Biomedical Engineering Reference
In-Depth Information
The presentation of the uncertainty in a three-dimensional dose
distribution presents a challenging problem due to the plethora
of data. One approach is that described in Urie
et al.
(1991) and
Goitein (1985), an example of which is shown in Figure 8.9. In this
case, because of the large target volume and limitations on field size,
abutting fields had to be used. In Figure 8.9, three dose distributions
are juxtaposed: (a) the nominal (most likely) dose, (b) the upper-
bounds on the dose, and (c) the lower-bounds on the dose at each
point at the stated probability level. This figure highlights the scale of
potential problems that can arise at a beam juncture due to possible
treatment uncertainties and, in Figure 8.9d, how the size of these
uncertainties can be reduced by beam feathering.
Figure 8.10. Uncertainty in a three-field plan employing proton beams.
(a) The dose distributions in a transverse section for the three individual
beams together with the composite (nominal) dose distribution.
(b) The “worst case” dose distribution in the same section (see text).
Reproduced with permission from ICRU78 (2007).
An alternative approach has been developed by Lomax (ICRU78,
2007). In this method, dose distributions are calculated for a number
of translated or rotated CT data sets, and, potentially, from data sets
with altered CT numbers to simulate density uncertainties. A hybrid
dose distribution, that indicates the worst-case dose at any point, is
then computed as follows. For points within the PTV, the dose is set
to the lowest dose at that point in any of the plans. For those points
outside the PTV and, hence, within normal tissue the dose is set to the
highest dose in any of the plans. This one display thus shows both
potential cold spots within the tumor and potential hot spots within
normal tissues. The result of such an analysis is shown in Figure
8.10; the potential cool regions in the tumor, colored blue corres-
ponding to a 10 to 20% dose reduction, are due to possible junction
