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Broad spectrum low avg. el energy (1.12Mev). Emax 2.18MeV. ‘Sr90 + Y90’ source sheet of 53cmx2cm of 24Ci Treatment distance is 40cm. Treatment time is 15mn. for 2Gy for scanning patient surface of 60cm x180cm Long treatment time, lesser avg. energy for penetration, poor uniformity…
Van de Graff accelerator used. Fix position vertically down Energy 1.5 – 4.5Mev Patient translated horizontally under beam Beam slit of 45cm x 1cm used Beam flatness ±8% at 1.2m 4 to 8 fields
Single electron beam of 6.5 Mev of L.A. Treatment plane at 7m. Titanium scattering foil 0.15mm at 10cm from exit window. Polystyrene scatterer at 7 m. Effective energy = 4 Mev & doserate 3Gy/m Flatness of ±8%
Pion is the ionization recombination correction for the present measurement. whereL/P is the average restricted mass collisional stopping power of electrons. Prepl is a replacement factor that corrects for perturbation in the electron and photon fluences at point P as a result of insertion of the cavity in the medium
Total skin electron therapy
• 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
• 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.
• 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
• Psoralen photochemotherapy
• Topical chemotherapy
• Radiation therapy
Role of RT:
To deliver uniform dose to the entire body
surface while protecting the underlying organs.
Objectives of TSET
• Uniform dose
• Upto Limited depth
• over whole skin area
• Sparing normal structures
• The irradiation beam requirements involve characteristics
of the treatment electron beam, the disease entity and the
• They include specification of:
▫ field size,
▫ 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
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.
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.
Sequential two-day treatment cycle illustrating
the angular orientation of the six dual-fields.
Patient position - the anterior,
posterior, and two of the angled dual field
Techniques of TSET
Basically two general categories
• 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).
• 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.
2) Narrow rectangular beams
(William et al)
• 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.
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.
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.
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%
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%
six-dual field technique
Day 1. Day 2.
Transverse View of TSET
(Anterior, posterior and four field oblique at 60o with
dual Gantry angles)
• Single fields gives variation in intensity at both edges
• 2-3 fields usually used
• Low energy electron beams are widened by scattering in air.
• 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
▫ to a larger uniform field,
▫ sufficient to cover a patient from head to foot.
• 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
• X-ray contamination is present in every therapy electron
beam and becomes a limiting factor in total skin
• 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
• 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.
• 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.
• 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
▫ 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
• 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.
• 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
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.
• 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
• This dose for a single dual field is called the
calibration point dose, DP.
• A plane-parallel chamber, embedded in a
• Positioned to first measure the depth dose
distribution along the horizontal axis for the single
• The surface dose measurement is made at a depth of
• Suppose M is the ionization charge measured;
• The calibration point dose to polystyrene, (DP)Poly, is
• 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
• Prepl can also be equated to unity for the plane-parallel
• 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.
• 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.
Calibration set up
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
• 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.