于IGRT程序的成像剂量。
