Interstitial Brachytherapy

# Interstitial Brachytherapy

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## Interstitial Brachytherapy

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1. Interstitial Brachytherapy Paris Dosimetry System

2. sites treated Tongue Floor of mouth Breast & chest wall skin Anal canal Prostate Interstitial BrachytherapyImplantation of radioactive sources into the tissue

3. Paris system • Use Iridium wire or hairpins • System defines permissible geometry • System defines method of calculating dosimetry

4. Implant rules 1. Sources should be straight and parallel No crossing sources 2. Sources should be of equal length 3. Equal separation between sources. Separation may be between 5mm and 20mm.

5. Implant rules (cont.) 4. In cross section, sources should follow the following patterns. Single plane Double plane Triangles Squares

6. Wires Central plane Paris system calculation 1. Dosimetry is calculated on the central plane 2. We define a set of dose points (Basal Points) on the Central plane.

7. Position of Basal Points Basal Points between wires, at point of lowest dose Single plane Midway between each pair of wires

8. Position of Basal Points Basal Points between wires, at point of lowest dose Two plane, triangles At the centre of gravity (centroid) of each triangle

9. Position of Basal Points Basal Points between wires, at point of lowest dose Two plane, squares At centre of each square

10. Paris system calculation (cont.) 3. Calculate the doserate at each Basal Point 4. Calculate the mean of the individual Basal Point Doserates (known as the Basal Doserate) 5. Calculate 85% of the Basal Doserate (known as the Reference Doserate 6. Calculate the treatment time based on the Reference Doserate and the dose required

11. 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 10 10 20 20 30 30 40 40 50 50 6.0 6.0 5.0 5.0 4.0 4.0 3.0 3.0 2.0 2.0 1.00 1.00 0.80 0.80 0.60 0.60 0.40 0.40 0.20 0.20 0.10 0.10 0.08 0.08 0.06 0.06 0.04 0.04 0.03 0.03 0.02 0.02 0.01 0.01 Distance mm Iridium-192 wire length 60mm AKR 1μGyh-1m2 Dose Rate Gy/h Crossline 0 10

12. Paris system calculation note: 1. Dose rate data available as cross-line curves. These are available for UNIT STRENGTH OF WIRE. so readings have to be corrected for strength used. 2. T1/2 = 74 days, so decay correction for wire strength needed. Two methods to do this: a) Paris ‘literature’ has a correction table - add hours depending on treatment time b) Calculate wire strength for (approx) middle of implant. Eg. if implant to last 6 days, calculate using wire strength on day 3.

13. How to choose the central plane Correct Incorrect

14. How to choose the central plane Correct Incorrect

15. How to choose the central plane a/2 b/2 b a Correct Incorrect

16. How to choose the central plane Correct Incorrect

17. x x x x Thickness of treated volume Single plane implant 0.5 x sources separation Reference isodose ~ 6mm 12mm between wires

18. x x x x x Thickness of treated volume Two plane “triangular” implant 1.2 x sources separation Reference isodose 15mm ~ 18mm

19. Thickness of treated volume Two plane “squares” implant 1.5 x sources separation Reference isodose x x 12mm 18mm

20. x x Length of treated volume Single and two plane implants 0.65 x sources length 60mm ~ 39mm This relation is approximate, and depends a little on the wire separation, being relatively smaller for shorter wires. Another way of stating this is to say that for a particular target volume length, the sources should be about 20-30% longer, at each end, than the target volume.

21. x x x x Lateral margin Single plane implant 0.37 x sources separation Reference isodose 4.4mm 12mm between wires

22. x x x x x Lateral margin Two plane “triangular” implant 0.15 x sources separation Reference isodose 15mm 2.25mm

23. Lateral margin Two plane “squares” implant 0.28 x sources separation Reference isodose x x 12mm 3.36mm

24. 15 mm x x x 1 2 3 4 Example 1 An iridium wire implant consists of 4 wires, (straight and parallel), each 50mm long, separation 15mm. Wire strength (at mid implant), AKR, = 450nGy.h-1mm-1 at 1 metre Side view P1 P2 P3 50mm 1 2 3 4 central plane

25. P1 P2 P3 W1 7.5 22.5 37.5 W2 7.5 7.5 22.5 W3 22.5 7.5 7.5 W4 37.5 22.5 7.5 Calculation Look at each Basal Point in turn (P1,P2 and P3) and calculate the dose rates Measure the distances from each wire to each point

26. Calculation (cont.) From these we can use the “50 mm” crossline curve to read off the dose rates (in Gy.h-1) P1 P2 P3 W1 0.38 0.083 0.035 W2 0.38 0.38 0.083 W3 0.083 0.38 0.38 W4 0.035 0.83 0.38 Totals 0.878 0.926 0.878 (Gy.h-1)

27. Calculation (cont.) Correcting for the wire strength used : ie. 450nGy.h-1mm-1 at 1metre (= 0. 45μGy.h-1mm -1 at 1metre) Therefore multiply the totals by 0.45 P1 P2 P3 Corrected Basal Point Dose Rates 0.395 0.417 0.395(Gy.h-1) Mean Basal Dose Rate = 0.402 (Gy.h-1) Reference Dose Rate = 0.85 x 0.402 = 0.342 (Gy.h-1) Time for 65Gy = 65/0.342 = 190 hours

28. Example 2 2 plane implant, 5 wires, triangular cross section Each wire 70mm long. Separation 20mm. Calculate the Air Kerma (AKR) of the wire required to give 25 Gy in 2.5 days

29. 20mm 300 x 20mm P2 20mm y P1 P3 300 20mm 20mm W1 W2 Example 2 (cont). Use trigomometry to obtain the required distances W3 W4 W5 eg. P1 from Wire 1 (and 4 and 3) 10/x = cos300 therefore x = 10/ cos300 = 11.55mm eg.P2 from Wire 3 (and 5) 20/y =cos300 therefore 20/ cos300 = 23.1mm

30. Dose Rates for 1μGy.h-1mm -1 at 1metre P1 P2 P3 Dist DR Dist DR Dist DR 1 11.55 0.24 11.55 0.24 23.1 0.095 2 23.1 0.095 11.55 0.24 11.55 0.24 3 11.55 0.24 23.1 0.095 30.5 0.062 4 11.55 0.24 11.55 0.24 11.55 0.24 5 30.5 0.062 23.1 0.095 11.55 0.24 Totals 0.877 0.910 0.877 Mean Basal Dose Rate = 0.888 Gy.h-1 Reference Dose Rate = 0.85 x 0.888 = 0.7548 Gy.h-1

31. Dose Rates for 1μGy.h-1mm -1 at 1metre BUT we wish to give 25Gy in 2.5 days (ie. 60 hours) Therefore the reference dose rate required is 25/60 = 0.417 Gy.h-1 Therefore the AKR of wire required = 1 x 0.417 0.7548 = 0.552 μGy.h-1mm -1 at 1metre (= 552 nGy.h-1mm -1 at 1metre)

32. Example 3 A lesion has dimensions of 40mm x 30mm x 5mm 1. Suggest a suitable wires arrangement, using the Paris system. 2. Calculate the wire strength, in air kerma rate at 1 metre per mm of wire, that is needed to treat this lesion to a prescribed dose of 65Gy in 6 days.

33. Interstitial Brachytherapy Manchester Dosimetry System

34. Manchester System(Paterson-Parker Rules) For calculation of moulds and implants Based on the use of radium sources Reference: Radium Dosage - The Manchester System, Ed by W J Meredith, Pub by E & S Livingstone Ltd, 1967

35. Manchester System(Paterson-Parker Rules) 1. Dosage tables - to calculate amount of radium required 2. Distribution rules - to determine how the radioactive material is to be distributed

36. Unit of Dosage Mg.hrs per 1000R Tables give the product of the amount of radium (in mg) and the time (in hours) needed to give 1000 roentgens to the treated surface

37. Manchester System(Paterson-Parker Rules) • Planar moulds • Sandwich moulds • Cylinder moulds • Planar implants • Volume implants Rules for planar implants are a simplified version of the mould rules

38. Planar implants • Sources implanted in a single plane • Dosimetry is specified on a parallel plane, 5mm from sources plane Sources plane Dosimetry planes 10mm

39. Distribution rules for planar implants (1) The implanted plane is divided into the “periphery” and the “area”. Sources are arranged, as uniformly as possible, on each of these categories, the proportion depending on the area. Distance between sources should not exceed 1cm

40. Distribution rules for planar implants (2) Area of Peripheral Area implant Fraction Fraction <25 cm2 2/3 1/3 25 to 100 cm2 1/2 1/2 >100 cm2 1/3 2/3

41. Distribution rules for planar implants (3) A common arrangement is a row of parallel needles, with the ends “crossed” by needles at right angles.

42. Distribution rules for planar implants (4) If ends are “uncrossed”, deduct 10% from area for “table-reading” purposes, for each uncrossed end.

43. Distribution rules for planar implants (5) • For 2-plane implants: • Planes should be parallel • Area to be used for table reading is the average of the 2 areas • Total activity divided pro-rata between the planes • Dose mid-way between the planes is low by 10% - 30% depending on separation and area

44. Paterson-Parker RulesExample (single plane implant) Size of implant: 4cm x 5cm = 20cm2 To give 6500R in about 7 days From table, mg.hrs per 1000R = 368  mg.hrs for 6500R = 2391 mg required = 2391/168 = 14.2 mg Ra Area < 25cm2, so 2/3 (9.4mg) on periphery and 1/3 (4.8mg) over area.

45. Distribution rules for volume implants (1) The implanted volume is divided into the “rind” and the “core”. The activity determined from the table is divided into 8 parts, and distributed as follows:

46. Distribution rules for volume implants (2) Cylinder: Belt - 4 parts Core - 2 parts Ends - 1 part each

47. Distribution rules for volume implants (3) Sphere: Shell - 6 parts Core - 2 parts

48. Distribution rules for volume implants (4) Cuboid: Each side - 1 part Each end - 1 part Core - 2 parts

49. Distribution rules for volume implants (5) • Sources should be spaced as evenly as possible on each surface, and within the volume • Not more than 1 to 1.5cm between needles • Correction made for “elongation” when the volume dimensions are unequal • Correction made for uncrossed ends if necessary • (-7.5% per uncrossed end)

50. Paterson-Parker RulesExample (volume implant) Elliptical cylinder, Height 3.6cm, Cross section 3 x 4cm. One uncrossed end. To give 7000R in 7 days. Volume = 33.9cm3 - less 7.5% = 31.4cm3 mg.hrs per 1000R = 340 Radium required = 340 x 7/168 = 14.2mg 4.0cm 3.0cm 3.6cm