dose values were measured in water, a table of the percentage of the maximum dose as a function
of depth (on-axis) in the water could be deduced. A similar table could be generated for off-axis
ratios (OARS) by measuring lateral profiles at a variety of depths. The absorbed dose at a given
point P in the patient could then be estimated by knowledge of the maximum dose for the
treatment, and applying a percent depth dose (PDD) correction for the depth of P in the patient,
and an off-axis correction for the distance to P from the axis at that depth4
Tables of PDDs were made available for various sources (incident energies), field sizes,
depths in the phantom, and SSDs. To modify the dose for an SSD that was different from the
reference SSD, the ratio of inverse distances squared would be applied. For a change in field
size, tables of field-size-factors would be subsequently applied. To compensate for the curved
surfaces of a patient, as opposed to the flat surface of the phantom, isodose lines could be shifted
along rays proj ected from the source.
It is important to point out that these techniques are often quite successful for predicting
absorbed dose in a homogeneous volume. In fact, in some cases they may be superior to
alternative simplified methods (for homogeneous volumes) because they are based on
measurements from a particular machine; thus, they circumvent the problem of how to model the
photon distributions emerging from the source and incident on the patient/phantom, which can be
a significant obstacle in accomplishing computational radiation transport based models.
The real challenge for these empirical-based methods, then, lies in correcting the
homogeneous water absorbed dose distribution to account for the many inhomogeneities present
in the human body, including air cavities, bone, dense muscle mass, and sparse lung tissue.
One way of addressing this problem is to apply "equivalent-thickness corrections" to the
water measurements. These corrections were based on compressing or stretching isodose lines by