1. BEAM MODIFIERS IN RADIOTHERAPY

Transcription

1 CHAPTER-1 1. BEAM MODIFIERS IN RADIOTHERAPY INTRODUCTION In radiotherapy, tumours under varying body contour are very often encountered. In such cases, due to the irregular body surface, the dose distribution within the target volume will be non-uniform. But, beam dose distribution data which are used for treatment planning, are obtained under standard conditions such as perpendicular beam incidence, homogeneous unit density medium and flat surface etc. During treatment the beam may be obliquely incident with respect to the surface and in addition the surface may be curved or irregular in shape as mentioned above. Under such situations, the standard dose distributions cannot be applied without proper modifications or corrections. Hence, beam modification is attempted to achieve a uniform dose to the target volume and tumour volume. The important types of beam modifiers are : Internal beam modifiers known as flattening filters, beam modifiers added externally to a treatment unit, asymmetric collimators, multileaf collimators, dynamic wedges etc. Flattening filter is virtually incorporated into the machine and remains unchanged and was first described by Chester and Meredith < 1>. The second is an external beam modifier which is added externally to the machine. 1

2 1.1. Internal beam modifiers or Flattening filters A flattening filter is used to reduce the amount of radiation in the centre of the beam relative to that at the periphery. When such filters are placed in the path of the beam, results in beam hardening at the centre of the field with appreciable modification of the depth dose. Because of this, Kemp and Oliver < 2> reported the use of a filter composed of low atomic number material such as perspex for orthovoltage beam. This attenuated the orthovoltage beam without significant modification of its quality. In cobalt teletherapy machines where the isodoses are more uniform, the need for flattening filters is less marked. Still they could be attempted with a view that the variation between the peripheral doses and central axis doses is a minimum Flattening filter for megavoltage x-rays The use of flattening filters is absolutely essential in the case of linear accelerators. To ensure flattening of the beam at the appropriate depth, it is sometimes necessary for the peripheral dose near the surface to be larger than at the central axis. Without this filter, the isodose curves will be conical in shape, showing increased x-ray intensity along the central axis and a rapid reduction transversely. The function of the flattening filter is to make the beam intensity distribution relatively uniform across the field. Therefore, the filter is thickest in the middle and tapers towards the edges. As beam energy increases, the thickness of the flattening filter also increases. The cross-sectional variation of the filter thickness also causes variation in the photon spectrum or beam quality across the field owing to selective hardening of the beam by the filter. In general the average energy of the beam is somewhat lower 2

3 for the peripheral areas compared with the central part of the beam. This change in quality across the beam causes the flatness to change with depth. However, the change in flatness with depth is caused not only by the selective hardening of the beam across the field but also the changes in the distribution of radiation scatter as the depth increases. Beam flatness is usually specified at a depth of 10cm with the maximum limits set at the depth of maximum dose. By careful design of the filter and accurate placement in the beam, it is possible to achieve flatness to within ±3% of the central axis dose value at 10cm depth < 11 31l. This degree of flatness should extend over the central area bounded by at least 80% of the field dimensions at the specified depth or 1 cm from the edge of the field. This specification is satisfactory for the precision required in radiotherapy. A high dose area near the surface need to be accepted to obtain an acceptable flatness at a depth of 10 cm. Though the extent of the high dose regions or horns varies with the design of the filter, lower energy beams exhibit a larger variation than higher energy beams. In practice, it is acceptable to have these "superflat" isodose curves near the surface provided no point in any plane parallel to the surface receives a dose greater than 107% of the central axis value < 4> Flattening filters for electron beam In the case of electron beam, the beam is flattened by the interposition of a thin foil of low atomic number, the thickness of which can be calculated at any point knowing the energy of the electrons and the amount of change in the isodose surface which is necessary. The scattering effect of the material traversed by the electrons is 3

4 such as to cause a marked spreading of the isodose surface beyond the geometrical beam. This feature prevents electron beams from treating deep seated tumours. Uniformity of the electron beam is usually specified in a plane perpendicular to the beam axis and at a fixed depth. The International Commission on Radiation Units and Measurements(ICRU)(5)specifies beam flatness in terms of a uniformity index. This is defined in a reference plane and at a reference depth as the ratio of the area where the dose exceeds 90% of its value at the central axis to the geometric beam crosssectional area at the phantom surface. The uniformity index should exceed a given fraction eg:-0.80 for a 1Ox1 0 cm 2 field size and at depth of dose maximum. In addition the dose at any arbitrary point in the reference plane should not exceed 103% of the central axis value. Because of the presence of lower energy electrons in the beam, the flatness changes significantly with depth. Therefore it has been recommended that the uniformity index be defined at the depth of half the therapeutic range(6).eg:- half the depth of 85% depth dose. Furthermore, it is defined as the ratio of the areas inside the 90% and 50% isodose lines at this depth. A uniformity index of 0.7 or higher is acceptable with field sizes larger than 100cm 2. The peak value in this plane should be less than 103%. The American Association of Physicists in Medicine (AAPM)(7) recommends that the flatness of an electron beam be specified in a reference plane perpendicular to the central axis, at the depth of the 95% isodose beyond the depth of dose maximum. The variation in dose relative to the central axis should not exceed. ±5%, optimally to be within ±3% over an area confined within lines 2cm inside the geometric edge of fields equal to or larger than 1Ox1 Ocm 2. Beam symmetry compares a dose profile on one side of the central axis to that on the other. The American 4

5 Gi 3oc,C)b Association of Physicists in Medicine (AAPM) recommends that the cross-beam profile in the reference plane should not differ more than 2% at any pair of points located symmetrically on opposite sides of the central axis. Accelerators with magnetically scanned beam do not require scattering foils. Others use one or more scattering foils, usually made of lead, to widen the beam as well as to give a uniform dose distribution across the treatment field. Present day accelerators employ a dual foil system for uniform electron beam. The first foil widens the beam by multiple scattering, the second foil is designed to make the beam uniform in cross-section. The thickness of the second foil is differentially varied across the beam to produce a desired degree of beam widening and flattening. Studies by Werner et.a1 < 5> confirmed that there is close agreement between the dual foil systems and scanning beam systems in minimising angular spread and hence the effect on dose distribution characteristics EXTERNAL BEAM MODIFIERS The most important external beam modifiers used in radiotherapy are wedge filters, compensators and beam shaping blocks Wedge filters Wedge filters were first used by Frank Ellis and he established the validity of the concept <3 >. This is a wedge shaped absorber which causes a progressive decrease in the intensity across the beam, resulting in a tilt of the isodose curves from their normal positions. The degree of tilt depends on the slope of the wedge filter. The wedge is normally made of a dense material such as lead, lipowitz, 5

6 ' ILICTIIION UAM ILECTIION 11AM l -X-RAY TARGET... UlY COlll.. 4TOtl LATTININQ,u, Tltl SCA ntlllng II Oil c ou1u--,;i!s,"'!'!"..,,,;/"""""""'"" I _...,!!. I : : ', SlCONDAIIIY CCK.LIMAfel'I,,,, I I I I ' E I I I 1 I l,j j I I I \.- Jl.OT WIDGU JLAnlNED _; I I I I \ ILOCKI. COMPINU.TOIIS ll 114\'...,\_,_J \ I I I I I I I I I I I I I I I 1 1 I I I I I 1 I I I I I I I I I I I J_tttii_ I A ntnt B PATIENT Figure 1.1. Components of treatment head. A, X-ray therapy mode. B, Electron therapy mode.. Source d Figure 1.2. Schematic representation of a compensator designed for an irregular surface. 5.\

7 brass, copper, steel etc. and is mounted on a transparent plastic tray or light metal holder which can be inserted in the beam at a specified distance from the source. This distance is chosen such a way that the wedge tray is always at a distance of at least 15cm from the skin surface to avoid electron contamination of the beam. Otherwise it will reduce the skin sparing effect of the megavoltage beam. The use of wedge filters with orthovoltage radiations was limited due to the presence of hot spot under the thin end of the wedge and the bulge of the isodoses away from the axis of the beam due to the scattering of the radiation. But the usefulness of wedge filters is well demonstrated with mega voltage radiations due to higher depth doses, associated greater focus to skin distance, less side and back scatter and skin sparing effect due to buildup effect Wedge angle The term wedge isodose angle or simply wedge angle refers to the angle through which an isodose curve is tilted at the central ray of a beam at a specified depth. ie, Wedge angle is the angle between the isodose curve and the normal to the central axis. As the scattered radiation causes the angle of isodose tilt to decrease with increasing depth in the phantom, the wedge angle should be described at a specific depth. However, there is no general agreement to the choice of the reference depth. Some choose depth as a function of field size such as one-half or two-third of beam width etc. while others define wedge angle as the angle between the 50% isodose curve and the normal to the central axis. However the latter choice becomes impractical when high energy beams are used. For example, the central axis 6

8 depth of the 50% isodose curve for 1 OMV beam lies at about 18cm for a 1Ox10cm field and 100cm SSD. This depth is too large in the context of most wedge filter applications < 11>. The wedge filters are mostly used for treating tumours, not more than 10 cm deep. Therefore, the current recommendation is to use a single reference depth of 10cm for wedge angle specification <9 > Tissue Compensators Bolus Bolus is a tissue equivalent material placed directly on the skin surface to even out the irregular contours of a patient to present a flat surface normal to the beam. This use of bolus should be distinguished from that of a bolus layer, thickness of which is sufficient enough to provide adequate dose build up over the skin surface. The latter should be called as build-up bolus. Placing bolus on the skin surface is desirable for orthovoltage radiations and even for cobalt-60 gamma radiations. This ensures that to reach a tumour at a known depth, the amount of matter traversed by the beam corresponds to that for the isodose measurements. The isodose surface at the tumour is then perpendicular to the central ray Compensating filters (Compensating wedges or C-wedges) In the case of megavoltage radiations, the use of bolus may neutralise the lack of build up in the surface tissues, with consequent loss of skin sparing. To preserve this advantage, but to get the effect of bolus, several methods are possible. In such situations, a compensating filter may be used which approximates the effect 7

9 of the bolus as well as preserves the skin sparing effect. A prefabricated compensating filter may be placed in the path of the beam at a suitable distance to retain the build up advantage. It should also be of such a density and of such dimensions that each pencil of radiation is attenuated as much in the compensating wedge as it would have been in the patient if there had been no surface obliquity. This method is justifiable when there is a slight curvature in one dimension as is usual with the thorax and abdomen. To attempt to compensate for curvature in two directions, Fulton used two brass wedges of suitable dimensions placed at a sufficient distance from the skin to get the build up advantage. They could be rotated relative to each other and to the beam collimators so as to compensate for curvature in two directions simultaneously. If a uniform dose is to be delivered to the target volume, an account of the differences between patient and different parts of the body must be taken into account. Contours and heterogeneity of the body tissues both modify the beam and tend to reduce the precision with which dose can be delivered. These modifications by the tissues of the patient should be corrected as far as possible. The oblique surface modifies the beam inside the patient so that the isodose curves, instead of being at right angles to the central ray are more nearly parallel to the surface. If the tissue curves in other directions, corresponding distortions can occur. Thus surface irregularity gives rise to unacceptable non-uniformity of dose within the target volume or causes excessive irradiation of sensitive structures. To overcome this problem, many techniques have been employed including the use of wedged fields or multiple fields and the addition of bolus material or compensators. 8

10 A wedge filter can be effectively used as a compensator. A wedge is primarily designed to tilt the standard isodose curves through a certain wedge angle. The wedge filter isodose curves must be available and used to obtain the composite isodose curves before the filter is used in a treatment set up. The C-wedge on the other hand is used just as a compensator so that the standard isodose charts can be used without modification. Also, no wedge transmission factors are required for the C wedges. An important advantage of C-wedges over wedge filters used as compensators is that the C-wedges can be used for partial field compensation. i.e., The C-wedge is used to compensate only a part of the contour, which is irregular in shape. A wedge filter, in this case, could not be used as a compensator because it is designed to be placed in the field in a fixed position Design of Compensators The use of a compensator is to provide the required beam attenuation which would otherwise occur in the "missing" tissue when the body surface is irregular or curved. Because the compensator is designed to be positioned at a distance from the surface, the dimensions and the shape of the compensator must be adjusted for (a) the beam divergence, (b) the relative linear attenuation coefficients of the filter material and soft tissues, and (c) the reduction in scatter at various depths when the compensator is placed at a distance from the skin rather than in contact with it. To compensate for this scatter, the compensator is designed such that the attenuation of the filter is less than that required for primary radiation onll 3,14,15,16,17,18 &19). Minification of the compensating filter for geometric divergence of the beam has been achieved in 9

11 , 0 er (/) (/) LJ.J 2 ::.:: (.) i f >- f- vi r '-----'----J L _----l 10 -L DISTANCE d OF ABSORBER (cm) Figure 1.3. A plot of density ratio or thickness ratio as a function of compensator distance of a uniformly thick compensator. 6 Co y-rays, field size =loxlo, SSD=80 cm, Compensation depth =7 cm, and tissue deficit =S.O cm.

12 many ways. One method is to construct the compensator out of aluminium or brass blocks, using a matrix of square columns corresponding to the irregular surface < ). The dimension of each column is minified according to the geometric divergence correction which is calculated from the SSD and the filter-surface distance. If the thickness of a tissue equivalent compensator is equal to that of the missing tissue, it will overcompensate.i.e., the dose to the underlying tissues will be less than that indicated by the standard isodose chart. This decrease in depth dose, which is due to the reduction in scatter reaching a point at depth, depends on the distance of the compensator from the patient, field size, depth and beam quality. To compensate for this decrease in scatter, one may reduce the thickness of the compensator to increase the primary beam transmission. The compensator thickness should be such that the dose at a given depth is the same whether the missing tissue is replaced with the bolus in contact or with the compensator at the given distance from the skin surface. The required thickness of a tissue-equivalent compensator along a ray divided by the missing tissue thickness along the same ray may be called the density ratio or thickness ratio (h'/h) < 20>. Thickness ratio (-r) depends in a complex way on compensator-surface distance, thickness of missing tissue, field size, depth and beam quality. A detailed study of this parameter has shown that -c is primarily a function of distance of absorber( d) for d Ocm and that its dependence on other parameters is relatively less critica1 < 1 1>. Thus a fixed value of -c, based on a given d usually 20cm, 1 Ox1 Ocm 2 field, 7cm depth and a tissue deficit of 5cm can be used for most compensator work. The concept of thickness ratios also reveals that a compensator 10

13 cannot be designed to provide absorbed dose compensation exactly at all depths. If for a given irradiation conditions, 1 is chosen for a certain compensation depth, the compensator overcompensates at shallower depths and undercompensates at greater depths. Considering the limitations of the theory and too many variables affecting 1, an average value of o.7 for 1 may be used for all irradiation conditions provided d 2': 20cm < 11>. In actual design of the compensator, the thickness ratio is used to calculate the compensator thickness (tc) at a given point in the field tc = TD.(1/pc) where TD is the tissue deficit at the point considered and pc is the density of the compensator material. A direct evaluation of thickness (1/pc) for a compensator system may be made by measuring dose at an appropriate depth and field size in a tissue equivalent phantom with a slab of compensator material placed in the beam at the position of the compensator tray. Pieces of phantom material are removed from the surface until the dose equals that measured in the intact phantom, without the compensator. The ratio of compensator thickness to the tissue deficit gives the thickness ratio. Another term compensator ratio (CR) has also been used in the literature to relate tissue deficit to the required compensator thickness < 21 >. It is defined as the ratio of the missing tissue thickness to the compensator thickness necessary to give the dose for a particular field size and depth. The concepts of compensator ratio and thickness ratio are the same, except that the two quantities are inverse of each other Compensators for tissue heterogeneity Compensators for tissue heterogeneity may 11

14 be attempted and should be achieved if possible. In a case of esophageal carcinoma, it was shown by Ellis < 32> that compensation for tissue heterogeneity as checked by small thermoluminescent dosimeters in the lumen reduced by 30% the dose in the oesophagus when compared with the dose without the compensation. Large air spaces produces appreciable change in the dosage values as in the case of lungs, it less commonly realised that air spaces in the larynx and in the trachea can also introduce dosage changes, particularly on the surface of a tumour, when a megavoltage beam first traverses the air cavity. If the beam passes through the tumour before passing through the cavity, this effect, which is due to loss of scattered electrons, is much less. Compensators can be designed to compensate for tissue heterogeneity from the knowledge of cross-sectional anatomy using transaxial tomography or a photographic film Compensators for total body irradiation including lung compensators were described by Khan et.al <22 >. Compensators have also been used to improve dose uniformity in the fields where non-uniformity of the dose distribution arises from the sources other than contour irregularity such as reduced scatter near the field edges and unacceptable high dose regions or "horns" in the beam profile. Another method of providing compensation for tissue heterogeneity has been suugested by Ellis et.a1 < 33> which involves two radiographs at right angles in the position and with the beams to be used < 12 >. Each radiograph indicates the amount of matter traversed by each pencil of the beam since with suitable film, blackening is proportional to the dose at the film. This can be plotted by a scanning densitometer. Modifying filters for two pairs of parallel opposed fields at right angles can be 12

15 constructed from the densitometer data. The division of the compensation between the fields on each parallel opposed pair is decided by inspection of full width ordinary radiographs, from which can be estimated the proportion of the absorbing tissue on each side of the tumour in the path of the beams. When wedge fields are being used so as to treat from one side of the body only, the proportion of compensation can be estimated in the same way, but it is only necessary to use the appropriate amount on the side from which the treatment is being given Positioning of Compensators A compensator must be placed at a distance of 15cm or more away from the skin surface to preserve the skin-sparing properties of the megavoltage beams. Because the dimensions of the compensator are reduced compared to the bolus in the plane perpendicular to the beam axis to allow for beam divergence, the filter must be placed at the filter-surface distance for which it is designed. In addition, the nominal SSD should be measured from the plane perpendicular to the beam axis, containing the most elevated point on the contour included in the field. For isocentric treatments, it is most convenient to use field dimensions projected at the isocentre in compensator design. Accordingly, the depth of the isocentre is measured from the level of the most elevated point on the contour to be compensated Beam Shaping Blocks Beam shaping blocks are used in producing irregular fields in all clinical situations where they happened to be useful or necessary for confining the radiation to the target volume or protecting important structures. For example, In treating some 13

16 orbital or sinus carcinomas and some intracranial tumours it is desirable to protect the eye or the middle and internal ear. The protection achieved by removing part of the primary beam, also diminishes the amount of scattered radiation and thus affects the dose distribution in other parts of the beam. The beam shaping is primarily dictated by tumour distribution i.e., Local extension and regional metastases. Not only should the dose to vital organs not exceed their tolerance, but the dose to the normal tissue, in general, should be minimised. As long as the target volume includes, with adequate margins, the demonstrated tumour as well as its presumed occult spread, significant irradiation of the normal tissue outside this volume must be avoided as much as possible Thickness of blocks Usually high Z materials such as lead or lipowitz are used for shielding. The thickness of the material required to provide adequate protection of the shielded areas depends on the required transmission through the blocks and the beam quality of the radiation under question. A primary beam transmission of 5% through the block is considered acceptable for most clinical situations. To obtain this transmission ratio, the number (n) of half value layers (HVL) needed is (%)" 2" n log2 n = = = = = / log20 log 20/ log 2 14

17 Table 1.1 Recommended Minimum Thickness of Lead for Shielding. Beam Quality 1.0 mm Al HVL 2.0 mm Al HVL 3.0 mm Al HVL 1.0 mm Cu HVL 3.0 mm Cu HVL 4.0 mm Cu HVL 1a1cs soco 4MV 6MV 10 MV 25 MV Required Lead Thickness 0.2mm 0.3 mm 0.4 mm 1.0 mm 2.0 mm 2.5 mm 3.0 cm 5.0 cm 6.0 cm 6.5 cm 7.0 cm 7.0 cm * Approximate values to give s5% primary transmission.

18 = 4.32 Thus a material thickness below 5 HVL is adequate for primary transmission less than 5% and is recommended for most of the clinical situations < 11l. Shielding for superficial and orthovoltage beams can be readily achieved by thin sheets of lead where as for megavoltage beams the thickness of lead increases substantially. The blocks are placed above a transparent acrylic tray called shadow tray. The recommended values of shielding thickness of lead for various energy beams < 11> are given in Table 1.1. The primary transmission of the blocks can be reduced further by increasing the thickness of the block. But the dose to the shielded area may not be significantly reduced due to the presence of scattered radiations from the adjoining open areas of the field. Ideally, the beam shaping blocks used should be tapered to match the geometric divergence of the beam. This minimises the block transmission penumbra. However, divergent blocks offer little advantage for beams with large geometric penumbra. Hence, for cobalt-60 beams, the sharpness of the beam cut off at the beam edge is not significantly improved by using divergent blocks. Divergent blocks are most suited for beams having small focal spots ASYMMETRIC COLLIMATORS A part of the radiation fileld is sometimes shielded asmmetrically with respect to ioscentre without changing the position of the isocentre by using asymmetric collimators. The asymmetric collimators have idependently movable jaws. Some machines have one independent jaw, some others have two independent jaws, 15

19 and some have all four jaws as independent. The independent jaw option is interlocked to avoid errors in the setting of symmetric fields, in which case the opposite jaws open or close symmetrically. One of the effects of asymmetric collimation is the change in the physical penumbra and the tilt of the isodose curves toward the blocked edge. This effect is simply the result of blocking which eliminates the photon and electron scatter from the blocked portion of the field, thereby reducing the dose near the edge. The same effect would occur on the isodose curves if the blocking were done with a lead or lipowitz block on a tray. When asymmetric fields are used, special considerations must be given to the beam flatness and the dosimetric parameters used to calculate monitor units < 23> _ 1.4. MULTILEAF COLLIMATORS A multileaf collimator (MLC) for photon beams has multiple vanes with more than 40 pairs of collimating blocks or leaves that can be driven automatically, independent of each other. The MLCs can create any shape by jagged stepwise boundary of leaves to conform to any non-geometrical shape of the tumour or treatment volume. The thickness of the leaves along the beam direction is sufficient to provide acceptably low beam transmission. The width of each leaf is usually about 1 cm as projected at the isocentre. The field edges are therefore formed stepwise, 1 cm wide. The double-focussed MLC systems provide sharp beam cut off at the edge. The use of MLCs for the stationary fields is the conformity between the planned field boundary, which is continuous, and the jagged stepwise boundary created by the MLC. The degree of conformity between the two depends not only on the projected 16

20 leaf width but also on the shape of the target volume and the angle of rotation of the collimator < and, 36>. The MLC system is used in place of custom made cerrobend blocking, automatic beam shaping for multiple fields, dynamic conformal radiotherapy for which beams are shaped as they are rotated and modifying dose distributions within the field by computer controlled dwell time of the individual leaves. Recent studies have demonstrated that three dimensional MLC conformal radiation therapy allows an increase in the dose to the target volumes than with conventional radiotherapy. The improved accuracy of tumour coverage and the increase in tumour dose are expected to improve local tumour control. This reduces the normal tissue complications as well. Studies by Zvi Fuks et. < 10> al showed that patients with carcinoma of prostate and nasopharynx demonstrated an excellent acute tolerance for a dose of 81 Gy for the prostate and 75.6 Gy for nasopharynx respectively DYNAMIC WEDGES The concept of the dynamic wedge was originally proposed by Kijewski et. a1 < 2 5) who postulated that a computer connected to a linear accelerator could be used to step a collimator jaw across the intended treatment field while radiation treatment was in progress, creating a wedge shaped beam profile. Until very recently, when computer controlled consoles were introduced to control linear accelerator collimator jaws, dynamic wedge capabilities were not available. Leavitt et. al. < l successfully implemented a dynamic wedge for Varian accelerators. One difficulty in the clinical implementation of the Varian dynamic wedge is the strong 17

21 variation of its output factor with field size < > as well as discontinuity in this variation. The output factor is defined as the ratio of output measured for a rectangular collimator setting, Cx X C y, to that for Cx = C y = 10 cm. Sets of lower and upper collimator jaws are represented by x and y respectively. The dynamic wedge, as it is implemented in a Varian accelerator, uses Segmented Teatment Tables (STis) to control dose rate and jaw movement. Each STI contains information on subsequent jaw position versus cumulative weighting of a monitor unit. A total of 132 STis were created for the four wedge angles, 15,30,45 and 60 ; including 33 different STis for each wedge angle for field sizes, y, from 4 to 20 cm. In principle, the STI allows the determination of all dosimetry characteristics of the dynamic wedge if the basic dosimetric parameters for open fields are explicitly known. Any changes in STis will change the output factor of the dynamic wedge. Since there are so many STis for each wedge angle, it is desirable to calculate the output factor for the dynamic wedge from the STis directly. Waldron et. a1 < 29 > have shown that the wedge factor in water for the Varian dynamic wedge can be calculated simply by noting the percentage of accumulated monitor units (MU) delivered up to the central axis with the moving collimator jaw. Bidmead et. a1 ( 25 > have characterised the field size dependence of dynamic wedge factors by experimentally measuring the output for each STI. Liu et. a1 ( 24> characterised the behaviour of the output factor for a dynamic wedge and introduced a normalisation factor which simplifies the output calculations for the Varian dynamic wedge. The amount of data required to commission dynamic wedges and quality assurance issues has 18

22 discouraged institutions from clinically implementing this modality CONCLUSION Beam modification is an integral part of present day radiation therapy without which in most tumours under varying bogy contour a uniform dose distribution cannot be achieved. The subject of beam modification is a vast one and at all energies involves a great deal of insight into physical processes. The primary aim of beam modification should be such that throughout the target volume, the tolerance of normal tissues within that volume should not be exceeded except with serious consideration of the possible consequences. Within the tumour volume the given dose should be as high as is possible compatible with clinical requirements and technical feasibility. 19

23 References 1. Chester AE and Meredith WJ. The design of filters to produce "flat" x-ray isodose curves at a given depth. British Journal of Radiology, 18:382 ( 1945). 2. LAW.Kemp and R.Oliver, British Journal of Radiology,25:500(1952) 3. Ellis F, and Miller H. The use of wedge filters in deep x-ray therapy. British Journal of Radiology 17:904 (1944). 4. Nordic Association of Clinical Physics.Procedures in external beam radiation therapy dosimetry with electron and photon beams with maximum energies between 1 and 50 MeV.Acta Radiology Oncology, 19:58( 1980) 5. International Commission on Radiation Units and Measurements (ICRU): Radiation dosimetry:electron beams with energies between 1 and 50 MeV.ReportNo.35.Bethesda,MD.(1984). 6. Almond PR Characteristics of current medical electron accelerator beams.ln:chu F (editor). Proceedings of the symposium on electron beam therapy. Newyork: Memorial Sloan Kettering Cancer Centre,43( 1979) 7. Khan FM, Doppke K,Hogstrom KR et alclinical electron beam dosimetry. Report of American Association of Physicists in Medicine(AAPM) Radiation Therapy Committee Task Group No.25, Medical Physics18:73(1991 ). 8. Werner Bl, Khan FM, Deibel FC. Model for calculating depth dose distributions for broad electron beam, Medical Physics 10:582(1983) 9. International Commission on Radiation Units and Measurements (ICRU): 20

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