Biomedical Engineering Reference
In-Depth Information
levels will yield a different tumour control, depending on the extent of the lesion.
A
boost
is a dose applied through small portals to residual disease.
Shrinking field
technique
is the use of progressively size-reduced portals in order to apply higher
radiation doses to the central portion of the tumour.
Ionizing radiations induce changes in normal tissues surrounding a tumour such
as structural alterations, tissue destruction, severe dysfunction and death. They are
related to the cell's radiation sensitivity and kinetic characteristics and depend on
the total radiation dose delivered, the fractionation schedule used and the tissular
volume treated. The
Minimal Tolerance Dose
TD5/5 is the radiation dose that will
cause a severe complication rate of no more than 5% in normal tissues within 5 years
of treatment [
10
]. There is a correlation between dose and both tumour control and
the probability of complications. The
therapeutic ratio
is the optimal dose that will
produce the maximum probability of tumour control with a minimal frequency of
complications.
To increase the therapeutic ratio, various fractionation schedules are used in
radiation therapy so that the total dose delivered is divided into a number of
fractions. Dose-time factors express the interdependence of total dose, total time
in which it is delivered and number of fractions. From a radiobiological point of
view, the advantages of dose fractionation are that fractionation favours repair of
sub-lethal damage, repopulation of cells between fractions, redistribution of cells
throughout the cell cycle and reoxygenation. Dose fractionation allows a reduction
in the absolute number of tumour cells by the initial fractions, reducing the number
of hypoxic cells through cell killing and reoxygenation so that the amount of
oxygen per remaining cell increases. This effect still increases because blood vessels
previously compressed by a growing cancer are decompressed; this permits a better
oxygenation reducing the distance that oxygen must diffuse trough tissue with each
fraction. Fractionation exploits the difference in the recovery rate between normal
tissues and neoplasic tumours and the patient's tolerance improves when applying
fractionated irradiation [
11
].
It is important to remember that an inadequate fractionation schedule with a
prolonged course of therapy and small daily fractions may allow the growth of
rapidly proliferating tumours and may decrease early acute reactions but will not
protect of serious late damage to normal tissue. The standard fractionation for
radiation therapy is the delivery of five weekly fractions of 150-200 cGy. Altered
fractionation schedules are:
Hyperfractionation A large number of dose fractions smaller than conventional are
given daily. The total dose administered daily is 15-20% higher than for standard
fractionation within a unchanged total period of time. The total dose delivered is
higher than for standard fractionation [
12
].
Accelerated fractionation In multiple daily fractions, several conventional radiation
dose fractions are delivered over a shorter total period reaching a similar total
administered dose. With the concomitant boost, a standard dose fraction is delivered
daily together with an additional dose to the final target volume (boost) during the
schedule of general radiation therapy.
