# Factors Affecting Implementation of Radiological Protection Aspects of Imaging in Radiotherapy
## 3 Results
### Imaging during the Treatment Cycle
#### 3.2.2 Frequency of Imaging  
The frequency of advanced imaging (i.e., CT, kV- and MV-CBCT, and fluoroscopic imaging) has a significant impact on the trade-off between doses to tissues surrounding the target from the therapy beam and those from imaging exposures. When a misalignment is identified by IGRT, the patient is typically repositioned to correct this and, in some cases, imaging may be repeated [17]. Increasing the number of fractions in which imaging is performed will reduce the likelihood of undetected patient alignment errors which could decrease the success of radiotherapy or lead to radiation injury to healthy tissue. The most common option used for all treatments was imaging at every fraction (Figure 2), as this ensures that delivery of treatment is as accurate as possible. A study of lung cancer treatments undertaken with varying frequencies of on-board mega-voltage CT (MVCT) imaging concluded that performing IGRT less than daily increased the rate of geographic misses when small (3 mm) uncertainty margins are chosen and recommended to increase such margin to \(\geq\)5 mm when imaging less frequently [21].  
The frequency was generally higher in countries with higher HDI numbers, although in Germany, the country with the highest HDI, only half of the radiotherapy centres chose imaging at every fraction as the option used most frequently for treatments of the trunk. The frequency of imaging may be chosen to match the standard of care within each individual country and the resources available in specific radiotherapy centres. General trends in frequency of imaging practices within individual countries tended to follow similar patterns for most types of treatment. However, practices did vary with the location of the tumour being treated, and will depend on the treatment protocol and local departmental resources. Centres in Egypt that did not have kV CBCT only imaged either once per week or once during the course of treatment for most tumour types and only carried out imaging once during the course of radiotherapy for breast, gynaecology, and brain treatments.  
For some treatments of the head, imaging might be performed twice or even three times during one fraction (Figure 3). This was particularly noticeable for stereotactic radiotherapy in which higher doses of radiation were delivered to the target from many angles, but with fewer fractions. Figure 3a,b shows all the treatment options used in each centre for treatment of the head and Figure 3c,d the ones used most frequently. This indicates that imaging more than once per fraction was only used for a proportion of patients being treated for both brain, and head and neck lesions.  
Figure 3: The frequencies of imaging used more commonly by each radiotherapy centre during treatment of the brain (**a**,**c**) and head and neck tumours (**b**,**d**), as a proportion of the centres surveyed in the country; (**a**,**b**) representing all the frequency options used, and (**c**,**d**) the ones used most frequently [14]. The countries are ordered using a temperature scale from white to dark red indicating increasing HDI.  
Figure 2: The frequencies of imaging used most commonly by each radiotherapy centre during treatment of tumours in the trunk, as a proportion of the centres surveyed in the country [14]. The countries are ordered using a temperature scale from dark red, through orange and yellow to white, indicating decreasing HDI.  
Imaging at every fraction is the choice favoured by most radiotherapy centres for most types of treatment providing close monitoring of variations over time and enabling the avoidance of random errors. A substantial number of centres in countries with HDIs between 0.75 and 0.9 also image for the first three fractions and then cut back to weekly, a strategy that helps to identify systematic differences between treatment planning and delivery.