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multiple filed arrangement in Radiotherapy, Medical College Kolkata


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multiple filed arrangement in Radiotherapy, Medical College Kolkata

  1. 1. Dose distribution in matter: Multiple and wedge field technique Dr. Dipanjan Majumder MD PGT Dept. of radiotherapy Medical college,kolkata
  2. 2. <ul><li>From single to multiple fields: </li></ul><ul><li>a) Uniform dose distribution in tumor [ ± 5%] </li></ul><ul><li>b)Maximum tissue dose not more than 110 % of prescribed dose </li></ul><ul><li>c)Normal critical structures not receive dose near or beyond tolerance. </li></ul><ul><li>in clinical practice single field rarely used. </li></ul><ul><li>Used only few cases. </li></ul><ul><li>Supraclavicular region. </li></ul><ul><li>Internal mammary nodes </li></ul><ul><li>Spional cord(direct posterior ) </li></ul>
  3. 3. <ul><li>Parallel opposed fields </li></ul><ul><li>pair of fields directed along the same axis from opposite sides of treatment volume. </li></ul><ul><li>Advantages: </li></ul><ul><li>Simplicity and reproducibility of set up </li></ul><ul><li>Homogenous dose to the tumour </li></ul><ul><li>Less chance of geometric miss </li></ul><ul><li>If field size large enough adequate lateral coverage to the tumour volume. </li></ul><ul><li>Disadvantage </li></ul><ul><li>Excessive dose to normal tissue and critical organ </li></ul><ul><li>Above and below tumour. </li></ul>
  4. 4. <ul><li>Calculations: </li></ul><ul><li>Composite isodose in parallel opposed obtained by adding depth dose contribution of each field. </li></ul><ul><li>Resultant distribution shows combined isodose distribution normalised to the individual beam weights </li></ul><ul><li>SSD technique: beams are weighted in dose units 100 at D max. </li></ul><ul><li>SAD technique: beam weights refer to doses delivered to the isocenter. </li></ul>
  5. 6. <ul><li>Patient thickness versus dose uniformity: </li></ul><ul><li>Patient thickness beam energy central axis dose near surface increases relative to midpoint dose. </li></ul><ul><li>this is called TISSUE LATERAL EFFECT. </li></ul>
  6. 7. Co- 60 4-6 MV beam thickness  15 cm dose uniform if  20 cm 10 MV or higher energy needed.
  7. 8. <ul><li>Edge Effect (lateral tissue damage) </li></ul><ul><li>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. </li></ul><ul><li>Larger thickness  20 cm if treated with one field daily using lower energy beam  6 MV </li></ul><ul><li>Normal tissue toxicity is severe. </li></ul>
  8. 9. <ul><li>Integral dose: </li></ul><ul><li>Total energy absorbed in the treated volume </li></ul><ul><li>When mass receives uniform dose then integral dose= mass×dose </li></ul><ul><li>But normaly tissue receives non-uniform dose for this reason Mayneord formulated following expression. </li></ul>
  9. 10. Unit of intgral dose is gram-rad or kilogram-gray Or simply joule. 1000 rad delivered at midpoint of a 25 cm thick pt. Field size 10 cm d ; SSD = 100 cm.
  10. 11. <ul><li>Integral dose to be kept at minimum </li></ul><ul><li>Provided adequacy of tumour irradiation & sparing of critical organs are not compromised. </li></ul><ul><li>Provides qualitative guidelines for treatment planning </li></ul><ul><li>Selecting beam energy </li></ul><ul><li>Field sizes </li></ul><ul><li>Multiplicity of fields. </li></ul>
  11. 12. <ul><li>Multiple fields: </li></ul><ul><li>Goals </li></ul><ul><li>Appropriate field size. </li></ul><ul><li>no. of fields or portals. </li></ul><ul><li>Proper field direction </li></ul><ul><li>Adjust beam weights </li></ul><ul><li>Appropriate beam energy </li></ul><ul><li>Beam modifier like wedge compensators use </li></ul>
  12. 14. <ul><li>Limitations of multiple fields: </li></ul><ul><li>Certain beam angles are prohibited due to critical organs at beam directions. </li></ul><ul><li>Setup accuracy inferior than parallel opposed beams. </li></ul><ul><li>Inferior reproducibility. </li></ul><ul><li>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 </li></ul>
  13. 15. 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 SSD  SAD isocentric method can be used Advantage: setup accuracy Machine isocentricity dependence rather skin marks
  14. 16. <ul><li>Rotation therapy: </li></ul><ul><li>Isocentric technique with continuously moving beam about patient. </li></ul><ul><li>Best applicable for small deep seated tumors </li></ul><ul><li>Tumor confined halfway from the center of contour cross section. </li></ul><ul><li>External surface not markedly differ from cylinder </li></ul><ul><li>May be used in esophagus,bladder,prostate gland,cervix & brain tumor . </li></ul>
  15. 17. Calculations: D iso = D o ×S c ×S p ×TMR Diso=dose rate at isocenter Do=Dmax dose rate for 10,10 field at the SAD Sc=collimator scater factor Sp=phantom scater factor.(for given field size at isocenter) TMR=average TMR over all deapths.
  16. 18. <ul><li>Modern machines automatic speed adjustment preset MU delivery. </li></ul><ul><li>Composite isodose curve summing isodose values at selected points placing charts at deferent angles. </li></ul><ul><li>Past pointing: oblique fields are directed through one side of patient they should be aimed at suitable distance beyond the tumor area. </li></ul>
  18. 22. <ul><li>Wedge field techniques: </li></ul><ul><li>Superficial tumors can be irradiated by two wedged beams from the same side of patient </li></ul>In wedge fields dose falls rapidly beyond region of overlap or plateau which is desirable.
  19. 24. <ul><li>Isodose curves from each field parallel to the bisector of hinge angle. </li></ul><ul><li>Above equation not valid for irregular shaped contour. </li></ul><ul><li>This problem can be solved by compensators </li></ul><ul><li>Part of wedge angel can be made compensator </li></ul>
  20. 25. <ul><li>Uniformity of dose distribution: </li></ul><ul><li>High-dose region up to +10% within tumor volume accepted. </li></ul><ul><li>Wedge suitable for tumor 0-7 cm from skin </li></ul><ul><li>Open and wedge field can be combined to compensate dose dropoff of open field </li></ul>
  21. 26. Separation of adjacent fields
  22. 27. <ul><li>Common uses: </li></ul><ul><li>Mantle and inverted Y field in HD. </li></ul><ul><li>Orthogonal craniospinal fields in treatment of medulloblastoma. </li></ul><ul><li>In head & neck cancer lateral neck fields anerior supraclavicular field . </li></ul><ul><li>Problems: </li></ul><ul><li>Large dosage errors across the junction. </li></ul><ul><li>Tumor recurrence if underdosed complications if overdosed . </li></ul>
  23. 28. Lance & Morgan angled beam technique.
  24. 29. Split beam technique:
  25. 30. 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
  27. 32. <ul><li>Problem of three field overlap region: </li></ul><ul><li>Created when bigger fields diverge on opposite smaller fields. </li></ul>
  28. 33. Calculations: Three-field overlap dS=S1-S2 dS=0 then L1/L2=SSD1/SSD2 If 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
  29. 34. 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 .
  30. 35. <ul><li>Orthogonal field junction: </li></ul><ul><li>Fields in which the central axis of beams are orthogonal. </li></ul><ul><li>For superficial tumor(head & neck) fields not separated unless junctions are free. </li></ul><ul><li>Beam splitter may be used to abut beam along central axis. </li></ul><ul><li>If cord came in the field may be blocked anteriorly or laterally if no tumor in shielded region. </li></ul><ul><li>Separation possible for deep seated </li></ul>
  31. 36. S=½.L.d/SSD
  32. 37. CSI field matching technique:
  33. 38. <ul><li>Procedure: </li></ul><ul><li>Patient positioned prone chest abdomen resting on hard styroform block. </li></ul><ul><li>Caudad field margin of cranial field kept parallel with the diverging cephalad margin of spinal field by collimator rotation θ coll of cranial field. </li></ul><ul><li>For adjustment of divergence of cranial fields couch to be rotated by θ couch. </li></ul>
  34. 39. θ coll =arc tan(½.L 1 .1/SSD) Θ couch =arc tan(½.L 2 .1/SAD) L1=length of posterior spinal field. L2=length of lateral cranial field SSD=for spinal field SAD=for cranial field
  35. 40. <ul><li>Guidelines for field matching: </li></ul><ul><li>Site of field matching not to contain tumor or critically sensitive organ. </li></ul><ul><li>Superficial tumor with critical organ at depth tilted beam or beam splitter </li></ul><ul><li>Beams separated on skin for deep seated tumor. </li></ul><ul><li>Field matching technique must be verified dosimetrically. </li></ul>
  36. 41. Thank youss Thank you