A Monte Carlo investigation of dose length product of cone beam computed tomography scans.

The dose length product (DLP) provides a measurement related to energy imparted from a computed tomography (CT) scan. The DLP is based on the volume-averaged CT dose index (CTDI ), which is designed for fan beams. The aims of this study were to investigate the use of DLP for scans with wide beams used in cone beam CT (DLP ) in radiotherapy that would be analogous to the DLP of fan beam scans (DLP ), and to compare the efficiencies of DLP and DLP in reporting the total energy imparted in patients. A validated Monte Carlo model of a kV imaging system integrated into a Varian TrueBeam linac was employed. The DLP was assessed by multiplying the CTDI for a 20 mm fan beam by scan length, and the DLP determined through multiplying the CTDI , estimated for wide beams using a correction factor based on free-in-air measurements, by the beam width. Two scan protocols for head and body were investigated for tube potentials between 80 and 140 kV and a range of scan lengths/widths. Efficiency values were estimated by normalising the DLP and DLP with respect to the corresponding dose profile integrals (DPIs), which were evaluated within 900 mm long phantoms. The results show that the DLP values were within 1% of those for DLP of similar length performed on the same system, and the efficiencies decrease with tube potential. However, whereas DLP values for fan beams are approximately proportional to scan length, those for wide beams decrease by ∼2% between beam widths of 20 and 320 mm. As a result, while the DLP efficiency is similar over all scan lengths, that for DLP increases slightly with beam width. The DLP and DLP underestimated the total energy imparted by comparable amounts with efficiencies within the range of 80-81% and 80-83% for the head scans, and 71-76% and 70-77% for the body scans, respectively. The results indicate that the DLP can be considered as an analogous dose index to the DLP . It could, therefore, be used for quantification of doses from imaging in radiotherapy and provide a valuable tool to aid optimisation.