Biomedical Engineering Reference
In-Depth Information
Split course Two conventional dose fractions are given daily, separated by a time
brake between them. Total dose is unchanged or higher than for standard treatment
and total treatment time is higher. I wonder if a larger line break is needed here in
order to show the description of special items is finished?
Some principles are pertinent when selecting a fractionation schedule of radiation
therapy to treat a patient [
13
]. Multiple daily fractions can be more effective in
rapidly growing tumours with a cell line exhibiting a high growth fraction. Fraction-
ation to less than one treatment per day and higher doses can be more efficacious for
slowly-growing tumours or tumour cells with a large repair capacity for sublethal
damages. Normal tissues behave as actively proliferating cells concerning acute
reactions but as slowly proliferating cells concerning the tissue's manifest late
injury; 4-8 hours should be allowed between fractions for a maximum repair of
normal tissues. When applying accelerated fractionation over a shorter total period,
some reduction in the total dose must be introduced. These schedules appear to
be preferable for use with hypoxic cell sensitizers or other chemical modifiers of
radiation response that require the presence of a high concentration of the compound
in the tumour at the time of radiation exposure. Finally, the aim of hyperfractionation
is to achieve the same incidence of late effects on normal tissue that is observed with
a conventional regime, with an increase in tumour control probability.
The
dose rate
can significantly influence the biological response to a given dose.
This effect is more evident for dose rates between 1-10 Gy/h. The biological effect
achieved by a given irradiation dose decreases as the dose rate diminishes, allowing
for an increase in cell repair. For high dose rates, the tumour dose must be decreased
in comparison to that delivered at low dose rates because of the effect on normal
tissues. The dose-rate-effect has special interest in brachytherapy and external cobalt
units because its numerical value diminishes with time in these radiation therapy
techniques [
14
].
24.1.3
Process of radiation therapy
The clinical use of radiation is a complex process that, from the moment when
he/she is first referred to a radiation therapy unit until the moment they finish
treatment, includes the following steps for a patient. It involves many health pro-
fessionals with a variety of interrelated functions (radiation oncologists, physicists,
technicians and nurses).
Clinical evaluation An initial evaluation of the patient is made and extent and nature
of the tumour is determined by a complete physical examination and a review of all
diagnostic studies. The full extent of the lesion should be determined and staging
should be established accordingly. The radiation oncologist must be aware of the
biological and pathological characteristics of the tumour, as well as any clinical
manifestations, so that micro-extensions of the tumour can be included in the treated
volume.
