Chapter 14 Electron Beam Therapy 1 Electron Interactions

Chapter 14 Electron Beam Therapy (1)

Electron Interactions Coulomb force interactions Inelastic collisions with atomic electrons (ionization and excitation) Inelastic collisions with nuclei (bremsstrahlung) Elastic collisions with atomic electrons Elastic collisions with nuclei Ionization Low Z ionization High Z bremsstrahlung Bremsstrahlung Excitation

Stopping Power and LET Delta-ray cutoff energy = D Ionization Bremsstrahlung (Radiation) S= = Scol + Srad = collisional SP + radiative SP Scol = Scol, <D + Scol, >D LET = Scol, <D LET (linear energy transfer) = restricted collisional stopping power

Collisional Losses Depending on the electron density of the medium Z Mass stopping power (S/ , Me. Vcm 2/g) Z electrons/g Z tightly bound electrons Fig 14. 1 1 Me. V, the energy loss rate in water 2 Me. V/cm

Radiation Losses Energy loss/cm Ee, Z 2 The probability of radiation loss relative to the collisional loss Ee, Z

Polarization of the condensed medium A high E electron loses more energy (J/g·cm 2) in a gas than in a more dense medium. Atoms close to the electron track screen those remote from the track. The ratio of S/ of water to air varies with Ee the dose conversion factor for an air ionization chamber in water varies with depth.

Absorbed Dose The “restricted” collision stopping power = the linear energy transfer (LET) =energy loss per unit path length in collisions in which energy is “locally” absorbed.

Energy Specification Almost monoenergetic before striking the collimation A spectrum of energies at the phantom surface Usually characterized by the energy at the phantom surface Measurement of threshold energy for nuclear reactions Range measurements The measurement of Cerenkov radiation threshold

Most Probable Energy (Ep)0 = C 1 + C 2 Rp + C 3 Rp 2 (Ep)0 the most probable energy at the phantom surface Rp the practical range in centimeters For water, C 1=0. 22 Me. V, C 2=1. 98 Me. V cm-1, C 3=0. 0025 Me. V cm-2 Rp is the depth of the point where the tangent to the descending linear portion of the curve intersects the extrapolated background.

Mean Energy for water, C 4= 2. 33 Me. V Energy at Depth The most probable energy and the mean energy of the spectrum decreases linearly with depth. Harder’s equation It is important in dosimetry to know the mean electron energy at the location of the chamber.

Output Calibration (1) Ion Chamber Plane-parallel ionization chambers for energies less than 10 Me. V Plane-parallel or cylindrical chambers for higher-energy beams Phantom Water, or plastic phantoms such as polystyrene and Lucite Dimensions large enough to provide full scatter for all field sizes and energies

Output Calibration (2) Reference depth The point of maximum dose on the central axis The deepest part of the maximum To avoid low-energy electron contamination problems close to the phantom field size 10 10 cm as reference field The maximum dose at dmax set at 1 c. Gy/MU Another cone is expressed as an output factor.

Output Calibration (3) Chamber voltage Under saturation conditions with a known ioncollection efficiency Less than 1% loss in charge by ion recombination The collection efficiency The ratio of current for the given voltage to that for the infinite voltage The ion recombination correction (Pion) Two sets of measurements

Output Calibration (4) Effects of chamber polarity The difference between the charges measured at positive and negative polarity should be less than 5%. Perturbation or replacement correction (Prepl) Depending on the electron energy and the size of the cavity The mean energy at the point of measurement calculated using Harder’s equation

Output Calibration (5) Displacement correction The effective point of measurement upstream toward the source from the center of the chamber For a thimble chamber 0. 45 r for 3 -10 Me. V electrons 0. 6 r for 10 -30 Me. V electrons AAPM 0. 5 r for all electron energies and depths

Absorbed Dose Calculation If the measurements are made in a medium other than water, then

Depth Dose Distribution (1) Ionization chambers Depth ionization curves depth dose curves Silicon diodes To give depth dose distribution directly Film Energy independence Relative dosimetry Sensitometric curve

Depth Dose Distribution (2) Phantoms Water is the standard phantom Polystyrene and electron solid water A depth dose distribution measured in a nonwater phantom The water equivalent or the effective density

Central Axis Depth Dose Curves A rapid dropoff of dose X-ray contamination 90% E/4 cm 80% E/3 cm Dmax does not follow a linear relationship with energy. The percent surface dose for electrons increases with energy. In clinical practice, isodose distributions for an individual machine, cone, and/or field size is required.

because the target volume has been drawn with rounded corners at depth, it may be treated with a 90 mm!90 mm field in this case. Calculation of Monitor Units. The dose prescription point is on the central axis of the treatment beam, at the depth of dmax, i. e. 22 mm for this linear accelerator. This means that a dose of 2. 75 Gy will be given to 100% on the central axis, 22 mm deep. If a 90 mm!90 mm insert is not available, one may be produced by attaching lead strips onto the edges of a 100 mm!100 mm insert to reduce the field size appropriately. The output (in Gy per monitor unit) of the newly fashioned insert should be

Isodose Curves Depending on the energy, field size, and collimation For the low-energy beams All the isodose curves show some expansion For the higher energies Only the low dose levels bulge out Higher isodose levels tend to lateral constriction, which becomes worse with decreasing field size.

Field Flatness The flatness changes with depth ICRU Uniformity index at the depth of half therapeutic range AAPM At the depth of the 95% isodose beyond the depth of dose maximum Not exceed 5% ( 3%) over an area confined within lines 2 cm inside the geometric edge of fields

Field Symmetry The AAPM recommends that the crossbeam profile in the reference plane should not differ more than 2% at any pair of points located symmetrically on opposite sides of the central axis.

Beam Collimation The dual-foil system 1. To widen the beam by multiple scatter 2. To make the beam uniform in crosssection The beam-defining collimators A primary collimation close to the source that defines the maximum field size. A secondary collimation close to the patient to define the treatment field

Principle of dual-foil system

Field Size Dependence (1) The dose increases with field size because of the increase scatter from the collimator and phantom. Various size cone with a fixed jaw opening minimizes the variation of collimator scatter. If the x-ray jaw setting changed with the field, the output would vary widely, especially for lower-energy beam.

If the x-ray jaw setting changed with the field, the output would vary widely, especially for lower-energy beam.

Field Size Dependence (2) If the distance between the point of measurement and the edge of the field is shorter than the range of the laterally scattered electrons phantom scatter The effects of field size on output and the central axis depth dose curve is significant.

Electron Source (1) Virtual source An intersection point of the backprojections along the most probable directions of electron motion at the patient surface. Pohlit’s method-a grid of copper wires A multipinhole technique-double conical holes Meyer method-FS magnification on film The use of the virtual SSD to predict dose variation with distance requires the inverse square law factor and correction factors as a function of FS and energy.

Electron Source (2) Effective SSD To correct air gap A function of energy and FS

X-ray Contamination The tail of the depth dose curve Bresstrahlung interactions of electrons with the collimation system and the body tissues In a modern Linac 6 -12 Me. V 12 -15 Me. V 15 -20 Me. V 0. 5 -1% 1 -2% 2 -5% Critical for total body electron irradiation

Chapter 14 Electron Beam Therapy (2)

Choice of Energy and Field Size Energy The target volume lies entirely within the 90% isodose curve. Field size A significant tapering of the 80% isodose curve at energies above 7 Me. V The constriction of the useful treatment is worse for the smaller fields. A larger field at the surface may be necessary to cover a target area adequately.

Corrections for Air Gaps Beam divergence Inverse square law correction from a virtual or effective SSD The relative depth dose distribution is unchanged. The absolute value of the dose is decreased at all depths.

Corrections for Beam Obliquity Increase side scatter at the depth of dmax. Shift dmax toward the surface. Decrease the depth of penetration

Corrections for Beam Obliquity Pencil or slit beams Obliquely incident beam The point at the shallow depth receives greater side scatter from the adjacent pencil beam. The point at the greater depth receives less scatter.

Corrections for Beam Obliquity The depth dose in an obliquely incident beam is affected both by the ”pencil scatter effect” and the “beam divergence”.

, OF( , d) Rp E (Me. V)/2

the calculated distribution using a pencil beam algorithm v. s. the measured distribution obtained in a cylindrical polystyrene phantom

Sharp Surface Irregularities Electrons are predominantly scattered outward by steep projections and inward by steep depressions.

Tissue Inhomogeneities It is difficult to determine dose distribution within or around small inhomogeneities becausfe of enhanced scattering effects For large and uniform slabs, dose distribution can be corrected by using the of equivalent thickness (CET) method. deff = d-z(1 -CET)

Tissue Inhomogeneities Bone CET of a compact bone = 1. 65 CET of a spongy bone = 1 Lung CET=0. 5 Relative e =0. 2 -0. 25 Reduced scatter from the lowdensity cork The increased penetration overtakes the reduced scatter A water-cork system

Small Inhomogeneities A material M’ of a higher mass scattering power A decrease in the electron fluence behind the slab Increase the dose in the medium M

Small Inhomogeneities Pohlit and Manegold To evaluate the maximum values for increase and decrease of dose behind the inhomogeneities -the position of the maxima of reduction and of increase of dose -the mean angle at which the effect is negligible

Pohlit and Manegold Method Under the inhomogeneity but outside angle the regular CET method The maxima and minima of dose along the boundaries of angle estimated by a maximum change, Pmax

Pmax The influence of an edge increases with increasing Ee.

Use of Bolus and Absorbers Bolus is used to Flatten out an irregular surface Reduce the penetration of the electrons in parts of the field Increase the surface dose Equivalent to tissue in stopping power and scattering power Decelerators Low Z To reduce the energy of an electron beam

Adjacent Fields The decision based on the uniformity of the combined dose distribution across the target volume

electron field v. s. photon field A hot spot on the side of the photon field A cold spot on the side of the electron field Outscattering of electrons from the electron field The electron beam SSD=100 cm The electron beam SSD=120 cm

Field Shaping Lead cutouts To give shape to the treatment area To protect the surrounding normal tissue or critical organs Placed on the skin or at the end of the treatment cone For lower-energy electrons (<10 Me. V), less than 5 mm thickness of lead is required ( 5%).

External Shielding Allowable transmission – 5% If the lead is too thin, the transmitted dose may be enhanced directly behind the shield. FS , the desired thickness of lead

Measurement of Transmission Curves A parallel-plate ion chamber A polystyrene phantom The measurement depth 5 cm Broad beam Bremsstrahlung the transmission dose E (Me. V)/2=the minimum thickness of lead The required thickness of cerrobend is 20% greater than that of pure lead.

Effect of Blocking on Dose Rate The magnitude of the effect depends on The extent of blocking The thickness of lead The energy The reduction in dose also depends on the depth of measurement.

Effect of Blocking on Dose Rate If a field produced by a lead cutout is smaller than the minimum size required for maximum lateral dose buildup, the dose in the open portion is reduced. Rp as the lower limit for field diameter

Internal Shielding For the treatment of lip, buccal mucosa, and eyelid lesions The electron backscatter from lead enhances the dose to the tissue near the shield 30% - 70% in the range of 1 – 20 Me. V, having a higher value for the lowerenergy beams For the polystyrene-lead interface ESF =1+0. 735 e(-0. 052 Ez)

Electron Backscatter v. s. Z

The dose enhancement drops off exponentially with the distance from the interface on the entrance side of the beam.

The relative backscatter intensity v. s. the thickness of absorber (polystyrene) To dissipate the effect of electron backscatter, a suitable thickness of low Z absorber may be placed between the lead shield and the preceding tissue surface.

Electron Arc Therapy For superficial tumors along curved surfaces “Pseudoarc technique” Field is defined by the x-ray jaws The electron collimation is provided on the patient’s skin surface large number of overlapping fields Directed isocentrically

Calibration of Arc Therapy Beam Dose per arc can be determined Integration of the stationary beam profiles An isodose distribution as well as the dose rate calibration of the field (under stationary beam conditions) for arc The dose per arc at P is given by Direct measurement A cylindrical phantom

Treatment Planning Beam energy For small scanning field width, the depth dose curve shifts slightly and the beam appears to penetrate somewhat farther than for a stationary beam. The surface dose is reduced. The bremsstrahlung dose at isocenter is increased.

Treatment Planning Scanning field width Small scanning fields give lower dose rate and greater x-ray contamination. Simplifying dosimetry 4 -8 cm Location of isocenter Equidistant from the surface contour for all beam angles Greater than the maximum range of electrons

Treatment Planning Field shaping

Total Skin Irradiation 2 -9 Me. V Translational technique A horizontal patient is translated relative to a beam of electrons. Large field technique A standing patient is treated with a combination of broad beams produced by electron scattering and large SSDs (2 -6 m).

The six-field Stanford technique An acrylic scatter plate ( 1 cm) to provide additional scatter