Biomedical Engineering Reference
In-Depth Information
Treatment delivery device: gantry
Because proton beam therapy originated
in physics laboratories, for decades only
fixed horizontal beams were available.
These are still used for specialized
treatments such as those of ocular
melanomas, but isocentrically rotating gantries have now become the
beam delivery device of choice. In photon linacs, the entire system,
except for the supporting power supplies and rf amplifiers but
including the accelerator and beam-transport system, is packed into
the gantry. With protons, the accelerator and beam transport are
generally separate
3
and the gantry's function is: a) to allow the beam
to be directed towards the patient from any direction in the plane of
rotation; and b) to carry the beam delivery system, of which more
below.
Two types of gantry have been developed: a large-throw gantry with a
diameter of the order of 10 to12 meters; and a compact gantry with a
diameter in the range of 4-7 meters, such as the prototype realized at
the Paul Scherrer Institute in Switzerland and now in clinical use
there. Both consist of a large mechanical structure which is supported
on rollers or roller bearings and can rotate by somewhat more
than 360
courtesy of IBA
. The structure supports a series of magnets - really just an
extension of the beam-transport system - and the beam delivery
system, which is discussed below.
Additional flexibility in beam direction and positioning capability is
provided by a treatment couch that has, in proton therapy, evolved to
have all six degrees of freedom of movement; in addition to the linac
couch's three directions of translation and rotation in the horizontal
plane about isocenter, pitch and roll motions are provided. These
additional degrees of freedom allow for easy correction of the
patient's orientation without requiring large motions of the gantry and
couch.
Tumors frequently abut, or are very close to, critical normal tissues.
This leads to a very tight requirement on overall beam-pointing
accuracy of 1 mm or better - which requires that the gantry and
patient positioner both be extremely reproducible and that the patient
°
3
Although, at the time of writing, gantry-mounted accelerators are being
considered for “single room” proton therapy.
