Biomedical Engineering Reference
In-Depth Information
because of the greater ease and, perhaps, safety with which they can
be delivered.
Determination of beam direction(s)
Generally, it makes little sense to use two photon beams that cover
the same volume but are separated by only a small angle, say 15
or
less; their combined dose distribution will not be very different from
that of a single beam.
Quite frequently, the geometry of the target volume and/or of a
critical OAR will suggest a particular approach. This is the case, for
example, with the beam illustrated in Figure 6.9 in Chapter 6 where
the obliquity of beam direction has been chosen so that the beam's
edge runs near-parallel to the medial surface of the target volume.
Naively, one might think that all beams should avoid all OARs.
2
However, this is never possible; one cannot achieve zero or very low
°
dose except in a limited volume. One then makes a conscious
decision to include a particular OAR in one or more beams, subject
only to the requirement that the dose thus delivered be below the
constraints established in the planning aims. The directions of other
beams are then picked to avoid the OAR in question to the extent
possible.
Modern linacs have a rotating gantry that allows beams to be directed
towards a central point (isocenter) from any direction within a plane
perpendicular to the axis of the gantry's rotation (see Figure 1.1 of
Chapter 1). The patient couch, in addition to being able to move
the patient laterally, longitudinally and vertically, can rotate in a
horizontal plane about isocenter, thereby enabling the use of beams
directed out of the transverse plane. However, this degree of freedom
is rarely used
3
and most plans used in practice feature coplanar beams
2
The concept that there are radiosensitive and non-radiosensitive OARs, and
that only the sensitive ones need to be worried about, is somewhat
dangerous. All tissues are affected by radiation and one should not ignore
the dose to any OAR.
3
In earlier times, when I visited another radiotherapy department, I would
often ask that the treatment couch be rotated off the straight-ahead
position. Almost inevitably there was a pile of dust where the couch had
been, suggesting that it had not been rotated for a long time; otherwise
the cleaning crew would have seen the dust and taken care of it. The
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