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Manchester system_Dr.Supriya

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brachytherapy systems

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Manchester system_Dr.Supriya

  1. 1. Classical Brachytherapy System of Interstitial Implants : Manchester system Dr. Supriya Sonaje
  2. 2. Henri Becquerel discovers radioactivity on February 26, 1896 15 dec 1852 -25 august1908
  3. 3. RADIUM :Marie Curie in 1898.
  4. 4. RADIUM DECAY α α α βγ βγ Uranium Ra Rn RaA RaB RaC Pb 1620yrs 3.83D 3.05Min 26.8min 19.7min stable Gamma rays of different energies are emmited – 0.6, 1.1 and 2.2 MeV , Average energy being – 1 MeV . Ra B emits beta ray of 0.65 MeV . Ra C emits beta ray of 3.17 MeV .
  5. 5. • AND SOON BECAME THERADIOACTIVE QUACKERY !!! • Few examples – • Radiothor – salt containing radium endorsed to have curative properties. • Toothpastes contianing radium – to make the teeth shiny . • In bathing water , drinking water .
  6. 6. • Danlos and Bloc performed the first radioactive implant in 1901. • First schools of brachytherapy were at Holt radium institute , Paris.
  7. 7. Brachytherapy consists of placing sealed radioactive sources very close to or in contact with the target tissue.
  8. 8. Types of brachytherapy 1. Intracavitary insertion consists of positioning applicators (bearing RAS) into a body cavity in close proximity to target tissue 2. Interstitial insertion consists of surgically implanting small RAS directly into the target tissues 3. Surface dose ( mould ) applications consist of an applicator containing an array of RAS usually designed to deliver an uniform dose distribution to a skin/mucosal surface 4. Transluminal application consists of inserting a single line source into a body lumen to treat its surface & adjacent tissues
  9. 9. • Dose rate  LOW DOSE RATE (LDR)  0.4-2 Gy/hr  MEDIUM DOSE RATE (MDR)  2-12 Gy/hr  HIGH DOSE RATE (HDR)  > 12 Gy/hr  ULTRA LOW DOSE RATE  0.01-0.3 Gy/hr
  10. 10. • Loading method- • Preloading • afterloading
  11. 11. Clinical advantages of brachytherapy - • High biological efficacy • Rapid dose fall off • Tolerable acute reactions • Decreased risk of tumor population • High control rate • Better cosmesis
  12. 12. Limitations of brachytherapy • Difficult for inaccessible regions • Limited for small tumor size • Invasive • Greater conformation • Needs skilled personnel
  13. 13. • Radioactivity :The activity, A, of an amount of radioactive nuclide, in a particular energy state and at a given time,is defined by equation: A= d N/ d T • where dN is the expectation value of the number of spontaneous nuclear transitions from that energy state in the time interval dt.
  14. 14. • Problems with the radium • Daughter element – radon (gas)- possibility of leakage which is not detected by visual check . • The practical maximum activity concentration (the specific activity) of radium salt is low (approximately 50 MBq mm~3 of active volume). Sources of higher activity are therefore bulky and not suitable for afterloading systems. • Need for heavy fitration and shielding .
  15. 15. Properties of ideal brachytherapy sources • Pure γ emitter – less α/β emission. • Medium γ energy – high enough to target tumor with homogenous dose & low enough to avoid normal tissues & reduce shielding needs • High specific activity – small size & suitability for HDR • Stable ( not liquid/gaseous) daughter product • Long t ½ /medium t ½ for permanent/ temporary implant • Should be available in a form which doesn’t powder or disperse if source is damaged or dispensed
  16. 16. RADIUM SUBSTITUTES NAME ORIGIN T1/2 γ ENERG Y-MeV β ENERGY β FILTRATIO N HVL (Pb - mm) SPECI. ACTI. DECAY PRODUC T Rn 222 NATURAL 3.83 days 0.83 Stainless steel 12 Pb 206 Cs 137 FISSION 30.1 7 yrs 0.662 0.512 1.17 - do - 6.5 87 Ba 137 Co 60 NEUTRON ACTIVAT. 5.26 yrs 1.17, 1.33 0.38 - do- 11 1020 Ni 60 Ir 192 - do - 73.8 day 0.136- 1.06 0.079- 0.068 Platinum 4 7760 Pt 192 Tn 182 - do - 115 yrs 0.67 - Platinum 12 - - Au 198 - do - 2.7 days 1.088- 0.412 0.96 St. steel 3.3 - Hg 198 I 125 - do - 59.4 days 0.274, 0.314 No Titanium 0.01 10th - Te 125 Pd 103 - do - 16.9 0.21 No Platinum 0.03 - Ru 103
  17. 17. Systems in Interstitial brachytherapy • “System” denotes a set of rules which takes into account the source types & strengths, geometry & method of application to obtain suitable dose distributions over the volume(s) to be treated. • Implant should follow both source distribution rules and method of dose prescription & specification of a system • Consist of 1. Distribution rules 2. Dose specification & implant optimization criteria 3. Dose calculation aids
  18. 18. Manchester system
  19. 19. Need for system • Two questions • How much radium will be required? • and how must it be arranged?
  20. 20. How much of radium ? • A unit of dosage rather than of intensity was preferred. • Because -total dose is the most important single factor in therapy (provided due consideration is given to the duration of radiation). • the clinician finds it very much easier to think in terms of • total dose delivered rather than in terms of intensity and time, separately
  21. 21. How much of radium? • For dosage unit of exposure – Roentgen (r) is chosen . • Roentgen is ‘the amount of Xray or gamma radiation such that the associated corpuscular emmision , per 0.001293 gramme of air, produces in air ions carrying 1 electrostatic unit charge of either sign ‘
  22. 22. • To determine exposure rate ‘R’ at a given point from a point source ‘A’ mCi whose specific gamma ray constant is ‘Γ’ at a distance ‘d’ • R = Γ x A (Roentgen/mCi hr.) d2 • Thus exposure rate of 1 mg of Ra at 1 cm with standard 0.5mm filtration thru Pt is found to be 8.4 R/ hr. • Line source : We divide the line into multiple point sources and the exposure rate simply added. > 1 cm 1 cm > 1 cm
  23. 23. 1. Amount of Ra depends on 1. Value of desired exposure 2. Area being treated 3. Treating distance 2. Clinical working unit was 1000 Roentgens 3. Exposure rate was in R/hr 4. Exposure Rate constant Γ= 8.4 R/mg hr 5. Table compiled for Mg hr/1000R for a given area/actual implant volume at a given distance 6. Filtered by 0.5mm of platinum (2% filtration correction factor for every 1mm) 7. A radiation field could be described as ‘uniform’, if variation is not more than +/-10 %. 8. R to cGy factor = 0.957
  24. 24. • The amount of radium needed for is determined – for a perticular area , treating distance , total exposure and treatment time for the designed dosage chart.
  25. 25. For example- • If an area of 20 cm2 is to receive 6000R over 50 hrs from a mould for which h=1.0 cm , taking the values from dosage chart- • The milgramme hours(mgh)per 1000Rfor 20 cm2 and h=1 cm is 641 . • Therefore , 6000R: 641x6=3846mgh will be needed . • And radium required for 50 hr treatment will be 3846/50=76.9 mg .
  26. 26. The distribution-governed by the inverse square law . • “for a point source , the radiation intensity at any place varies inversely as the square of the distance from the source to the place at which intensity is being considered”. • Dose varies more rapidly in 1st cm of distance
  27. 27. Geometrical terms : • The plane formed by the parallel sources - radioactive plane. • The centre of the radioactive plane is called the centre of the implant. • The central plane through this point is located at right angles to the needles and thus at right angles to the radioactive plane. A third plane parallel to the sources and at right angles to the radioactive plane, will be referred to as the coronal plane
  28. 28. • The length of the implant is the length of the radioactive plane in the direction of the sources . • The width of the implant is the dimension at right angles to the length, in the radioactive plane or parallel to it. • The thickness of the implant is the dimension at right angles to the length and to the radioactive plane, in the central plane or parallel to the central plane.
  29. 29. MOULDS:rules
  30. 30. PLANAR MOULDS • The word ‘planar’implies that the radium sources are mounted on a flat or curved surface parallel to the area being treated ,the curved surface being less than a semicircle . • Mould arrangement-Circular/square/rectagle /random • Treatment surface-Flat/convex/concave
  31. 31. For moulds The term mould is used to describe the situation in which the radioactive sources are positioned external to the patient, usually at a distance from the patient's skin known as the treatment distance and represented by the letter d. The treatment dose is prescribed to the plane which is at distance d from the sources and the dose in this plane will be delivered with a 10% accuracy if the rules are followed.
  32. 32. For circular mould • The distribution of radium depends upon the D/d. (D- diameter of the treatment area , d- treatment distance ) • For ‘ideal circle’ central and periferal doses are equal and the variation elsewhere across the circle is minimal. • If D/d is less than 3 , a single circle of radium will produce uniform irradiation over the treatment area , within the defination of ‘uniformity’ • Most uniform distribution is achived with D/d =2.8 .
  33. 33. %Radium D/d <3 3- <6 6- <7.5 7.5<1 0 10 In outer circle 100 95 80 75 70 In inner circle - - 17 22 27 In central spot - 5 3 3 3
  34. 34. For square mould : • Source is devided in to periphery and in lines, called as bars. • The number of bars is such that area is devided in to strips of width not greater than 2 d . the amount of radiumdistribution depends upon the number of bars .
  35. 35. For square moulds • If one bar suffices , its linear density (miligrammes per cm) should be half of that in periphery. • If two or more bars are needed , linear density should be 2/3rd of that of the periphery. • Attempt is made to maintain the space between two active ends of the sources less than the treating distance (d)
  36. 36. For rectangular moulds • The bars are taken in a line parallel to the longer side . • To get the vallue of mgh per 1000R , elongation correction factor is applied Ratio of sides of rectangle 1.5:1 2:1 3:1 4:1 Elongatio n correctio n factor 1.025 1.05 1.09 1.12
  37. 37. For curved surface The smaller area is chosen.
  38. 38. Dose below the treated surface • As tumors treated by moulds are superficial and thin , the dose at the distance d’ below the surface can be calculated by adding d’ and d, and looking for the value of mghper 1000R for treatment distance (d’+d) in the dosage chart .
  39. 39. Interstitial implant
  40. 40. Planar interstitial implant • Considered as direct development of planar mould with d=0.5 cm , inside the tissue . • They are implanted inside the tissue to get the uniform dose distribution on either side within distance of 0.5 cm . • For the values of mgh per 1000R , values from the charts for mould can be used .
  41. 41. Distribution rule for single planar implant • The proportion of the radium used on periphery depends upon the area of the treatment itself . Area Under 25 cm2 25-100 cm2 Over 100 cm 2 Peripheral fraction 2/3 1/2 1/3
  42. 42. 45 Importance of Differential Loading 1. All sources equal 2. Weaker sources in interior prescribed dose does not conform to target volume Dose conforms to target volume
  43. 43. 2. The needles should be arranged in parallel rows 1 cm apart with the ends crossed(active ends < 1 cm from crossed needles) Areas of under dosage End source fills in under dosed areas Shape of isodose line around a linear source
  44. 44. 3.. If the ends of the implant are uncrossed, the area should reduced by 10 % for each uncrossed end for table reading purposes A. Both ends crossed. The area treated is (a x b) B. One end uncrossed. The area treated should be considered as 0.90 (a x b) C. Both ends uncrossed. The area treated should be considered as 0.80 (a x b)
  45. 45. 4. If two planes are to be used, the separate planes should be arranged as for single planes, parallel to each other, and if they differ in area, then the average area is used to determine the mg-hrs and the activity is proportioned to each plane.
  46. 46. Volume interstitial implants • To calculate the amount of radium required , it follows the various set of charts . • M=34.1 V2/3 x f • M- miligramme per 1000R • V- implanted volume in cubic cm • f – correction factor
  47. 47. Rules for distribution in a cylindrical implant
  48. 48. CORE RIND/BELT UPPER END LOWER END 1. Any volume can be visualized as having two components : outer surface(RIND) and CORE. Total amt. of Radium is divided into 8 parts and distributed for the various shapes:
  49. 49. If these rules followed exposure throughout treatment volume will not drop by > 10% or rise by > 15% above stated exposure 2. The sources on each surface should be spaced as evenly as possible, with 1 -1.5 cm separation 3. For the core, the sources should be spread as evenly as possible throughout the volume and not all of it at the center. 4. In cylinders, belt should consist of not less than 8 needles, the core not less than 4. 5. If crossing not possible, volume of implant must be reduced by 7.5% for each uncrossed end. Crossing needles inserted at active ends, not at tips.
  51. 51. • Introduced the points at which dosage can be calculated . • Such a point must be anatomically comparable in the patients and should be at which dosage is not highly sensitive to small , clinically unimportant alteration in the source geometry and should be in the region of limiting radiosensitivity. • Introduced – ovoids and intrauterine tubes.
  52. 52. Point A- a point 2 cm latersl to the centre of uterine canal and 2 cm superior to the mucous membrane of lateral fornix , in the plane of uteru Point B - 5 cm laterally to midline at the same level as the A points
  53. 53. • Point A - The dose at point A is representative of the dose throughout much of the malignant tissue. Corresponds to the the point where uterine artery crosses the uterus . Point B – anatomically proximal to the obturator glands . Represents the lateral fall off of the dose . • The dose at point B is approximately 20-25% of the dose at point A and is of importance when calculating the total dose when brachytherapy is combined with external-beam irradiation
  54. 54. • Not more than about a third of total exposure rate at Pint A should be delivered from the vaginal sources
  56. 56. Showed that biological changes in tumor as well as normal tissue not only depend upon the dose of radiation but also on the duration over which this dose is spread. • The dose required to produce lethal change rises as the total period of radiation increases. • “It has been found that, subject to the proviso regarding duration of exposure already discussed, a dose of 6,000 ‘r,’ if delivered to the whole of tumour and tumour-bearing zone, causes permanent resolution of the great majority of epitheliomata. This figure may, therefore, be taken as a serviceable in vivo ‘tumour-lethal’ dose”. • Data regarding the other tumors is not extensive.
  57. 57. Suggested readings : • 1. Patterson, R. and Parker, H.M. (1934) A dosage system for gamma ray therapy. Br.J. Radiol., 7, 592. • Fundamental Physics of Radiology by W.J. Meredith & J.B. Massey (dosage calculation for plesiotherapy) • Principles and practice of brachytherapy –CAF Joslin THANK YOU