GLOSSARY

Definition of terms

NOTE : All definitions and terms that are quoted in the following section are direct quotations from:

THE PHYSICS OF RADIOLOGY

(Fourth Edition)

by

Harold Johns and John Cunningham

published by

Charles C. Thomas

Springfield Illinois

Page number references are from that book.

Axis Ratio (dimensionless)

This terminology is used on the monitor of timer setting sheet. This factor represents the dose at the axis of an isocentric setup relative to the calibration dose at a depth of Dmax at the calibration setup SSD.

It is not the Tissue -maximum ratio as may be used by some for hand calculations in that the dose is relative to the calibration dose at SSD + Dmax, rather than the dose at the axis at SAD with Dmax overlying the point yielding a new shorter SSD of SSD -Dmax (where the calibration has NOT been specified).

BSF "backscatter factor" (dimensionless)

page 347: "The term "backscatter factor" is applied to the tissue- air ratio for depth Dm largely for historic reasons. Backscatter factor was proposed and measured for radiation in the so-called orthovoltage range long before tissue-air ratios were though of. For these beams, since Dm = 0.0 the scatter to the surface is truly backscatter, At higher energies, where Dm>0, not all photons reaching the reference point are scattered back but the terms still continues to be used"

Dose Ratio (dimensionless)

This is a term we developed to facilitate the easier verification of the monitor unit calculations obtained from a complicated treatment plan. This “Dose Ratio” is the Dose that would be delivered at Dmax at the calibration distance in a calibration phantom if the machine were left on for the same time that is required to give the prescribed cGy dose.

Expert System ™

A trade marked term referring to Medicalibration’s systems which epitomize the quality of excellence and reliability in providing the correct answer. As in ...

“ex.pert (ek’spert): 1) very skillful; having much training and knowledge in some special field; 2) excelling above all others in that special field of knowledge or capability; 3) able to be relied upon to give the correct answer.”

Field Size Ratio “FSR” ( dimensionless)

This term is the relative dose at Dmax depth as a function of field size as set on the collimator. Usually the reference field size is a 10x10 cm field as used for calibration purposes. This ratio is the dose at Dmax for the specified field size divided by the dose at Dmax for the reference field size. The field size ratio includes the effect of Backscatter (see: BSF) at the reference distance and depth and the effects of scatter from the collimator and other such sources.

FSD “Focal-spot Skin Distance” ( cm. )

Similar to SSD (defined later) except that this terminology has historically been applied to X-ray machines (as opposed to Cobalt-60 teletherapy machines). This term refers to the distance from the focal spot source of the radiation to the distance to the patients skin.

Normalization (dimension: percentage)

Normalization is simply the re-expressing of isodose distribution in terms of percents. All dose distribution points are re-adjusted (re-normalized) to the specified 100% point and then the distribution of isodose lines is able to be selected in terms of percentages.

There are several points in the dose matrix that can be defined as being "100 percent". All other points will then be normalized to the selected point and able to be expressed as a percentage of the selected point.

These programs are especially capable and powerfully unique in that the normalization point may be user specified as:

a) 100% to matrix maximum

b) 100% to user specified point

. the Isocenter

. any other point

c) 100% to user. specified dose line

. Tumor minimum

. Tumor average

. User specified isodose line

NOTE: Compare Normalization with Prescription and Weighting

This method of normalization is especially useful. Separating the ability to normalize to 100% at any point from the weighting allows you to adjust the distribution by adjusting the weighting without disturbing the normalization.

Also separating the weighting and the normalization from the prescription, allows you to specify the normalization and also change the weightings and yet maintain a specified prescription to the specified point.

Prescription (dimension: dose in cGy)

This is the physicians prescription dose in terms of centiGray per time interval and area of treatment.

A complete prescription must be specified in terms of the time interval over which the specified dose is to be delivered, such 180 cGy per day or as cGy. per total course of treatment. In many cases, the physician will specify the prescription in terms of both a daily dose and a total cumulative dose.

Further, a complete prescription must also specify the point, volume or area to which the specified dose is to be delivered.

These programs are especially versatile in their ability to allow the physician to prescribe in any way desired, yet allow the weightings to be independently changed to adjust the dose distribution and while allowing the percentage specification to remain valid.

These programs give the ability to specify the dose to:

a) a percentage, which is further able to be specified as percent of:

. Matrix maximum

. Tumor minimum

. Tumor average

. Any user specified line

. Any user specified point

. The isocenter

. Any other user specified point

b) Any user specified raw Isodose Line Value

c) Any user specified point

. The isocenter

. Any other user specified point

Range (For electrons) (dimension: cm. )

Page 197: "Two ways of specifying the range of electrons are indicated,

R50 is the distance traveled by one-half of the electrons and is sometimes referred to as the average range.

Rp is determined by extrapolating the straight descending part of the curve to meet the background due to X-rays. It is called the practical, or extrapolated, range and is the easiest to measure

We use the extrapolated range as the required depth of the deepest cross axis scan for electrons. The precision of this depth for our purposes is not critical.

rIDL “Raw Isodose Line” (dimensionless)

The unnormalized isodose line values as generated by the beams as they positioned and as they are weighted (see: normalization and weighting).

SAR "Scatter Air Ratio"(dimensionless)

page 372: "A field size of zero area would be a narrow beam and would contain no scattered radiation. The difference between the tissue-air ratio for a field of finite area and that for a zero area would be a measure of the contribution from scattered radiation. A tissue-air ratio can therefore be thought of as having two components ... (equation included) ... the first term is the zero-area tissue-air ratio and the second has been called the scatter-air ratio"

SDD “Source Definer Distance” ( cm.)

Also :Source Diaphragm Distance”. This is the distance from he source of the radiation to the position of the collimator blocks or the beam shaping blocks. This is often considered to be the tray distance, but in the case of PA fields for example, where the blocks are placed on top of the inverted tray, the SDD will be at the tray distance PLUS the thickness of the blocks themselves.

This SDD value is necessary to be known for the use in producing the shaped blocks in the correct size and divergence or for producing a template of the correct configuration.

SFD “Source Film Distance” ( cm. )

The source to film distance that was used to take a port film. Knowing the SSD, this value can be used to determine the magnification factor of the film. This is necessary to determine field sizes or block positions from port films and to determine the position of radioactive isotopes in a brachytherapy implant.

SMR “Scatter Maximum Ratio (dimensionless)

Compare Scatter Air Ratio (SAR). This term has been proposed for high energy machines which have no “in-air” dose that can be measured. Instead, the ratio is calculated on the maximum dose at the depth of maximum buildup.

SPR “Scatter Phantom Ratio” (dimensionless)

Another variation on this ratio is the Scatter Phantom Ratio. Because the depth of maximum build up can change with field size for the higher energy machines, it has been proposed to define a ratio based on the dose at some constant depth in a phantom material where the dose can be routinely measured. This term is not used in the Compute-Rx-Plan or PC3D but is include here for completeness.

SSD “Source Skin Distance” ( cm. )

This is the distance from the source of the radiation to the surface of the patient’s skin. Historically the term SSD was applied to Cobalt-60 radiation producing equipment with its radioactive “source”. The source of the radiation was not necessarily the surface of the radioactive material, but rather some effective center within the radioactive material. This terminology is now used also with accelerators which instead have a focal spot for the source of radiation, but the term is usually applied to any machine which produced teletherapy beams.

TAR "Tissue -Air Ratio" (dimensionless)

page 432: "When the tissue-air ratio was first introduced, it was defined in terms of a ratio of exposures: the exposure in the phantom divided by the exposure at the same point in the absence of the phantom. This is certainly valid procedure, but has the disadvantage that it could not be used for high energy radiation since for these the use of exposure is not recommended. The use of absorbed dose also makes tissue-air-ratios conveniently consistent with other functions used in the calculation, such as percent depth dose. It is also in line with the recommendations of the ICRU."

page 347: "... (Tissue-air ratios) can also be obtained from measured percent depth doses and backscatter factors using relations between these quantities given in equation 10 -5 " (which follows: )

P(d,Wm,F)= 100 Ta I = 100 Ta(d,Wd ) æ F+Dm ö ^ 2

B B(Wm) è F+d ø

where:

P = Percent Depth Dose

d = depth of tissue

Wm = Beam Width at depth Dmax

F = FSD (or SSD)

Ta = Tissue-air ratio

I = Inverse square factor

Wd = Beam Width at depth d

B = Backscatter factor

TARO “Tissue Air Ratio for zero field area” (dimensionless)

The TARO “Tee Ae Are Zero” is a special case of the TAR.

page 372: "The zero-area tissue-air ratio is a mathematical abstraction. It cannot be measured directly. It can only be obtained by extrapolation. This extrapolation is not precise, but is helped by the fact that (the tissue-air ratio for zero field area) as a function of depth d, has the same shape as a narrow beam attenuation curve, which can be measured."

Wedge Factor (dimensionless)

The wedge factor is used in the determination of monitor unit or timer settings. It is the ratio of the dose at the calibration point with the wedge in place relative to the dose at the same place without the wedge in place.

The wedge factor is often measured at a depth of perhaps 10 cm. in a phantom, in order to also include to some slight measure the effect of any possible beam hardening that may be caused by the presence of the wedge in the beam.

Weighting (dimensionless)

The relative amount of radiation that is applied from the specified beam. The number is relative and may be expressed in a variety of ways. For example: “Equal Weighting” could be expressed as “fifty:fifty”, “one to one”, “100 each”, or any combination which indicates an equal “weighting”.

A complication of this is that the weighting may be defined at various points which will result in a variety of machine “ON” times for what otherwise might seem to be the same “weighting”.

The weighting may be defined as the dose to the calibration setup at the nominal SSD plus Dmax depth as used for calibration. In this case equal weighting will yield equal machine “ON” times.

The weighting may be defined at the isocenter. In this case the machine times will be adjusted so the dose at the isocenter from each beam is equal. This will necessitate the least efficient field (most tissue overlying the isocenter) to be required to deliver the greater amount of radiation.

The weighting may be defined at any other user definable reference point. In this case the machine “ON” time will be adjusted to deliver the same dose to the specified point from each beam. This can be seen to be a serious problem if the point is not very carefully chosen. For example, if the selected point is under a block or near a region of high dose gradient, then the machine “ON” times may vary quite unpredictably.

The weighting methods that are available in this planning system are:

a) Calibration dose, dose at SSD+Dmax

b) Isocenter dose (this is always used for arcs)

c) Any other user defined point (e.g.: Rx Point)

d) Monitor Units

e) Dmax at entry distance

CALCULATION OF MONITOR UNITS

The calculation of the required monitor units for a specified prescription depends of several well defined factors. It is necessary only to know the desired prescription dose and the dose rate at the prescription point. The monitor units (or timer setting) is simply the quotient of the two:

The simplicity of the above relationship is then complicated only by the difficulty in accurately determining the dose rate at the prescription point. The dose rate at the prescription point is a combination of several factors: The amount of radiation coming from the therapy unit and the amount of that radiation that actually penetrates to the prescription point.

We can consider these two general considerations separately; first, the Machine output: The machine output is able to ,be determined at very exact calibration conditions that are reproducible and consistent and measurable. Typically the output is specified at the depth of Dmax at a specific SSD setup distance. The output varies with the field size and fields shape and also with the effect of various interposed absorbers such as compensators, wedges or trays. Each of these effects can be expressed as a ratio of the output with and without the effecting parameter.

FSR The effect of the field size can be expressed as the FSR

TF The effect of a tray is expressed as a tray factor

WF The effect of a wedge is expressed as a wedge factor

CF The effect of a compensator is expressed as a compensator factor

EqSq An irregular fields can be included by the use of an equivalent square field

ISR The effect of different distances is expressed as an inverse square factor

In each case the [Machine Output Dose Rate] starts with the [Calibrated Dose Rate]. This [Calibrated Dose Rate] can be cGy per minute value as for a Cobalt-60 teletherapy machine (which must then also be adjusted for the decay of the activity of the source from the date of initial calibration) or this can be cGy per monitor unit which can be adjusted and reset as necessary on a periodic calibration schedule to be a constant value of 1.00 cGy per monitor unit (or as close to constantly that value as may be practical).

The methods for adjusting the machine output for irregular fields by using an equivalent square and the methods for adjusting for changes in distance by using the Inverse square factor (as used for SSD setups) or the “inverse square relationship” (as for SAD setups) are topics for later discussion.

The machine output is also effected by the selection of the appropriate FSR which requires the consideration of the effect of blocking. The wedge factor may also be a variable factor which changes with the field size or the equivalent square field size.

The machine output is also effected by the position in the field as must be considered when multi-leaf collimators or independent jaws are used and the prescription point may not be along the central axis of the teletherapy units primary collimators.

[Machine Dose Rate] = [Calibrated Dose Rate] x FSR(for EqSq) x TF x WF x CF x InvSq

further, for Cobalt-60: [Calibrated Dose Rate] = [Initial Dose Rate] x [Decay Factor]

After you have successfully determined the machine output dose rate (as generally outline above) then it is only necessary to know the fraction of that dose which actually reaches the specified prescription point. The fraction at the prescription point [Fraction at Rx Point] is also dependent on many factors, all of which need to be known in order to determine the necessary therapy unit timer or monitor unit setting that are required to deliver the desired prescription dose. For treatment plans which have more than one beam which are being used to apply the total prescription dose (as is usually the case), the fraction is also dependent on the relative weight of the beams and further complicated by the method of defining that relative weighting.

It is also necessary to account for the various methods of prescribing the dose in any of several possible different ways and the procedure for determining the [Fraction at Rx Point] is different for each prescription method.

1) Prescribe to a percentage, which can further be:

. a) percent of Matrix maximum

. b) percent depth dose of a SSD set-up

. c) percent of some point, which can further be:

. I) the isocenter point

. ii) any other user defined point

2) Prescribed to a raw isodose line

3) Prescribing to a point, which can further be:

. a) the isocenter (if there is one)

. b) or any other arbitrary user defined point.

All of these prescription and weighting methods can be used to determine the fraction of Rx at the prescription point. In some cases, there is no “point”, but rather a prescription volume, percentage or isodose line. However, the fractional value need for the determination of the necessary machine “on” times can be determined for any situation. We will now discuss only the most typical situations and the methods of prescribing for them:

1) 1b above: Prescription to a depth as a percent depth dose for SSD setup

2) 3a above: Prescription to a isocenter with TMR calculations for SAD setup

3) 1a above: Prescription to a volume with a covering percent line

Perhaps the most simple calculation for the determination of the [Fraction at Rx Point] is by the use of a percent depth dose factor. These tables are generally quite well established and are easily able to be verified by physical measurement on the therapy machine. The [Fraction at Rx point] is simply determined by looking up the value in a printed table of values for the appropriate field size and for the appropriate depth.

This is not too difficult if the patient is setup at the distances for which the depth dose table has been determined and if the field size is roughly a rectangle and if the Rx point is approximately on the central axis of the therapy beam. If any of these conditions is not met, then special care must be used in determining the appropriate percent depth dose value. If these conditions are met, then:

If multiple fields are to be used, then the [Desired Dose (cGy)] will not be the total prescription dose, but rather the individual dose which is to be delivered via the beam for which the SSD calculations are being made.

Another simple method of prescribing a dose is to specify the [Desired Dose (cGy)] to an isocenter. A modification of this method is to specify the [Desired Dose (cGy)] to a percentage of the isocenter. Either of these case is also fairly simple, requiring in each case to know the desired dose at the isocenter. If the prescription is to a percentage of the isocenter, then first the [Desired Dose (cGy)] at the isocenter must be determined by knowing the desired dose and the percentage that that dose is on the isocenter. For example: if the prescription is to give 180 cGy to 90 percent of the isocenter, then is able to be determined that the isocenter must, in that case, have a total [Desired Dose (cGy)] of 200 cGy.

For isocentric calculations, there are a couple of methods that can be used to determine the [Fraction at Rx Point]. One method that is typically used is by using a TMR value. The TMR is the Tissue Maximum Ratio value which expresses the dose at some depth in a water equivalent medium (the Tissue) relative to the dose at the same distance from the radiation source, but with just the minimum amount of material overlying the point that would be necessary to provide full buildup (the Maximum). This ratio is referred to as the “Tissue Maximum Ratio”

The Tissue Maximum Ratio value can be looked up in a prepared table of values for the appropriate field size and for the appropriate depth. The limitations and restrictions to using the TMR values are that the point of calculation needs to be at the specified “axis” distance (usually the nominal axis distance of the therapy machine = 100 cm.) and that the ratio is relative to the dose at the depth of maximum buildup (often not the calibration distance) for that energies. If these conditions are met, then:

However the more typical case is that the calibrated output is determined with the surface of the calibration phantom at the nominal SSD of the therapy machine and with the calibration point at a depth which puts it at an additional distance from the source, but Not at the isocenter where the TMR’s are meant to be measured. To compensate for this difference, the inverse square relationship must be applied to determine the output at the new position which is closer to the source by a distance equal to the Dmax depth.

In the TMR calculation situation it is therefore necessary to also include this correction factor to the output by correcting the output as determined from the Dmax at SSD setup conditions to the TMR requirements by adjusting the output by the additional axis factor:

This additional inverse square consideration is eliminated by the Compute-Rx-Plan and the PC3D, which have the ability to express the dose at the isocenter for an isocentric treatment relative, not to the dose at the isocenter with Dmax of build up, but rather at the true calibration setup distance under the calibration conditions at Dmax. This ratio is referred to as the ”Axis Ratio” and related the dose at the Axis (isocenter) relative to the standard calibration dose at the normal calibration depth and calibration SSD. In this case:

One of the more typical methods of stating a prescription is to specify a tumor or treatment volume of interest and then plan various approaches using various techniques, with various wedges or beam weighting or combinations, until a desirable dose distribution can be designed which will fairly uniformly cover the desired volume. After the desired combinations of beams and beam parameters is determined, then the prescription volume is indicated by specifying the percentage isodose line which adequately covers the necessary volume.

In this case, with various beams and various other parameters and with no specific point of prescription, it is impossible to calculate the dose to any one point simply because there is no single point to which you can calculate. The dose is a distribution which is a result of the overlapping of all of the beams which are involved in the treatment.

All of these factors combine in complicated interrelated fashion, which the Compute-Rx-Plan and PC3D system attempt to simplify for you by carefully combining the various parameters into one factor referred to as the “Dose Ratio” (see: Glossary in the users manual Appendix). This dose ratio combines all of the many factors in a single ratio which expresses the machine output cGy that is required from each field in order to contribute its necessary proper share to the total prescribed cGy dose. In this case, the [Fraction at Rx Point] is simply the “Dose Ratio”

All of the mathematical pieces that are necessary for the determination of the monitor unit setting or machine “on” times are able to be determined, it is only necessary to combine the appropriate factors in the correct order and relationship to determine the correct setting for the desired prescription