Dose distribution in matter:Multiple and wedge field techniqueDr. Dipanjan MajumderMD PGT Dept. of radiotherapyMedical college,kolkata

From single to multiple fields:a) Uniform dose distribution in tumor [± 5%]b)Maximum tissue dose not more than 110 % of prescribed dosec)Normal critical structures not receive dose near or beyond tolerance. in clinical practice single field rarely used. Used only few cases.Supraclavicular region.Internal mammary nodesSpional cord(direct posterior )

Parallel opposed fieldspair of fields directed along the same axis from opposite sides of treatment volume.Advantages:Simplicity and reproducibility of set upHomogenous dose to the tumourLess chance of geometric missIf field size large enough adequate lateral coverage to the tumour volume.DisadvantageExcessive dose to normal tissue and critical organ Above and below tumour.

Calculations:Composite isodose in parallel opposed obtained by adding depth dose contribution of each field.Resultant distribution shows combined isodose distribution normalised to the individual beam weightsSSD technique: beams are weighted in dose units 100 at Dmax.

SAD technique: beam weights refer to doses delivered to the isocenter.

Patient thickness versus dose uniformity:Patient thickness beam energy central axis dose near surface increases relative to midpoint dose. this is called TISSUE LATERAL EFFECT.

Co-60 4-6 MV beam thickness 15

cm dose uniform if 20 cm 10 MV or higher energy needed.

Edge Effect (lateral tissue damage) biologic effect in the normal tissue is greater if it receives alternating high and low dose fractions compared with medium but equal dose fractions resulting from treating both fields daily.Larger thickness 20 cm if treated with one field daily using

lower energy beam 6 MV Normal tissue toxicity is severe.

Integral dose:•Total energy absorbed in the treated volume•When mass receives uniform dose then integral dose= mass×dose•But normaly tissue receives non-uniform dose for this reason Mayneord formulated following expression.

Unit of intgral dose is gram-rad or kilogram-grayOr simply joule.

1000 rad delivered at midpoint of a 25 cm thick pt.

Field size 10cm d ; SSD =100cm.

•Integral dose to be kept at minimum Provided adequacy of tumour irradiation & sparing of critical organs are not compromised.•Provides qualitative guidelines for treatment planning•Selecting beam energy•Field sizes•Multiplicity of fields.

Multiple fields:

GoalsAppropriate field size. no. of fields or portals.Proper field directionAdjust beam weightsAppropriate beam energyBeam modifier like wedge compensators use

Limitations of multiple fields:•Certain beam angles are prohibited due to critical organs at beam directions.•Setup accuracy inferior than parallel opposed beams.•Inferior reproducibility.•POINT TO REALISE : TREATMENT PLAN DEPENDS NOT ONLY ON THE DOSE DISTRIBUTION ON PAPER BUT ALSO ON PRACTICAL FEASIBILITY,SETUP ACCURACY & REPRODUCIBILITY OF TREATMENT TECHNIQUE

Isocentric technique:Point of intersection of collimator axis and gantry axis - isocenter

Stationary beam:Isocenter placed within patient beams directed from different direction. SSD=SAD-d if SSDSAD isocentric method can be usedAdvantage: setup accuracyMachine isocentricity dependence rather skin marks

Rotation therapy:Isocentric technique with continuously moving beam about patient.Best applicable for small deep seated tumorsTumor confined halfway from the center of contour cross section.External surface not markedly differ from cylinderMay be used in esophagus,bladder,prostate gland,cervix & brain tumor .

Calculations:Diso=Do×Sc×Sp×TMRDiso=dose rate at isocenterDo=Dmax dose rate for 10,10 field at the SADSc=collimator scater factorSp=phantom scater factor.(for given field size at isocenter)TMR=average TMR over all deapths.

•Modern machines automatic speed adjustment preset MU delivery.•Composite isodose curve summing isodose values at selected points placing charts at deferent angles.•Past pointing:oblique fields are directed through one side of patient they should be aimed at suitable distance beyond the tumor area.

HOW TO TREAT SUPERFICIAL TUMOR BY MULTIPLE FIELDS?

Wedge field techniques:•Superficial tumors can be irradiated by two wedged beams from the same side of patient

In wedge fields dose falls rapidly beyond region of overlap or plateau which is desirable.

•Isodose curves from each field parallel to the bisector of hinge angle.•Above equation not valid for irregular shaped contour.•This problem can be solved by compensators•Part of wedge angel can be made compensator

Uniformity of dose distribution:•High-dose region up to +10% within tumor volume accepted.•Wedge suitable for tumor 0-7 cm from skin•Open and wedge field can be combined to compensate dose dropoff of open field

Separation of adjacent fields

Common uses:•Mantle and inverted Y field in HD.•Orthogonal craniospinal fields in treatment of medulloblastoma.•In head & neck cancer lateral neck fields anerior supraclavicular field.Problems:•Large dosage errors across the junction.•Tumor recurrence if underdosed complications if overdosed.

Lance & Morgan angled beam technique.

Split beam technique:

Field separation methods:Geometric:Two adjacent beams are considered.joined at given depth d. L1&L2 be the lengths, SSD1&SSD2 are source to surface distance.then

S=S1+S2=½.L1.d/SSD1+½.L2.d/SSD2

WHAT THE PROBLEM IN JUCTIONAL REGION: THREE FIELD OVERLAP

Problem of three field overlap region:•Created when bigger fields diverge on opposite smaller fields.

Calculations:Three-field overlap dS=S1-S2 dS=0 then L1/L2=SSD1/SSD2If field sizes different SSD s to be adjusted accordingly.If S1+S2 increased by dS three field overlap eliminated but cold spot at mid line.Practically dS* used; dS*=dS.[(d*-d)/d]d* =distance of critical organ from surface ; d= midline distance

Dosimetric:Separation of the fields can be determined by optimizing placement of the fields on contour so that composite isodose uniform.Hot and cold spots are acceptable.

Orthogonal field junction:Fields in which the central axis of beams are orthogonal.•For superficial tumor(head & neck) fields not separated unless junctions are free.•Beam splitter may be used to abut beam along central axis.•If cord came in the field may be blocked anteriorly or laterally if no tumor in shielded region.•Separation possible for deep seated tumor.ss

S=½.L.d/SSD

CSI field matching technique:

Procedure:•Patient positioned prone chest abdomen resting on hard styroform block.

•Caudad field margin of cranial field kept parallel with the diverging cephalad margin of spinal field by collimator rotation θcoll of cranial field.

•For adjustment of divergence of cranial fields couch to be rotated by θcouch.

θcoll=arc tan(½.L1.1/SSD)Θcouch=arc tan(½.L2.1/SAD)L1=length of posterior spinal field.L2=length of lateral cranial fieldSSD=for spinal fieldSAD=for cranial field

Guidelines for field matching:1.Site of field matching not to contain tumor or critically sensitive organ.2.Superficial tumor with critical organ at depth tilted beam or beam splitter3.Beams separated on skin for deep seated tumor.4.Field matching technique must be verified dosimetrically.

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