- : Purpose or Objective: There is a growing awareness of dose delivered to parts the body outside the target volume during external beam radiotherapy. This concomitant dose could arise from external linac head leakage and scatter, scattered therapy dose outside the target volume, as well as nontherapeutic doses from imaging for planning and delivery, such as CT planning scans. Total concomitant dose has increased steadily with the introduction of more imaging procedures to the treatment process and the drive for better images quality. Much of this exposure is only loosely monitored and it could be the case that the cumulative concomitant dose has a negative biological effect even within the context of radiotherapy . To quantify the dose contributed by CT planning scans, a retrospective dose audit was carried out on a TOSHIBA AQUILION LB multislice CT scanner at Derby Teaching Hospitals in July 2015. Material and Methods: A cohort of 200 patients were identified, twenty each from ten of the most frequently used CT scanning protocols who were scanned in the 12 months immediately prior to the dose audit. Patients undergoing CT planning scans were initially identified in the Mosaiq Oncology Information System (Elekta, Crawley, UK) and subsequently interrogated via the PACSWeb system, (Centricity Enterprise Web V3.0, GE Healthcare, Barrington, IL). Data harvested from PACSWeb included: Number of slices, slice thickness, CTDIVOL, DLP, Patient sex, Patient Age, total scan time, transverse width and AP width. Mean Effective Dose (E) was derived from values of DLP for each examination using appropriately normalised coefficients. As yet, there are no published UK national guidelines for planning CT scans. However, to put the results of this audit into context we have compared local DLP and CTDIvol to similar values published for a previous UK national (diagnostic CT) dose audit . The following relationships were reported: CSA vs Age, CTDIvol vs CSA, DLP vs CSA, CTDIvol by Patient, DLP by Patient. Results: The mean scan length, DLP, CTDIvol and Effective Dose by Protocol were found for each protocol. The most significant result was that the DLP values from the Head & Neck protocol were tightly clustered but higher than one would normally expect. The mean DLP was a factor of 4 greater than the head and neck reference level reported in the previous UK national (diagnostic CT) dose audit. Conclusion: The results from this CT dose audit can be used as local Radiotherapy Imaging Reference Levels (RIRL). They will be able to guide protocol optimisation, allow comparison with other similarly equipped radiotherapy departments and participation in regional and national audits. The higher than expected DLP values for the Head & Neck protocol highlighted here has prompted a reassessment of the scanning parameters and may lead to protocol optimisation. (Figure Presented).