TELETHERAPY DOSAGE DATA- II

KILOVOLTAGE RADIATION OUTPUT VALUES - Surface doses were limiting factor in treatment

with kilovoltage radiations- The standard reference point for dosage

statements is at the centre of the field on the surface

- Surface output: The exposure rate (R) at any point in an irradiated material is made up of two components –

- 1) Primary radiation( directly from the tube) - 2) Scattered radiation ( from the irradiated

material)

PRIMARY RADIATIONThe exposure rate of primary radiation

depends on- The amount of radiation generated at the

target of the Xray tube- The volume and type of material it has to

pass through to reach the surface- The distance of the surface from the target

I - Tube current in milliamperesE – Applied kilovoltageZ – Target atomic number

SCATTERED RADIATIONThe exposure rate of the scattered

radiation will always be directly proportional to exposure rate of the primary radiation

Percentage scatter: The exposure rate of the scattered radiation expressed as a percentage of the primary exposure rate which is producing the scatter

For a point on the surface its called as percentage back scatter

Depends on the size and shape of the Xray field and radiation quality

BEAM DIMENSIONS:- Percentage back scatter does not vary in

direct proportion with beam size ( The scattered radiation generated in outer parts of an irradiated zone suffers more attenuation in reaching the reference point)

- Saturation value: Point where a further increase in beam size produces practically no increase in the scattered radiation reaching the centre.

- Beam shape – Percentage backscatter is same for circle and square of the same area but lesser for rectangle of the same area

RADIATION QUALITY

The likelihood of radiation being scattered ( Scatter attenuation coefficient) decreases as the energy of radiation increases

The steady fall in the percentage backscatter is expected at the higher quality end (Megavoltage radiation) due to relative reduction in the amount of scattered radiation

At lower energies what is unexpected is the marked falling off of scattered radiation

SCATTER VERSUS RADIATION QUALITY

It is not only the scatter caused by the primary radiation but by any radiation that has to be taken into account (for large and rectangular beams)

The greater amount of scattering of the lower energy radiations is due to its much lower penetrating power

The magnitude of the total scattered radiation depends on contributing volume and how much radiation is scattered per volume at low energies

At higher energies, the percentage backscatter depends entirely on the amount of scatter produced only and so it decreases steadily with increasing energy

S.S.D Percentage backscatter is independent of

S.S.D except for very short values

SCATTER GENERATED BY A BEAM- COLLIMATION APPLICATOR

Beam defining devices like diaphragms and applicators may add scattered radiation to the beam which will be a contributory factor in the variation of tube output

How the magnitude of scattered radiation varies with beam size and shape can be determined only by measurements

With a well designed system the contribution of scattered radiation will be small but will vary from one equipment to another

Output calibration measurements of one equipment can seldom be applied to another

The surface is not the ideal place for making output calibration measurements

POSITION IN THE BEAM

The reason for the falling off of scattered radiation towards the beam edge is that this edge is further away from much of the beam than is the centre , therefore scattered radiation reaching the edge has generally suffered more attenuation

The primary exposure rate is smaller at the edge than at the centre of the field because of the inverse square law. The magnitude of the effect depends on beam size being much greater for larger beams

There is considerable amount of scattered radiation beyond the geometric edges of the beam ( in the region receiving no primary radiation)

Clinical implication: Organs and tissues outside the geometric beam may well be exposed to amounts of radiation that are not negligible While in the plane at right angles to the

electron stream the Xray emission is symmetrical on either side of the central axis, but in the plane of the stream more radiation emerges from the target side of the central axis of the beam than from the filament side. This effect is counteracted to some extent by different attenuation in the target.

SUMMARY OF OUTPUTPrimary exposure rate depends on 1) Tube current2) Tube kilovoltage3) S.S.D4) Any added filter5) Tube wall thickness and material6) Target material7) Voltage wave form

Percentage backscatter depends on 1) Size of the beam2) Shape of the beam3) Quality of radiation4) Design and detail of collimation

Depth dose data Percentage depth dose = Absorbed dose at the point x 100 Absorbed dose at the surface

Absorbed dose rate at any point = Exposure rate at that point X Exposure to absorbed dose conversion factor ( roentgen to rad)

Percentage depth dose = Exposure rate at the point X 100 Output

Factors influencing percentage depth dose values1) Depth of that point below the surface - The greater the depth the smaller the

percentage depth dose(D) - Explained by inverse square law and increasing

attenuation suffered with increasing thickness

2) Beam dimensions- Steady increase in D though not linear as area

increases- Smaller values of D for rectangular than square or

circular beams

3) Radiation quality- Penetrating power of the radiation- D increases with increasing half value layer- Magnitude of increase affected by scattered

radiation. Therefore more pronounced for small beams

- Not very efficient to increase PDD values by increasing filtration

So for greater PDD values, higher radiation energies are needed

To achieve this radiotherapy turned from kilovoltage range to

megavoltage range

4) S.S.D- D increases as SSD increases- Effect of SSD on output : Surface output is

inversely proportional to square of SSD- Device working in 200- 300 kV range : SSD is 50 cm

5) Position in the beam - The exposure rate falls to either side of the central

ray - Explained by inverse square law and attenuation - The PDD also varies: greatest at the central axis and

falling off towards the beam edge

ISODOSE CURVESDefinition : Lines joining the points of equal Percentage Depth

Dose (PDD). The curves are usually drawn at regular intervals of

absorbed dose Expressed as a percentage of the dose at a

reference point.

ISODOSE CHARTS :Contour maps of dosage distribution in and around

the beam which consists of a family of isodose curves

THE HEEL EFFECT

The beam is symmetrical about the central ray

Any falling off in output should prompt a distribution check since it may result from ‘pitting’ which may upset the distribution resulting in asymmetrical distribution

Routine checks of radiation distribution should be done periodically ‘ in air’ to ensure that satisfactory conditions are maintained or to detect any changes

MEGAVOLTAGE RADIATIONS> 1 MVGreater penetrating power Smaller scattering Scatter occurs in ‘forward’ directionDifference in ionization by the electronsDifference in spatial distribution

between the kilovoltage (primary electrons in all directions) and megavoltage radiations( primary electrons in forward direction)

IONIZATION:- The effect of X-radiation are produced by ionizations and

excitations produced in turn by electrons liberated when photons interact with matter

- Kilovoltage radiations liberate electrons which travel only a fraction of a millimetre in tissue , water, air . So the exposure at a point is a direct measure of the absorbed dose at that point- In Megavoltage radiations , electrons liberated travel

1 mm to 8 cm before being brought to rest. Because of the BRAGG

EFFECT the electrons produce most of their ionization towards the final end of their track. Here the absorbed dose at any point may arise from Exposure at a point some little distance away

Central axis depth dose values- Most striking difference between

kilovoltage and megavoltage radiation is the pattern of their respective absorbed dose variation with depth

- ‘The Build up’

The total ionization at any place will be the sum of all the effects shown and numbers representing this are given at the foot of the above diagram

The build up is the same for all depths greater than 4 mm

In practice the ionization decreases beyond the peak of the build-up because of the effects of the inverse –square law and photon attenuation

The electrons ejected by megavoltage photons travel predominantly ‘forward’ and there is an increase of ionization along their tracks towards the end of their range

‘Build up ‘ effect is counterbalanced by any scattered radiation travelling in the opposite direction( as its electrons are ejected towards the surface), so this build up effect is not seen in kV radiation

In megavoltage radiation ‘build up’ occurs because there is minimal scattered radiation

EXPOSURE AND ABSORBED DOSE

• Exposure which is a function of the beam is maximum at the surface and falls steadily with depth because of the joint influence of the inverse square law and beam attenuation• Beyond the peak of the build up, Exposure and absorbed dose vary in the same way• At these depths exposure (roentgen) can be directly converted into absorbed dose(Rad) as for lower voltage radiations

DOSAGE REFERENCE POINT - Not on the phantom surface - At a depth of the maximum of the depth dose curve on the central axis - For a 20 MV Xray beam its about 4 cm deep

OUTPUT

• Filtration effect is negligible in megavoltage radiations unlike kilovoltage radiations

• Transmission type target is always used in high energy tubes because Xrays are mainly produced in the forward direction. Therefore it undergoes inherent filtration before emerging out of the tube

• Output is independent of field shape and size because of minimal scattered radiation

PERCENTAGE DEPTH DOSE VALUESGenerating voltageS.S.DBeam size and shapeDepthPosition in the beamIsodose charts

Generating voltage

S.S.DDirectly proportional like in Kv radiations but change is

greaterThe smaller the scatter , the greater the influence of

S.S.D on percentage depth dosesMuch greater S.S.D values are used for megavoltage

radiationsFor eg, with 4 M.V apparatus 100 cm S.S.D is used

BEAM SIZE AND SHAPEMV beams PDD is almost independent of beam size and

shape due to minimal scatter

DEPTH-Slower rate of decrease with depth

POSITION IN THE BEAM

Beam flattening filter or compensator is necessary in MV radiations to eliminate the rapid variation of primary exposure rate across the beam

Compensators help in minimising the exposure variation across megavoltage fields

ISODOSE CHARTSStriking difference with kV radiation – Much

smaller limits of scattered radiation outside the geometric limits of the beam AND

smaller changes in PDD across the beam at any depthFlatter isodose curves are the result of the beam flattening compensators whose efficiency at considerable depths is due to the reduced scatter for high energy radiation

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