Electron Beam Physics for Total Skin Irradiation

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Electron Beam Physics for Total Skin Irradiation

• Faisal Siddiqui, Ph.D.
• July 23, 2008

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Total Skin Electron Beam TherapyApplications

• Cutaneous T-Cell Lymphoma (Mycosis Fungoides,
  Sezary Syndrome)
• Leukemia Cutis
• Kaposis Sarcoma
• Scleromyxoedema (Lichen myxoedematosus)

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Mycosis Fungoides

• Most common form of cutaneous T-cell lymphoma.
• In US, 1000 new cases per year.
• Men twice as often as women, and is more common
  in African-Americans than in Caucasian.
• Most common age is 50.
• Progression in phases
• Patch phase - The skin has flat, red patches
  very itchy. plaques).
• Skin tumors phase - Red-violet raised lumps
  (nodules) appear and may be dome-shaped (like a
  mushroom) or be ulcerated.
• Erythroderma stage - Large red areas that are
  very itchy and scaly and thickening of skin
  folds.
• Lymph node stage Spread to lymph nodes, and
  often to the liver, lungs, or bone marrow.
• Staging
• Stage IA/IB skin lt/gt 10
• Stage IIA peripheral adenopathy
• Stage IIB tumor
• Stage III erythroderma disease
• Diagnosis skin biopsy
• Treatment
• TSEBT
• Psoralen with UV light (PUVA)
• Extracorporeal photochemotherapy

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Leukemia Cutis

• Cutaneous manifestations of any type of leukemia
  with infiltration of neoplastic leukocytes or
  their precursors into the skin
• Skin nodules that move freely over subcutaneous
  tissue but are well-fixed to the skin.
• Skin involvement is characteristic of monoblastic
  and myelomonocytic leukemias (most commonly AML)
  ? poor prognosis, harbinger of BM relapse.
• Typical Treatment 4 times a week with 200 cGy
  per fraction to a total dose of 1200 cGy, using 6
  MeV electrons.

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Electron Beam Physics

• Electron Beam Production
• Central Axis Depth Dose Curve
• Isodose Curves
• Field Flatness and Symmetry
• Field Size Dependence
• X-Ray Contamination

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Electron Beam Production
The scattering foil (dual-scattering foils
separated 510 cm) to broaden the beam Secondary
(x-ray) collimator and electron applicator to
collimate the beam Ion chamber (actually dual,
segmented ionization chamber) used to monitor the
beam
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Electron Energy Spectrum
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Central Axis Depth Dose Curve

• Ds Relative surface Dose at 0.5 mm
• Dx dose due to x-ray component, bremsstrahlung
  tail
• G0 normalized dose gradient, G0 Rp/(Rp Rq)
• If G0 is large ? rapid fall off
• R100 distance corresponding to Dmax (R85, R50,
  Rp), zmax
• R90 E/3.2 (13/3.2 4 cm)
• R80 E/2.8 (13/2.8 4.6 cm)
• Dmax 0.46 x E0.67
• (0.46 x 130.67 2.56 cm)

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Depthdose curves in water for multiple electron
beam energies

• 1060 MeV for large fields at 200 cm SSD
• 5 MV (small-dashed line) and 22 MV (long-dashed
  line) x-ray beams for a 10 10 cm2 field at 100
  cm SSD
• Electrons lose energy at rate of 2 MeV/cm in
  water/tissue
• Surface dose for E beams increases with electron
  energy
• Lower electron energies ? steeper fall off due to
  scattering and conntinuous energy loss.
• Increased bremsstrahlung contamination

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Field Size Dependence

• The field-size dependence of percent depth dose
  is illustrated for an 18 MeV beam.
• Lateral Scatter Equilibrium
• Req 0.88 (E)1/2
• Req 0.88 (18)1/2 3.7 cm
• Fields smaller than Req, field size dependent
• Fields larger than Req, not field size dependent
• For rectangular fields, percent depth dose is
  best determined using the square-root method
• DX,Y DX,X DY,Y1/2

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Isodose Curve

• Central axis distribution
• Flatness
• Curvature near field borders (bulge)
• Lateral Constriction
• Differences in machines
• Different collimation systems, and
• Air column above the patient, cause
• Angular dispersion of the beam as well as the
  energy spread

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Field Flatness Symmetry

• Low Energy Beam greater expansion
• High Energy Beam
• only low isodose bulge
• Lateral constriction which becomes worse with
  decreasing field size

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Build Up Curve Skin Effect

• Low energy electron beam greater fluence due to
  scatter, greater skin effect, less skin sparing
• High energy electron beam smaller fluence.

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Treatment Planning

• Choice of energy and field size
• Corrections for air gap and beam obliquity
• Tissue heterogeneities
• Use of bolus and absorbers
• Problems with adjacent fields
• Field Shaping
• External Shielding
• Measurement of transmission curves
• Effect of blocking on dose rate
• Internal Shielding

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Patient Positioning

• Treatment area entire body surface to a limited
  depth and to a uniform dose using electrons.
• Field Size treatment plane must be
  approximately 200 cm in height by 80 cm in width
  to encompass the largest patient.
• Uniformity vertical 8 and horizontal of
  4 over the central 160 cm x 60 cm area

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Stanford Six Dual Field Technique

• Developed by C. J. Karzmark at Stanford
  University in 1960s.
• The patient was treated at a large distance
  (typically more than 400 cm) from the source in a
  standing position.
• A dual field consisting of two gantry angles was
  used, with each pointing up or down from the
  horizontal direction.
• To achieve a uniform dose over the entire body,
  the patient was rotated around the vertical axis
  six times at 60 intervals so that six dual
  fields had been delivered.
• Patient factors
• Variable thickness of the skin
• Surface irregularities of its surface
• Machine factors
• high output, large fields, and extended SSD.
• The Stanford technique requires the irradiation
  of the patient at 6 different positions.
• Unusual positions may be quite uncomfortable for
  the patients, especially for the elderly.
• They have to stand on their own throughout the
  long treatment procedure, with their eyes
  covered, often causing loss of orientation.

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Tissue Heterogeneity

• Coefficient of Equivalent Thickness (CET) of a
  material is given by its electron density
  relative to the electron density of water and is
  essentially equivalent to the mass density.
• Lung
• Density of 0.25 g/cm3 and a CET of 0.25.
• A thickness of 1 cm of lung is equivalent to 0.25
  cm of tissue.
• Solid bone has a CET of approximately 1.6

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TSEB Irradiation Dose

• High Dose
• 4 to 6 MeV beam 36 to 40 Gy over 8 to 10 weeks
• Up to 40 of pts treated with high dose TSEB
  irradiation remain relapse free for long periods
• High frequency of initial clearing after high
  dose irradiation, but lower continuous disease
  free survival
• Sustained disease free survival primarily for
  patients with Stage Ib or IIa MF

• Adverse Effects
• Erythema the MF lesions become red and
  pigmented, desquamated
• Temp scalp alopecia
• Temp nail stasis
• Hands/feet edema
• Minor nosebleeds
• Blisters on fingers and feet
• Anhidrosis, parotiditis, gynecomastia
• Corneal tears from internal eye shields

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References Electron Beam Physics
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References

• Stanford Technique
• Karzmark, C. J. Leo Vinger, R. Steel, R. E. A
  technique for large-field superficial electron
  therapy. Radiology 174633 1960.
• Karzmark, C. J. Large-field superficial electron
  therapy with linear accelerators. Br. J. Radiol.
  37302 1964.
• Karxmark, C. J. Physical aspects of whole-body
  superficial therapy with electrons. Front.
  Radiat. Tber. Gncol. 236 1968.

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References
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References

• INTERNATIONAL ATOMIC ENERGY AGENCY, The Use of
  Plane Parallel Ionization Chambers in High Energy
  Electron and Photon Beams, Technical Reports
  Series No. 381, IAEA, Vienna (1997).
• Absorbed Dose Determination in External Beam
  Radiotherapy, Technical Reports Series No. 398,
  IAEA, Vienna (2000).
• INTERNATIONAL COMMISSION ON RADIATION UNITS AND
  MEASUREMENTS, Radiation Dosimetry Electron Beams
  with Energies Between 1 and 50 MeV, Rep. 35,
  ICRU, Bethesda, MD (1984).
• JOHNS, H.E., CUNNINGHAM, J.R., The Physics of
  Radiology, Thomas, Springfield, IL (1985).
• KLEVENHAGEN, S.C., Physics and Dosimetry of
  Therapy Electron Beams, Medical Physics
  Publishing, Madison, WI (1993).
• VAN DYK, J. (Ed.), Modern Technology of Radiation
  Oncology A Compendium for Medical Physicists and
  Radiation Oncologists, Medical Physics
  Publishing, Madison, WI (1999).

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Effect of Field Size on DD
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MF TNM

• TNM(B) Definitions Primary tumor (T)
• T1 Limited patch/plaque (lt10 of skin surface
  involved)
• T2 Generalized patch/plaque (10 of skin
  surface involved)
• T3 Cutaneous tumors (one or more)
• T4 Generalized erythroderma (with or without
  patches, plaques, or tumors)
•  Note Pathology of T1T4 is diagnostic of
  cutaneous T-cell lymphoma (CTCL). When
  characteristics of more than one T-type tumor
  exist, both are recorded and the highest is used
  for staging, for example, T4(3).
• Regional lymph nodes (N)
• N0 Lymph nodes clinically uninvolved the
  pathology is negative for CTCL
• N1 Lymph nodes clinically enlarged,
  histologically uninvolved the pathology is
  negative for CTCL
• N2 Lymph nodes clinically unenlarged,
  histologically involved the pathology is
  positive for CTCL
• N3 Lymph nodes enlarged and histologically
  involved the pathology is positive for CTCL
• Distant metastasis (M)
• M0 No visceral disease
• M1 Visceral disease present
• Blood involvement (B)
• B0 No circulating atypical cells (lt1000 Sézary
  cells CD4 CD7-/mL)
• B1 Circulating atypical cells (1000 Sézary
  cells CD4 CD7-/mL)
• TNM Stage Groupings
• Stage IA T1, N0, M0