Radiobiology

Oral & maxillofacial radiology Radiobiology
Lec. 3 د.غسان علي

Radiobiology

Radiobiology: is the branch of science, which studies the effect of radiations on living tissues, these effects can be physical, chemical or both.
Adult human body consists of about 10 14 cells. In the human body, the platelet is the smallest cell having 3-4 µ diameter. Cell consist of nucleus and cytoplasm, certain cell don t contain the nucleus ( e.g. RBCs). The nucleus consist of chromosomes which contain the genes. DNA carries the genetic material and constitutes the genetic component of the cell.

Interaction of radiation with the cells:

a. Direct action:- the sensitive volume in the cell ( e.g. molecule, atom) is change by direct absorption of energy from the radiation. Direct action is explain in term of " target theory" where the sensitive volume is assumed as the target and production of the ionization.
b. Indirect action:- the sensitive volume is inactivated by transfer of energy from another volume which has absorbed energy from the radiation.

Radiolysis of water :

Human tissues consist of 85% of water. On radiation, most of the energy will initially be deposited in H2O, only a small proportion will be taken up by the materials such as skin, bone etc.
Ionization take place as follows:
H2O loss electron and becomes H2O+ . H2O H2O+ + e-

The electron captured by another molecule to give a negative molecule.
H2O + e- H2O-
This completes the formation of an a ion pair. Uptil this, the stability of molecule is maintained and comes under physical changes. After that the chemical changes occur.
H2O+ H+ + OH+

H2O- H- + OH-


Free radicals have an odd e-, they have a short life-time of about one microsecond and attack most of the organic substances. The free radicals can react with proteins, carbohydrates, hormones resulting in their breakdown.
The presence of oxygen at the time of irradiation acts as a sensitizing agent. The effect of radiation become low as the amount of oxygen contained in the tissues is decreased. and this called " oxygen effect".
Degree of susceptibility to radiation depending on : cellular differentiation and cellular reproduction. The following tissues are arranged according to their susceptibility to radiation.
• Blood forming cell.
• Reproductive organs.
• Bone and glandular tissues.
• Epithelium of alimentary canal.
• Skin and muscles.
• Nervous tissues.

Latent period :

There is always a time gap between the exposure and the clinical symptoms. And this is the latent period. It can be from hours to years, partially depending on the dose. More severe dose, smaller is the latent period. The end result of radiations may be either:

• Reversible : if the cell return to their re-irradiation stage.
• Irreversible : when the permanent change occurs.
• Conditional : when they are so affected that another similar and/or small dose will prevent their return to the normal.

Radiation effects on cell :
1. inhibition of the mitosis.
2. chromosome aberrations.
3. cell mutations.
4. cell death.

1. Inhibition of the mitosis :
This delay the onset of mitosis i.e. cell division. The severity of effect depend on the dose level and the rate of the dose. The effect can be temporary or permanent.


2. chromosome aberrations :
radiation can cause breakage in the chromosomes. Usually the break may be restitute and the damage may not be manifested. It may occur on defferent types:
• stickiness.
• Accentric or dicentric chromosome.
• Translocation
• Deletion

3. cell mutations :
the change is in the characteristics of the gene which will be manifested as genetic effect ( it refer to cell and not hereditary).

4. cell death:
irradiation can result in the death of the cell which may be due to the change in the physical properties of vital cell structures.

Lymphocyte, oocyte and spermatocytes are killed by few hundred rads while the organs where the rate of cell division is low, require several thousands of rads to cause death.[

Biological effects of radiations :

Biological effects can be considered in two aspects:
(a) somatic effects : it is occur in exposed individuals. It can be classify into:
• acute effects : this will be manifested within few hours to few days of acute irradiation and the severity of the effect will depend on the dose and the dose rate.
• chronic effects : are mainly due to low level of irradiation for longer period, or chronic irradiation

(b) Genetic: it is that which manifested in future generation of the exposed individuals.
When the patient’s teeth are exposed, it is said that 1/10000th of secondary/stray radiation are directed from the face to the reproductive organs in the males; and 1/7th of this in females. In children, the exposure is much more because of the short structure of the children.
Human embryo is most sensitive especially during 15-42 days of its life. Therefore, x-radiations in the pregnant women must be avoided. In heavy doses, sterility in human being has been reported.


Biological effects can be categorized into:
(a) Stochastic effects : for which the probability of an effect occurring rather than its severity is regarded as a function of the dose without threshold. Examples of stochastic effects are:
• Carcinoma
• Leukemia
• Hereditary effects
(b) Non-stochastic effect : for which the severity of the effect varies dose for which a threshold may matter. Examples of non-stochastic effects are:
• Cataract
• Shortening of life span
• Infertility


Lethal dose (LD 50/30): it is the whole body acute dose required to kill 50% of the exposed organisms within 30 days after irradiation. LD 50/30 for the man is 400-600 rem. It is much higher in fish 700, snail 10,000 and amoeba 100,000 rem.

Osteo radio necrosis :

It is an infection in the bone rendered necrotic by ionizing radiation. It is the frequent complication in the treatment of cancer of the oral cavity by irradiation.
Osteoradionecrosis result from either of the following or in combination :
(a) Radiation in massive doses
(b) Partial necrosis of the bone
(c) Trauma which cause infection
Radiation is delivered as a therapeutic measure for cure of malignant tumor of oral cavity e.g.
• Tongue
• Floor of the oral cavity
• Salivary glands
• Sinuses and neoplasms etc.
The small superficial lesions of face may cause osteoradionecrosis of mandible and maxilla. Necrosis and ulceration of soft tissues occur two to three months after the radiation.

Poor oral hygiene, periodontal diseases, residual roots, caries etc., are local factors which must be eliminated prior to irradiation to prevent osteoradionecrosis.
Systemic diseases which affect the state of health of the oral cavity, such as diabetes, anaemia etc., are predisposing factors.
Questionable teeth, which can cause infection later on must be removed prior to irradiation.
Any bone receive 5000-6000 rads radiations may not be recover from trauma in case of mandible, it must not exceed 2500 rads.
Dentist must question the patients before extractions regarding their irradiation dose and time.
Tooth in reasonable good condition may deteriorate after irradiation. Saliva is diminished and enzymes are altered. Teeth may become brittle and prone to caries.

Radiographic appearance

Radiographic image in case of osteoradionecrosis is very deceptive before infection. Trabecular pattern, size and configuration of medullary spaces show normal appearance. Once the infection enter, it become ragged radiolucent areas. Clinically, patient experiences excruciating pain and there can be suppurative discharge from sequestrated sites.

In dentistry, occupational hazard is with x-ray machine operators. A committee on radiation protection has set a maximum permissible dose (MPD) as 0.3 R/ week. However, the total accumulated dose should not be more than 5 R/year.

Chronic radiation affecting small area of the body:
This can lead to various types of hazards such as :-

? Radiation burns: its seen especially in cases where operator hold film in patients mouth. Continuous irradiation produces an erythema. Appearance is something like sunburns.
• Skin become dry, discolored with burning sensations
• Nails become friable with end broken.
• Cuticle around the nails is affected
• Slight change in blood supply of the sebaceous and sweat glands.
• Cracks appear which may lead to malignant changes.

? Loss of hair( Alopecia): it can result because too often or too long exposure to roentgen rays. Though the loss of hair is not permanent but one must be vigilant.

? Cataract: it can result from chronic exposure to x-radiation in and around the eyes.

? Effect on oral mucous membrane: it show area of redness and inflammation. With repeat exposures pseudo-membranes are formed because of breakdown of m.m., secondary infection by candida albicans is a common complication. Usually the m.m. heal rapidly once the irradiation is over. Otherwise after few months, the m.m. will tend to become atrophic and relatively avascular.

? Effect on taste-buds: they are very sensitive to radiations and soon degenerative changes begin. Loss of taste is very common.

? Effect on salivary gland: salivary gland come under exposure during treatment of cancer in the oral cavity and oropharyngeal region. There occurs acute inflammation involving serous acini. A marked increase in serum amylase. As the exposure progress, the gland demonstrate degeneratrion. salivary changes have perform influence on oral microflora and on dentition. Increase in streptococcus mutans, lactobacillus and candida, also xerostomia has been reported.

? Effect on teeth: the growth is retarded when teeth are irradiated during there development. If the radiation precedes calcification, the tooth may be destroyed. After calcification, if irradiation is continues, malformation can result. The root development is retarded. The tooth may erupt prematurely.

? Effect on bones: in dentistry, mandible most susceptible to be irradiated frequently during treatment of cancer. The predominant change occur in the marrow, lacunae of the compact bone are empty indicating early necrosis. Marked decrease in vascularity of bone because of irradiation, decreases the capacity of bone to resist infection. Osteoradionecrosis is also a complication because of exposures.

? Radiation caries: the decrease in the salivary flow, its pH and buffering capacity coupled with increased viscosity are the complications of radiation exposure which lead to rampant type of carious lesions. Histological features of these lesions are similar to those of typical carious lesions, they can be distinguish by their rapid attach. Topical application of 1% sodium fluoride and proper oral hygiene measure can reduce the radiation caries.

Personal monitoring
It is the evaluation of radiation doses received by the persons working in the department concerning radiation. Film badges are the commonly used device, it measure wide range of doses from 10 mR to 1000 R of different types of radiation, film badge worn on chest and give the whole body radiations under normal conditions. x-rays, b-rays, g-rays etc. are all measured with the film badges.

Advantages:
• Permanent record can be kept
• All types of radiations can be differentiated
Disadvantages:
• It can’t be read immediately
• It is not very accurate
• It cant record accidental exposures

Area monitoring
It is the assessment of radiation levels at different locations in the vicinity of the radiation sources. Most commonly used area monitoring device is the survey- meter based on the ionisation chambers. This meter usually does not function at the high radiation levels.

Radiation protection

Radiation protection philosophy follows principle of ALARA ( as low as reasonably achievable). According to this principle, the exposure to radiation is to be reduced as far as possible. Even with a single periapical film, the exposure to the patient is 217 mR.

Shielding:
Various shielding materials are used as iron, lead, concrete wall and hard plastic etc. the thickness of shielding material, which reduce the intensity of radiation to the half its original value (50%) is define as the half value thickness (HVT) of the material.
Even very large thickness of the shielding material will not completely attenuate the radiation to have zero- intensity.

Distance:
The exposure rate from a point source of radiation at a specified location varies inversely as the square of the distance. The exposure rates E1 and E2 at distance, D1 and D2 are related as:
E1 D2 2
E2 = D1 2
The basic formula is the larger the distance from the source, lesser is the radiation dose.

Time:
For a uniform distance and shield, the exposure from a source at a point will be directly proportional to the time during which the exposure was on.



Protection of patient and operator:

The recommendation is that the walls of the operatory room should be sufficiently thick or cover with black paper so that someone occupying the adjacent room should not received radiation greater than 10 R/week.
• The rule of six feet distance and the proper angle of the operator with respect to x-ray tube must be followed. An angle of 135? is recommended as a safer zone.
• The operator should never hold film in patients mouth.
• The tube should never be held by operator during the exposure.
• Personal monitoring devices should be used and checked every week.
• Protective guards especially around gonads should be used both by operator and the patient.
• Personal working in X- ray department should be kept inform of various newer materials and equipment and also the various protective devices.


Why we use vacuum tube inside the X-ray machine?

A. To facilitate the movement of electrons from cathode toward anode without any loss of its energies by interactions with atoms of air.
B. To prevent heat productionduring the electrons collisions with atoms of air.
C. To prevent ionization which if occur, it will difficult the movement of electrons toward the anode.
D. To prevent the interactions between x-ray ( after its production) with the air ( matter) inside the x-ray tube and this if happened it will affect the quality of primary x- ray beam.