Dose length product (DLP) is a measure of CT tube radiation output/exposure (measured in mGy.cm). It is related to volume CT dose index (CTDIvol), but CTDIvol represents the dose through a slice of an appropriate phantom. The dose length product accounts for the length of radiation output along the z-axis (the long axis of the patient). It may be described by the following equation:
DLP = (CTDIvol in mGy) . (length of scan in cm)
The dose length product does not take the size of the patient into account and is not a measure of absorbed dose. If the AP and lateral dimensions of the patient are available, then the size specific dose estimate (SSDE) can be used to estimate the absorbed dose.
The dose length product is also one of the two parameters (along with CT dose index) used to determine the diagnostic reference levels for radiation doses in various imaging modalities 4.
It is important to remember that the dose length product is not the patient's effective dose. The effective dose depends on other factors including patient size and the region of the body being scanned. One of the method is Monte Carlo method that collect data from large number of X-rays to calculate the probability of photoelectric absorption or Comptom effect in the phantom, assuming that the patient's interaction with X-rays is similar to the phantom. The results are used to calculate absorbed doses. The absorbed doses are then multipled by organ dose coefficients to get the effective dose 5.
To minimize the effective dose differences due to different calculation methods and data sources, k-factors have been estimated to convert dose length products directly into effective doses, depending on the body region 3,5.