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TSET

Total skin electron therapy
Hanuman Doke

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TSET

  1. 1. Total skin electron therapy HD
  2. 2. Introduction • This is also called as high dose total skin electron therapy(HDTSe). • It is used for the treatment of cutaneous T-cell lymphoma. • It is usually called mycosis fungoides but sometimes denoted as sezary syndrome. • Very large fields are needed for electron beam irradiation. • Extensive dosimetric measurements are required prior to initiate such procedure, one widely used technique is six dual fields. • A rigorous quality assurance program is needed because high electron dose rates at isocentre are usually employed to minimize treatment time in a plane several meters distant.
  3. 3. Mycosis Fungoides [MF] • Lymphoproliferative malignancy • Evolves from localized cutaneous infiltrates of neoplastic helper T cell. • Chronic disease of the skin • May result in secondary involvement of the lymph nodes and internal organs
  4. 4. • Psoralen photochemotherapy • Topical chemotherapy • Radiation therapy • Combination Role of RT: To deliver uniform dose to the entire body surface while protecting the underlying organs. Treatment modality
  5. 5. Objectives of TSET • Uniform dose • Upto Limited depth • over whole skin area • Sparing normal structures
  6. 6. Irradiation Requirements • The irradiation beam requirements involve characteristics of the treatment electron beam, the disease entity and the patient population. • They include specification of: ▫ field size, ▫ penetration, ▫ energy, ▫ dose, ▫ dose rate, ▫ field flatness in the treatment plane, ▫ X-ray background • To treat virtually the entire body surface to a limited depth and to a uniform dose using electrons with a low X-ray background.
  7. 7. Irradiation Beam Requirements • To treat virtually the entire body surface to a limited depth and to a uniform dose using electrons with a low X-ray background. • The field size of the composite electron beam at the patient treatment plane must be approximately 200 cm in height by 80 cm in width to encompass the largest patient. • The requisite penetration depth is usually ranges from 5mm to 15mm more at the 50% isodose surface encompasses most lesion. • The average energy of 2MeV/(gm/cm2) is approximately lost. • The incident mean electron beam energy E0 ave. at the patient treatment plane is usually in the range of 3-7MeV.
  8. 8. Geometrical arrangement of the symmetrical dual-field treatment technique. Equal exposures are given with each beam. The calibration point dose is at (x=0, y=0) in the treatment plane.
  9. 9. Sequential two-day treatment cycle illustrating the angular orientation of the six dual-fields.
  10. 10. Patient position - the anterior, posterior, and two of the angled dual field exposures.
  11. 11. Techniques of TSET Basically two general categories Translation technique- • In which a horizontal patient is translated to a beam of electrons of sufficient width to cover the transverse dimension of the patient. Large field technique- • In which a standing patient is treated with a combination of broad beams produced by electron scattering and large SSDs (2 to 6 m).
  12. 12. Translation technique • The patient lies on a motor-driven couch and is moved relative to a downward directed beam at a suitable velocity. • Alternatively, the patient may be stationary and the radiation source translated horizontally. 1) Beta particles(Haybittle et al) • A 24-Ci 90Sr β source, in the form of a 60-cm linear array, is used. • The source is contained in a shielded source housing and positioned above the couch. • The maximum energy 90Sr is 2.25 MeV. • due to the spectral distribution of β-ray energies, the effective depth of treatment in this case is only a fraction of a millimeter.
  13. 13. 2) Narrow rectangular beams (William et al) 120cm • Van de Graff accelerator used. • A well collimated monoenergetic electron beam is scattered just after leaving the vaccum window to improve uniformity. • Energy 1.5 – 4.5Mev • The beam is the collimated by an aluminium cone with a 5mmX45cm defining slit. • The patient is translated under this beam at a suitable speed.
  14. 14. 3)Scatter single Beam (Tetenes & Goodwin) • No applicator is used • Xray collimators are fully retracted • The patient is treated ant. and post. • The dose uniformity in the transverse direction is enhanced by suitable combining transverely overlapping fields. 700cm Titanium
  15. 15. Large Field technique • Large electron fields required for total body skin irradiation can be produced by scattering electrons through wide angle • Using large treatment distances • The field is made uniform over the height of the patient by vertically combining multiple fields or vertical arcing. • The patient is treated in a standing position with four or six fields directed from equally spaced angles for circumferential coverage of the body surface.
  16. 16. Pair of parellel beam (Szur et al)  2 horizontal parallel beams separate 150cm  Energy = 8Mev  Patient plane 200cm  Carbon energy degrader at exit window  Various penetration 2-25mm  Uniformity ±5% for 200cm height  X-ray contamination 2% 150 cm 200cm
  17. 17. Pendulum Arc (Sewchand et al) • Isocentrally mounted 8Mev LA • Rotating arc of 50o • Variable dose rate or gantry rotation speed for optimization of treatment • 6 arcing fields • X-ray cont 4.2% 5 cm Patient plane 3m 200cm
  18. 18. six-dual field technique Day 1. Day 2. I IIIII IV V VI Transverse View of TSET (Anterior, posterior and four field oblique at 60o with dual Gantry angles) (Standford) • Single fields gives variation in intensity at both edges • 2-3 fields usually used
  19. 19. Field flatness • Low energy electron beams are widened by scattering in air. Example • a 6-MeV narrow electron beam, after passing through 4 m of air, achieves a Gaussian intensity distribution with a 50% to 50% width of appro. 1 m. • This usually gives adequate uniformity over a patient's width. • If two such fields are joined together vertically at their 50% lines, the resultant field will be uniform over a height of approximately 1 m. • A proper combination of more such fields or a continuous arc can lead ▫ to a larger uniform field, ▫ sufficient to cover a patient from head to foot.
  20. 20. • Combination of three beam intensity profiles along the vertical axis to obtain a resultant beam profile. The central beam is directed horizontally, whereas the others are directed at 18.5 degrees from the horizontal.
  21. 21. X-ray Contamination • X-ray contamination is present in every therapy electron beam and becomes a limiting factor in total skin irradiation. • These x-rays are contributed by bremsstrahlung interactions produced in the exit window of the accelerator, scattering foil, ion chambers, beam-defining collimators, air, and the patient. • The bremsstrahlung level can be minimized if the electron beam is scattered by air alone before incidence on the patient. • This would necessitate some modifications in the accelerator, such as removing the scattering foil and other scatterers in the collimation system. • Various safety interlocks would be required to make this separation feasible for routine clinical use.
  22. 22. Field Arrangement • In the Stanford technique, the patient is treated with six fields (ant, post, and 4 obl) positioned 60 degrees apart around the circumference of the patient. • Each field is made up of two component beams, pointing at a suitable angle with respect to the horizontal.
  23. 23. Dose Distribution • The depth dose distribution ▫ In a single large field incident on a patient will depend on the angle of incidence of the beam relative to the surface contour. ▫ For an oblique beam, the depth dose curve and its dmax shift toward the surface. • When multiple large fields are directed at the patient from different angles. • The composite distribution shows a net shift with apparent decrease in beam penetration. • This shift of the relative depth doses closer to the surface, ▫ due to greater path lengths taken by the obliquely incident electrons in reaching a point.
  24. 24. Contd…… • Using the six-field technique ▫ a dose uniformity of ±10% can be achieved over most of the body surface ▫ areas adjacent to surface irregularities vary substantially due to local scattering. • Areas such as inner thighs and axilla, which are obstructed by adjacent body structures, require supplementary irradiation.
  25. 25. Dual Field Angle • A low-energy electron beam is considerably widened in size by scattering in air. • For example, ▫ a 9-MeV electron beam, after transversing 4 m of air and an acrylic scatter plate, ▫ attains a Gaussian dose profile measuring a 90% to 90% isodose width of about 60 cm, ▫ which is usually sufficient to cover a patient's width. Along the height of the patient, • Two fields, ▫ one directed toward the head and ▫ the other toward the feet, are angled such that in the composite dose distribution a ±10% dose uniformity can be obtained over a length of about 200 cm.
  26. 26. Calibration • A thin window (≤0.05 g/cm2) plane-parallel chamber is a suitable instrument for measuring depth dose distribution for the low-energy beams used for this technique. • The AAPM (112) recommends that the total skin irradiation dose be measured at the calibration point located at the surface of the phantom and the horizontal axis. • This dose for a single dual field is called the calibration point dose, DP.
  27. 27. Contd…… • A plane-parallel chamber, embedded in a polystyrene phantom. • Positioned to first measure the depth dose distribution along the horizontal axis for the single dual field • The surface dose measurement is made at a depth of 0.2 mm. • Suppose M is the ionization charge measured; • The calibration point dose to polystyrene, (DP)Poly, is given by:   replion poly air gasPTpolyp PP L NCMD           ,
  28. 28. • The calibration point dose to water, (Dp)W, can then be determined as • The electron fluence factor is approximately unity, because the calibration measurement is made close to the surface. • Prepl can also be equated to unity for the plane-parallel chambers. • The parameters and are determined for the mean energy of electrons at the depth of measurement, • Which is given by, • where z is the depth.     W Poly W poly polypWp SDD       W poly S       Poly air L                   p z R z EE 10
  29. 29. Contd… • The treatment skin dose is defined by, • The AAPM as the mean of the surface dose along the circumference of a cylindrical polystyrene phantom 30 cm in diameter and 30 cm high that has been irradiated under the total skin irradiation conditions with all six dual fields. • If (Dp)Poly is the calibration point dose for the single dual field, • where B is a factor relating the treatment skin dose with the calibration point dose, both measured at the surface of a cylindrical polystyrene phantom. Typically, B ranges between 2.5 and 3 for the Stanford-type technique.  polysD     BDD polyppolys 
  30. 30. Calibration set up Floor Perspex sheet Chamber Target Floor
  31. 31. In Vivo Dosimetry • Excessive dose (e.g., 120%–130%) can occur in areas with sharp body projections, curved surfaces, or regions of multiple field overlaps. • Low-dose regions occur when the skin is shielded by other parts of the body or overlying body folds. • From in vivo measurements, areas receiving a significantly less dose can be identified for local boost. • If eyelids need to be treated, internal eye shields can be used, but the dose to the inside of the lids should be assessed, taking into account the electron backscatter from lead. • TLDs are most often used for in vivo dosimetry.
  32. 32. Thank you

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