Biomedical Engineering Reference
In-Depth Information
they will enter the capillary bed of a target organ, but are too large to
pass completely through the organ. Thus, the target organ acts like a
filter in which the radioactive glass particle becomes trapped.
13.5.2 Useful Radioisotopes
An important advantage of
in situ
irradiation is that much larger,
more than 10 times, therapeutic doses of radiation can be delivered
using radioisotopes that emit the more localized (shorter-range) beta
radiation. In comparison, radiation administered externally has to pass
through other tissue, and shaped radiation beams have to be aimed from
several angles, meeting at the tumor in order to reduce the dose in the sur-
rounding, healthy tissue. To date, REAS and borate glass microspheres
containing beta-emitting
90
Y,
153
Sm,
165
Dy,
166
Ho, and
186
Re/
188
Re
have been tested in animals [5-7, 9, 10, 13, 14]. Only REAS micro-
spheres containing
90
Y are in commercial use (TheraSphere
TM
) to treat
patients with primary liver cancer, hepatocellular carcinoma (HCC).
In addition to the radioisotopes mentioned above, other radioisotopes
potentially useful for
in situ
irradiation are
32
P,
108
Pd,
113
In,
124
Te,
146
Nd,
168
Yb, and
177
Lu.
13.5.3 Radiation Dose
The maximum amount of radiation that glass microspheres can safely
deliver to target organs is still undetermined, but should be expected
to depend upon such factors as the specific organ being irradiated, the
specific radioisotope and type of radiation emitted, and the specific
activity of the microsphere at the time of injection. In general, the
radiation doses are much larger than those administered by an external
beam. For example, prior to use in humans, dogs were injected with
90
YAS glass microspheres that delivered up to 350Gy (35 000 rad) to
their liver with no ill effects [15]. Patients with inoperable liver cancer
(HCC) are now being injected with YAS microspheres,
100mg, that
deliver a dose calculated to be up to 150Gy (15 000 rad) to the entire
liver [16, 17]. Since the glass microspheres tend to locate preferentially
in the malignant tumor(s), as opposed to the healthy tissue, localized
regions in the tumor have received doses up to 3000Gy (300 000 rad), as
calculated from the number and distribution of YAS glass microspheres
found in a treated liver [18].
<
