Radiobiology is a branch of science that deals with the action of ionizing
radiation on biological tissues and their cellular and molecular
components (Hall and Giacca, 2006). The biological effects of radiation
result mainly from damage to the DNA, which is the most critical target
within the cell. Damage to DNA occurs by both direct and indirect action.
In direct action the radiation interacts directly with the DNA, but about
two-thirds of the biological damage by low-LET radiations (sparsely
ionizing radiations, called low linear energy transfer), such as X-rays or
electrons, is due to indirect action. In indirect action the radiation
interacts with other molecules and atoms (mainly water, since 80% of a
cell is composed of water) within the cell to produce free radicals that can,
through diffusion in the cell, damage the DNA. The indirect action can be
modified by chemical sensitizers or radiation protectors. Direct action is
the dominant process in interaction of high-LET particles, such as
neutrons and charged heavy particles, with DNA.
Irradiation of a cell can result in division delay (the cell is delayed from
going through division), apoptosis (the cell dies before it can divide or
after division by fragmentation into smaller bodies that are taken up by
neighboring cells), or reproductive failure (the cell dies when attempting
the first or subsequent mitosis). Radiation damage to cells can be lethal,
sublethal (damage can be repaired in hours unless additional sublethal
damage is inflicted and eventually leads to lethal damage), or potentially
lethal (can be manipulated by repair processes when cells are allowed to
remain in a nondividing state).
Much of our understanding of the mechanisms of the response of tissues
and organs to dose fractionation and dose rate has come from studies of
colony-forming cells in culture. A cell survival curve describes the
relationship between the surviving fraction of cells, that is, the fraction of
irradiated cells that maintain their reproductive integrity (clonogenic
cells), and the absorbed dose. Cell survival as a function of radiation dose
is conventionally represented graphically by plotting the surviving fraction
on a logarithmic scale on the y-axis against dose on a linear scale on the
x-axis.
The type of radiation influences the shape of the cell survival curves
(Steel, 2002). On the one hand, densely ionizing radiations exhibit a cell
survival curve that is almost an exponential function of dose, shown by
almost a straight line on the log-linear plot. For sparsely ionizing radiation,
on the other hand, the curves show an initial slope followed by a curving
shoulder region and then become nearly straight at higher doses. Factors
that make cells less radiosensitive are removal of oxygen to an hypoxic
state, the addition of chemical radical-scavengers, the use of low dose-
rates or multifractionated irradiation, and cells synchronized in the late-S
phase of the cell cycle. The linear-quadratic model is now most often used
to describe the cell survival curve assuming that there are two
components to cell killing by radiation. A constant a describes the linear
component of cell sensitivity to killing on a semi-log plot of (log) survival
versus (linear) dose, and b describes the increasing sensitivity of cells to
higher radiation doses. The ratio a/b is a measure of the curvature of the
survival curve, and is the dose at which the linear and quadratic
components of cell killing are equal. The a/b ratio is lower and the curve
on a semilog plot is more pronounced for homogeneous, slowly
proliferating cell populations, such as in slow-renewing organ systems like
kidney and spinal cord. The a/b ratio is higher and the survival curve is
straighter for heterogeneous, rapidly proliferating cell populations, such
as the regenerative target cell populations in oral mucosa and intestine.
One possible contributor to this straightening is the presence of
subpopulations with different sensitivities as a function of cell-cycle phase.
The a/b ratio is generally in the range 7–20 Gy for early reactions in
tissues (10 Gy is commonly used) and 0.5–6 Gy for late reactions (3 Gy is
commonly used).
Various chemical agents may alter the cell response to ionizing radiation,
either reducing or enhancing the cell response (Hall and Giacca, 2006).
Chemical agents that reduce cell response to radiation are called
radioprotectors. They generally influence the indirect effects of radiation
by scavenging the production of free radicals. Chemical agents that
enhance the cell response to radiation are called radiosensitizers and they
generally promote both the direct and indirect effects of radiation.
Examples are halogenated pyrimidines that intercalate between the DNA
strands and inhibit repair, hypoxic cell radiosensitizers that act like
oxygen, and bioreductive agents that are activated under hypoxic
conditions to kill hypoxic cells.
The presence or absence of molecular oxygen within a cell influences the
biological effect of ionizing radiation. Only several micromolar units of
oxygen are required to sensitize cells to radiation, and the maximum
amount of sensitization on a dosage basis is about 2.5–3-fold, as occurs in
well-oxygenated normal tissues and in oxic tumor cells. The sensitization
effect is smaller at doses lower than 1 Gy, and when high-LET radiation is
used, for example, neutrons, carbon ions. Cell sensitivity is increased with
high-LET radiation, and the relative biological effectiveness (RBE)
compares the dose of test radiation to the dose of standard radiation to
produce the same biological effect. The RBE depends largely on the cell
type and the dose per fraction being used. For therapeutic applications,
the RBE in the era of neutron radiotherapy was about 3, and it is also
around 3 for the newer dose-delivery patterns and protocols using carbon
ions.
The time scale involved between the breakage of chemical bonds and the
biological effect may be hours to years, depending on the type of damage.
If cell killing is the result, it may happen in hours or much later when the
damaged cell attempts to divide. Tissue and organ reactions can appear
early or late depending on the renewal characteristics of the tissues. If the
damage is oncogenic (cancer induction), then its expression may be
delayed for years. If the damage is a mutation in a germ cell, the effects
may be expressed after one or more generations.
Early reactions in tissues are characterized, for example, by inflammation,
tissue edema, denudation of epithelia, and hemopoietic depression (Steel,
2002). Late reactions are characterized, for example, by teleangiectasia of
blood vessels, dermal fibrosis, tissue atrophy, ulceration, and intestinal
stenosis.
Acute total body radiation exposure can result in one of several radiation
syndromes, leading to death, depending on the dose range. The bone
marrow syndrome occurs in the dose range 1 Gy < dose < 10 Gy, the
gastrointestinal (GI) syndrome occurs in the dose range 10 Gy < dose <
100 Gy, and the central nervous system (CNS) syndrome occurs after
doses > 100 Gy. Partial body irradiation to doses at the low end of the
dose range for the GI syndrome, of solely the kidneys, lung, or heart, can
also result in late lethal reactions. Chronic irradiation over several years to
accumulated doses of several Gy can also lead to a chronic radiation
syndrome (CRS), characterized by persistent reactions in the hemopoietic,
immune, and digestive systems.
The effects of radiation on tissue as a function of dose are measured via
dose–response curves. Functional endpoints for various tissues are
measured on a graded reaction scale or expressed as a proportion of
cases in which reactions are greater than a specified level. Early tissue
reactions can be alleviated by the application of radical scavengers before
irradiation and/or growth factors after irradiation. These include
hemopoietic growth factors in the case of bone marrow, epithelial growth
factors (e.g., KGF) for mucosa and epithelium. The mechanism is for
growth factors to accelerate the repopulation and differentiation of
precursor cells. The lesser reactions imply that higher radiation doses can
be tolerated, by up to double the radiation dose in the case of KGF and
oral mucosa. These radioprotective effects may be very useful in
accelerated radiotherapy in which early reactions are often more severe
than in conventional treatments. Late reactions can also be modified by
various vascular-associated compounds, such as essential fatty acids in
skin, and angiotensin II enzyme inhibitors or receptor blockers in kidney.
These agents at least delay the onset of functional injury, and may also
reduce the incidence. Hence in principle, this might not only reduce late
reactions that are dose limiting, but also allow the possibility of some dose
escalation, which would increase tumor control rates.
In general, reactions in tissues are greater when the volume irradiated is
increased. This effect is most marked for very small volumes, and in that
case it is due largely to migration of cells from the edges of the irradiation
fields, which has a greater influence on healing small than large volumes.
For larger irradiated volumes, most common in radiotherapy, the volume
effect is less marked but still important. For skin, the tolerance of a larger
irradiated region may be reduced although the reaction level may be little
increased. This is because the likelihood of any area within the irradiated
region not healing properly increases with an increase in the number of
such areas irradiated. Also, the architecture of organ systems has an
influence on the volume effect. Some organ systems comprise functional
tissue subunits arranged in parallel, such as nephrons in the kidney and
alveoli in the lung. In these cases, parts of the organ can be irradiated and
injured without causing functional defects, because the other regions can
compensate. In contrast, organs or tissues that comprise functional
components in a serial arrangement – for example spinal cord, intestine,
and blood vessels – can be functionally damaged to irreparable levels by
injury in one small region. The presence of a volume effect is the rationale
for new strategies in radiotherapy to reduce irradiated volumes. These
include dynamic imaging used to reduce irradiated tumor margins,
intensity-modulated radiotherapy used to shape the irradiated fields more
closely around the tumor, and protons or carbon ions that can be used to
provide a sharper edge to the irradiated volume than is possible using X-
rays, gamma-rays, or electrons.
The aim of radiotherapy is to deliver enough radiation to the tumor to
destroy it without irradiating normal tissue to a dose that will lead to
serious complications (morbidity) (Hall and Giacca, 2006). The basis of
dosefractionation practices is rooted in five primary biological factors
called the ‘five Rs’ of radiotherapy: radiosensitivity (mammalian cells have
different radiosensitivities), repair (mammalian cells can repair radiation
damage), redistribution (of cells in the different phases of the cell cycle),
repopulation (cells repopulate while receiving fractionated doses of
radiation), and reoxygenation (of hypoxic cells during a fractionated
course of treatment).
The different radiosensitivities between patients led to a search for
biological or molecular markers that would allow the tailoring of different
doses to patient subgroups and improve overall outcome (Steel, 2002).
The best assay, that is, with the highest positive predictive value, in this
case 90%, has been the use of the enzyme marker TGF-b to predict, in a
high percentage of lung cancer patients, the progression of marked
radiation-induced fibrosis. However, none of the assays has proved
sufficiently robust to be able to implement this type of dose-
prescriptionvariation strategy in regular practice. Hope is now pinned on
the new molecular-based assays.
The biological mechanisms of conventional fractionation treatments are
explained as follows. Each daily dose of 2 Gy kills around 50% of tumor
cells through reproductive failure and apoptosis. The multiple daily
fractions spare normal tissues through repair of sublethal damage
between dose fractions and repopulation of normal cells. The former is of
note for late-reacting tissues, and the latter applies to early-reacting
tissues. The sparing of early reactions also prevents the consequence of
an exacerbation of late reactions. Concurrently, dose fractionation
increases tumor damage through redistribution of tumor cells into more-
sensitive cell cycle phases (G2/M) and allows reoxygenation of hypoxic
tumor cells. The differential (the therapeutic ratio) is increased as much as
possible between the response of tumor and early and late-reacting
normal tissues, so that small doses per fraction spare late reactions
preferentially, and a reasonable schedule duration allows regeneration of
early-reacting tissues, few consequential late reactions, tumor
reoxygenation, and as little tumor cell repopulation as possible.
Current standard fractionation schedules are based on five daily
treatments per week and the total treatment time of several to many
weeks (Steel, 2002; Hall and Giacca, 2006). This regimen reflects practical
aspects of dose delivery to patients, successful outcomes, and
convenience to staff delivering the treatment. Other fractionation
schemes are employed in particular cases with the aim of improving
further the therapeutic ratio. The most common are accelerated
fractionation (more than five daily fractions per week) and
hyperfractionation (more than one fraction per day with a smaller dose
per fraction, <1.8 Gy).
Following irradiation and recovery of normal tissues, there is some
residual injury. For example, skin is generally more susceptible to
mechanical trauma, and bone marrow may be compromised in stem-cell
content and hence more susceptible to subsequent cytotoxic agents. This
indicates that retreatment using radiation may be possible, but by using
lower doses. There are now many cases of reasonably successful
treatments of recurrences in patients and many data in experimental
animal systems, including kidney and spinal cord, showing the gradually
increasing tolerance to retreatment at later times after the first course of
irradiation.
