Radiation Dose And Image Quality From Pelvic Localisation Computed Tomography In Oncology

Abstract

Introduction: Computed tomography (CT) in radiation therapy plays an important role in the accurate identification of the position of the tumour and organs at risk, through high geometric fidelity of the CT image. It has been determined that the radiation dose from CT is amongst the highest from all medical imaging. There is concern over the increased radiation dose from pelvic CT localisation scans, due to the increased scan length and the necessity for high image quality used in radiation therapy planning. The necessity for high image quality, while lowering the CT dose and honouring the ALARA principles, is essential. Four article-style studies, which evaluate the CT dose and image quality produced for pelvic localisation scans and that are ultimately aimed at publication, are presented in this research paper. Purpose: The purpose of this research was to determine the CT dose and image quality produced for pelvic localisation scans in a department of oncology, Free State. The aim was to measure the dose received by patients during pelvic CT localisation scans and to determine whether the dose is justified in terms of imaging requirements for radiation therapy planning. The objectives of the research were to (i) determine baseline dose level for pelvic CT scan utilising an anthropomorphic phantom, (ii) measure patients’ CT pelvic localisation dose by using size-specific dose estimates, (iii) to verify whether the field of view (FOV) modified image quality for patients of different sizes, utilising water phantoms and (iv) objectively examine image quality using the contrast-to-noise ratio (CNR) for patients’ CT localisation scans. The significance of this research is reflected in filling the gap in existing literature, as most published studies were conducted on diagnostic CT dose and image quality. Methodology: The research was conducted as a prospective research study, performed between January and June 2019, after ethical approval was obtained. All CT scans were produced on a TSX-201A (Toshiba Aquilion © Large Bore) CT scanner. The CT baseline dose level was established utilising an anthropomorphic phantom. The patient dose for pelvic CT localisation was calculated by the size-specific dose estimate (SSDE) that determines the dose, based on individual patient dimensions. The participants were divided into three body mass index (BMI) categories; these were underweight, normal weight and overweight. The CT image quality was examined based on scans of different sized water phantoms utilising CT quality assurance tests. The patients’ CT image quality was derived from the contrast-to-noise ratio (CNR). Results: A total of 131 participants met the inclusion criteria of the research study and were grouped according to their BMI into categories, these being categorised as: overweight-, normal- and underweight BMI category. For the baseline dose level, the best kV and FOV combination was determined for the 120 kV setting with a large (L), large-large (LL) or extra-large (XL) FOV, which calculated at 14.0 cGy using SSDE. In terms of the BMI patient category the median dose was determined as: 12.3 cGy for an overweight BMI; 14.8 cGy for a normal BMI, which is in line with the baseline CT dose and 17.1 cGy for the underweight BMI category. The image quality determined as per phantom indicated that the 135 kV demonstrated the highest quality as well as the highest dose. In terms of FOV the small and medium sized phantom could be scanned with any size FOV. However, the large phantom excelled with the L, LL and XL FOV. The results for the patients in terms of image quality, based on CNR illustrated that the normal BMI category patients had the highest quality. It was furthermore concluded that the overweight BMI patient category reflected the lowest image quality. Conclusion: The research questions were addressed and the objectives of this research were indeed met. In addition, this research addressed the gap in relevant literature, by determining results based on oncological CT scans and protocols in terms of dose and image quality. The fact that only one anthropomorphic phantom was available for dose calculations and that no tissue types were present in the phantom to utilise for the image quality, limited the research. The researcher recommends that protocols for patients in different BMI categories be established for CT pelvic localisation scans whilst simultaneously adhering to the ALARA principles. Research demonstrates that there is an industrial drive to decrease dose while maintaining image quality. Numerous techniques have been introduced to assist with the reduction of dose. One of these techniques was illustrated by Irish researchers who established national diagnostic reference levels (DRL’s) for breast CT protocols in oncology. It is believed that by utilising knowledge from both diagnostic and oncology CT scan techniques, the reduction of CT dose - while maintaining image quality - is an achievable goal.