# Factors Affecting Implementation of Radiological Protection Aspects of Imaging in Radiotherapy
## 1 Introduction  
Radiotherapy is an important method for treatment of patients with cancer, with 50-60% of patients undergoing radiotherapy either alone or in combination with surgeryor chemotherapy [1; 2]. In the last few decades, dramatic improvements have been made in the capability of external beam radiotherapy (EBRT) equipment to conform radiation treatment fields to any shape of tumour. Most radiation treatment plans for EBRT consist of 3D dose distributions calculated from computed tomography (CT) scans, while other medical imaging modalities are often necessary to assist with the identification of the treatment target. The development of tungsten multi-leaf collimators (MLCs) has enabled the radiation field delivered by the radiation source, most frequently a medical linear accelerator (linac), to be conformed to the shape of the tumour target. Conformal treatment fields are then delivered from multiple static directions or via dynamic arc radiotherapy to focus the therapeutic high dose on the tumour and spare the healthy surrounding tissue from the potentially harmful radiation side effects. However, the required high targeting accuracy can only be achieved by ensuring a daily reproducible patient position as close as possible to that of the treatment plan. The size of the high therapeutic dose region of the treatment field is typically larger than the tumour, as defined by the clinical target volume (CTV) and planning target volume (PTV) concepts [3; 4]. The CTV expands directly from the tumour or gross tumour volume (GTV) to account for any cancer cells lying on the periphery of the tumour. The PTV expands from the CTV to account for internal motion and variations in shape of the tumour, and the uncertainties in patient position during radiotherapy treatments. The treatment dose and PTV margin are adjusted to maximize the chance of cancer cure, while minimizing the probability of complications by maintaining doses to nearby organs at risk (OARs) below their respective tolerance doses [5]. When a patient is set up for treatment, images are acquired, and then compared and aligned with the planning images of the patient to reduce positional uncertainty. Pre-treatment imaging allows the size of the high-dose margin to be reduced (Figure 1). Set-up images are frequently recorded at many, if not all, of the fractions in which treatment is delivered. Cases involving motion management or high-dose treatments might also require intra-fraction imaging. This enables changes in patient anatomy to be monitored during treatment, and allows corrections to be made to treatment delivery if required. The process of using images in planning treatments and then taking further images at the time of treatment to guide the delivery is referred to as image guided radiation therapy (IGRT). Additional imaging during treatment planning and delivery can also be used to account for motion, with the recording of multiple images through breathing or other motion cycles. The application of IGRT around the world has increased considerably in the last decade. It should enable more precise treatment delivery and improved safety. IGRT has led to better patient outcomes, and this is still improving steadily [6]. The National Radiotherapy Trials Quality Assurance Group (UK) reviewed the current evidence on the use of IGRT for pelvic tumours and concluded that IGRT was essential for the safe implementation of highly conformal pelvic radiotherapy [7]. IGRT may in some cases decrease overall treatment time, reduce the number of treatment sessions required, and even enable certain patients to receive radiation therapy when that might otherwise not have been possible. However, increased imaging with X-rays exposes normal tissues surrounding a tumour to additional radiation. This carries a risk of raising doses to OARs close to the PTV boundary above the respective tolerance doses, as imaging doses are not accounted for in treatment planning [5], and thereby increases the risk of deterministic acute or long-term effects.