Linear Energy Transfer and Relative Biological Effectiveness

ByDr.Ayush Garg

Deposition Of Radiant Energy

• If radiation is absorbed in biologic material, the events(ionization) tend to localize along the tracks of individual particles in a pattern that depends upon the type of radiation involved.

• X-ray photons give rise to fast electrons carrying unit electrical charge and have very less mass. The primary events of x-rays are well separated in space and hence said to be sparsely ionizing.

• Cobalt 60-γ-rays are even more sparsely ionizing than x-rays

• Neutrons give rise to recoil protons carrying unit electrical charge but mass 2000 times greater than that of electrons. Neutrons are intermediately ionizing.

• α-particles carry 2 electrical charges and 4 times heavier than a proton. They are densely ionizing.

Densely vs. Sparsely ionizing Sparsely ionizing: ionizing events are well separately in the space, like: X-ray

Time

Sparsely ionizing radiation

High dose sparsely ionizing radiation

Time

Linear Energy Transfer• LET is the energy transferred per unit length of the track. • Unit -kiloelectron volt per micrometer (keV/um) of unit

density material. • In 1962, the International Commission on Radiological

Units defined this quantity as follows: – The LET (L) of charged particles in medium is the quotient of

dE/dl, where dE is the average energy locally imparted to the medium by a charged particle of specified energy in traversing a distance of dl.

• That is, L= dE/dl

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Low and High LET Radiations

• Low LET Radiation:– This is a type of ionizing radiation that deposit less amount of

energy along the track or have infrequent or widely spaced ionizing events.

– Eg. x-rays, gamma rays• High LET Radiation:

– This is a type of ionizing radiation that deposit a large amount of energy in a small distance.

– Eg. Neutrons , alpha particles

gamma rays

deep therapyX-rays

soft X-rays

alpha-particle

HIGH LETRadiation

LOW LETRadiation

Separation of ion clusters in relation tosize of biological target

4 nm

The Spatial Distribution of Ionizing Events Varies with the Type of Radiation and can be defined by LET

• High LET radiation ionizes water into H and OH radicals over a very short track. In fig 1, two events occur in a single cell so as to form a pair of adjacent OH radicals that recombine to form peroxide, H2O2, which can produce oxidative damage in the cell.

• Low LET radiation also ionizes water molecules, but over a much longer track. In fig 2, two events occur in separate cells, such that adjacent radicals are of the opposite type: the H and OH radicals reunite andreform H2O.

High vs Low LET Radiations

• High-LET radiations are more destructive to biological material than low-LET radiations.

• The localized DNA damage caused by dense ionizations from high-LET radiations is more difficult to repair than the diffuse DNA damage caused by the sparse ionizations from low-LET radiations.

• High LET radiation results in lower cell survival per absorbed dose than low LET radiation.

• High LET radiation is aimed at efficiently killing tumor cells while minimizing dose to normal tissues to prevent toxicity.

• Biological effectiveness of high LET radiation is not affected by the time or stage in the life cycle of cancer cells, as it is with low LET radiation.

• Track Average: calculated by dividing the track into equal lengths, calculating the energy deposited in each length, and finding the mean.

• Energy Average: is obtained by dividing the track into equal energy increments and averaging the lengths of track over which these energy increments are deposited.

Typical LET Values

Higher the energy

Lower is LET

Therefore lower its biologic

effectiveness

Relative Biologic Effectiveness(RBE)

• The National Bureau of Standards in 1954 defined RBE as:

• The RBE of some test radiation (r) compared with x-rays is defi ned by the ratio D250/Dr, where D250 and Dr are, respectively, the doses of x-rays

• and the test radiation required for equal biologic effect.

• Eg. A comparison of neutrons with 250kV x-rays in lethality of plant seedlings. The end point of observation being death of half of plants(LD50). Suppose if LD50 for x-rays is 6Gy and for neutrons is 4Gy then RBE of neutrons compared with x-rays is 6:4 or 1.5

Factors Determining RBE• Radiation quality• Radiation dose• Number of dose fractions• Dose rate• Biologic system or end point

Survival curves for mammalian cells exposed to x-rays and fast neutrons

•X-ray survival curve has large initial shoulder and neutron curve has smaller shoulder and steeper final slope•RBE increases with decrease in dose•RBE for fractionated regimen with neutrons is greater than for single exposure.•The little or no shoulder of neutron curve indicates less wastage of dose whereas wide shoulder of x-ray curve indicates wastage of a part of dose each time in fractionated regime

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RBE for different cells and tissues

• The intrinsic radiosensitivity among the various types of cells differ from each other.

• The curves demonstrate the variation of radiosensitivites for x-rays and markedly less variation for neutrons.

• X-ray survival curves have large and variable initial shoulder whereas for neutrons ,it is small and less variable

• Hence RBE is also different for different cell lines.

RBE as a function of LET

•As the LET increases from about 2keV/µm for x-rays upto 150 keV/µm for α-particles, the survival curve becomes steeper and the shoulder of the curve becomes progressively smaller.•Larger shoulder indicates the accumulation and repair of the large amount of sub-lethal radiation damage

RBE as a function of LET

As the LET increases, the RBEincreases slowly at first, and then more rapidly as the LET increases beyond 10 keV/µm. Between 10. and 100 keV/µm, the RBE increases rapidly with increasing LET and in fact reaches a maximum at about 100 keV/µm. Beyond this value for the LET, the RBE again falls to lower values.

The Optimal LET• LET of about 100keV/µm is optimal in terms of producing

biologic effect• At this density of ionization the average separation between

the ionizing events just about coincides with the diameter of DNA double helix(2nm) and has highest probability of causing DSBs by passage of a single charged particle.

In x-rays, probability of a single track causing a DSB is low and requires more than one track.

Much more densely ionizing radiations (eg. LET of 200keV) readily produce DBSs but energy is wasted as events coincide with

each other

The Oxygen Effect and LET• Oxygen enhanced ratio(OER) is the ratio of doses of

radiation administered under hypoxic to aerated conditions needed to achieve the same biologic effect.

• OER for different types of radiations are as follows:– X-rays: 2.5– Neutrons: 1.6– 2.5-MeV particles:1– 4-MeV particles: 1.3

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Survival curves for cultured cells of human origin in hypoxic and aerated conditions determined for four different types of

radiation.

OER AS A FUNCTION OF LET

At low LET (x- or y-rays) with OER between 2.5 and 3, as the LET increases, the OER falls slowly until the LET exceeds about 60 keV/µm, after which the OER falls rapidly and reaches unity by the time the LET has reached about 200keV/µm.

OER AND RBE AS A FUNCTION OF LET

•The rapid increase in RBE and the rapid fall of OER occur at about the same LET 100keV/µm .•Two curves are virtually mirror images of each other.

Example

• To achieve 50% survival fraction, 250 kV x-ray needs 2 Gy, but the tested particle needs 0.66 Gy only

RBE = D250/Dt 2 = 2 / 0.66 = 3RBE at survival fraction of 0.5 for the

tested particle is 3.

Radioprotectors

WAYS TO IMPROVE THE PROTECTION OF NORMAL

TISSUES

IDEAL RADIOPROTECTOR

• Preservation of the anti-tumor efficacy of radiation

• Wide window of protection against all types of toxicity

• High theraputic ratio

• High efficacy/toxicity profile(Low intrinsic toxicity profile)

• Easy and comfortable administration

• Reasonable cost-effectiveness

Chemical RadioprotectorsRadioprotector: A chemical compound that reduces the biologic consequences of radiation

Some may protect whole animals as they cause vasoconstrictions/upset metabolism to <O2 concentration. e.g. Sodium Cyanide, CO, epinephrine, serotonin, histamine

• The most remarkable group of true radioprotectors is the sulfhydryl (SH) compounds.

• The simplest is cysteine, an SH compound• containing a natural amino acid, the

structure of which is

• Bacq and his colleagues• in Europe independently discovered that

cysteamine• could also protect animals from total• body irradiation. This compound has a

structure represented by

• Animals injected with cysteamine to concentrations of about 150 mg/kg require doses of x-rays 1.8 times larger than control animals to produce the same mortality rate. This factor of 1.8 is called the dose reduction factor (DRF), defined as

MECHANISM OF ACTION

• The mechanisms most implicated in SH mediated cytoprotection include:

• 1. Free-radical scavenging that protects against oxygen-based free radical generation by ionizing radiations or chemotherapy agents such as alkylating agents

• 2. Hydrogen atom donation to facilitate direct chemical repair at sites of DNA damage

• Radioprotectors containing a sulfhydryl group exert their effect by scavenging free radicals and by reducing free-radical damage to DNA. They are most effective for radiations characterized by low linear energy transfer (LET), becoming progressively less effective with increasing LET because the amount of local damage is so great.

HISTORY OF DEVELOPMENT OF RADIOPROTECTERS

• After World War II, a development programme was initiated in 1959 by the U.S. Army at the Walter Reed Institute of Research to identify and synthesize drugs capable of conferring protection to individuals in a radiation environment, but without the debilitating toxicity of cysteine or cysteamine.

• Over 4,000 compounds were synthesized and tested.

Historically known fact

NH2

HS-CH2-CH COOH

Problem was their toxicity• Nausea and Vomiting

General structure: i. A free SH group at one end

ii. Strong basic function, i.e. an amine or guanidine at other

Toxicity of the compound decreased because the phosphate group is stripped inside the cell, and the SH group begins scavenging for free radicals.

• The important discovery was made that the toxicity of the compound could be greatly reduced if the SH group was covered by a phosphate group.

AMIFOSTINE (WR-2721) AS ARADIOPROTECTOR IN

RADIOTHERAPY• The only radioprotective drug approved by the U.S. Food and

Drug Administration (FDA) for use in radiation therapy is amifostine (WR-2721), sold under the trade name Ethyol for use in the prevention of xerostomia in patients treated for head and neck cancer.

• The RTOG conducted a phase III randomized clinical trial, which demonstrated the efficacy of amifostine in reducing xerostomia in patients with head and neck cancer receiving radiotherapy without prejudice to early tumor control.

• The drug was administered daily, 30 minutes before each dose fraction in a multifraction regimen. Three months post treatment, the incidence of xerostomia was significantly reduced

>30 min---NO difference

<30 min--- Difference present

Head & Neck Cancers

• SCC of H&N • 75% parotid gland was present in the fields • Dose was 200 mg/m2 daily,15–30 minutes

before each fraction of radiation therapy (1.8 –2.0 Gy/day, 5 days per week for 5–7

weeks, to a total dose of 50–70 Gy).

• Amifostine is a phosphorothioate that is nonreactive and does not readily permeate cells, primarily because of its terminal phosphorothioic acid group.

• It is therefore a “prodrug.” When dephosphorylated by the enzyme alkaline phosphatase, which is present in high concentrations in normal tissues and capillaries, it is converted to the active metabolite designated WR-1065.

• This metabolite readily enters normal cells by facilitated diffusion and scavenges free radicals generated by ionizing radiations or by drugs used in chemotherapy such as alkylating agents.

Amifostin (WR-2721)Phosphorothioate prodrug-inactive, does not readily

permeate cells.Dephosphorylation by ALP(expressed on

endothelial cell lining & proximal renal tubular cells)

Active thyol (WR 1065)

OxidationEnter in cell by facillited diffusion

WR – 33278(polyamine like disulphide metabolite)

Radioprotection

RADIOPROTECTORS ANDCHEMOTHERAPY

The experimental clinical use of amifostine has shown that the compound offers significant protection against nephrotoxicity, ototoxicity, and neuropathy from cisplatin and hematologic toxicity from cyclophosphamide.

DIETARY SUPPLEMENTS ASCOUNTERMEASURES TO

RADIATION• Example is the soybean-derived serine protease

inhibitor known as the Bowman-Birk inhibitor (BBI), which has long been proposed as a cancer chemopreventive agent.

• Another possibility is a cocktail of common antioxidants, including

• L-selenomethionine• Ascorbic acid• Nacetyl cysteine• Alpha-lipoic acid• Vitamin E succinate• Coenzyme Q10.

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