APPENDIX
VAPPENDIX
V
The 2.5 cm margins (for the boost fields) are three-dimensional:
superiorly, inferiorly, medially and laterally as well as anteriorly
and posteriorly. (See Diagrams A & B)
Please note that it is important that the dose inhomegeneity
be minimized particularly between the central axis dose and the
tumor minimum dose. The protocol dose should be prescribed at
the center of the target volume.
The inferior margin of the temporal lobe follows
the outline of the sphenoid sinus. In order to encompass a 2.5
cm margin around most temporal lobe tumors, the entire temporal
fossa usually needs to be included. In order to deliver a full
dose to the inferior portion of the temporal fossa, the inferior
border of the treatment portals should be below the bottom of
the sphenoid sinus. (See Diagram B).
Diagram A Diagram B
Diagram 1 illustrates that even with 6 MV photons utilizing parallel opposed portals, if the treatment portal encompasses the posterior occiput or the frontal region, it is obvious that without wedges the target minimum dose is 10% lower than the prescribed central axis dose, and there is a hot spot in the thinner portion that can be 10% to 15% hotter. These differences can be minimized with the use of wedges as in Diagram 2. The tumor minimum is only 2% lower than the central axis dose, and the hot spots are smaller and of lower dose. Isodose distributions are required for parallel opposed fields.
Diagram 3 is an example of a composite plan for an
anteriorly located lesion with significant edema. By combining
large 15 wedged parallel opposed fields to 57.60 (Diagram 4),
it was possible to even out hot spots and treat the target volume
to a high dose throughout with minimal gradient. Optimization
of individual plans and the submission of composite plans is an
essential requirement of this study.
