Biomedical Engineering Reference
In-Depth Information
should be placed on the PRVs. However, this concept has not caught
on, and it is rarely the case that PRV requirements are imposed.
The requirements on OARs are almost always stated as constraints. A
dose or dose
−
volume value is given which must not be exceeded,
always with the idea that even lower values would be desirable.
The following normal tissue requirements are typical, alone or in
combination:
•
the maximum dose to the optic disc may not exceed 50 Gy;
no more than ⅓ of the kidney may receive more than 60 Gy and,
no more than ⅔ of the kidney may receive more than 30 Gy;
•
the dose to the PTV may not exceed 80 Gy.
The last of these constraints is designed to set an upper limit on the
dose delivered within the PTV on the grounds that it may include
normal tissue stroma whose preservation may be important to avoid
long term complications.
In addition to the constraints just listed, one can set biological
constraints, such as have been discussed in Chapter 5. For instance:
•
•
the NTCP for pneumonitis of the lung should not exceed 10%;
the NTCP for myelitis should not exceed 0.2%;
•
the EUD for the bladder should not exceed 40 Gy.
There may also be constraints on the dose fractionation. The total
dose allowed to be given to a specific OAR may need to be qualified,
for example:
•
•
the maximum dose per fraction delivered to the optic disc may
not exceed 1.5 Gy.
Other requirements
A radiation treatment may contain two or more sequential
segments
where one segment, for example, might treat the primary tumor and
regional nodes to a dose of 50 Gy, followed by a second segment in
which the primary tumor is given a boost dose of 20 Gy for a total
tumor dose of 70 Gy. The treatment aims and the prescription need to
be separately supplied for each segment, including the
segment dose
which is, in essence, the prescription dose for the segment. Each
segment of a radiation therapy course is represented by one and only
one plan. The overall treatment plan, then, is the composite sum of
these plans.
