Electronic portal imaging (EPI) has been used in the assessment of the accuracy of radiotherapy treatments in the pelvis. The daily reproducibility of any treatment is primarily dependent on the effectiveness of the set-up method. Treatments of radical planned volumes within the pelvis are subject to field placement errors (FPE) which could potentially compromise the successful outcome of radiotherapy treatment. Increasing use of shaped treatment fields to limit the dose delivered to surrounding normal tissues has prompted a more detailed examination of set-up methods.

Within the radiotherapy department at the Leicester Royal Infirmary it was noted that tattooed marks on the anterior or posterior and the lateral skin surface, marked at simulation, could not always be aligned for daily radiotherapy treatment.

An assessment of the relative merit of the anterior/posterior or the lateral tattoos in determining the isocentre position in the superior-inferior (cranio-caudal) plane is presented. This study showed that changes in the use of tattoo marks during set-up would have reduced the incidence of FPE >5 mm for a small sample group of patients. Implementation of changes in clinical practice, or research using a larger sample group is now needed to verify any improvement in accuracy using a modified treatment set-up technique.