Total Skin Electron Therapy_PRESENTED_10!12!11

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Total Skin Electron

Therapy

Wednesday, October 12th, 2011

Michael S. Curry, M.S.


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TSET

TG-30

Introduction

Requirements

Techniques

LINAC Operating Conditions

Dosimetry and Instrumentation

Patient Considerations

Commissioning

TSET at MDACCO


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Introduction to TSET

Diseases most commonly treated (require shallow Tx depths)

Cutaneous T-cell Lymphomas

Micosis Fungoides

Sezary syndrome (less so than Micosis Fungoides)

Less commonly treated diseases Kaposis sarcoma

Dose Scheme (TG-30):


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Requirements

Beam

Specification of:

Field size, penetration, energy, depth, dose rate, field flatness in

treatment plane, X-ray background, and the need and nature of boost

fields

Room

Careful consideration of:

Space, shielding, ventilation, electron ranges


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Beam Requirements

Field size

200 cm high by 80 cm wide

Penetration depth

Varies with stage, type of disease, and over the body surface

Typically 515mm or more at the 50% isodose line

Energy

3 to 7 MeV at patient treatment plane

4 to 10 MeV at beam exit window

Dose rate

In order to reduce Tx time high dose rates are desirable

Range from 0.25 Gy/min to several Gy/min


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Beam Requirements

Field flatness in the treatment plane

Vertical uniformity of +-8%, Horizontal Uniformity of +-4% over the central

160 cm x 60 cm area

X-ray background

Penetrating and forward directed

Exposes most of body and should be ALARA

Can be reduced by angling beams such that the x-ray peaks lie outside the

body

Desired to be 1% or less averaged over the entire body (typically 1-4%)

EORTC spec < 5%

Need and nature of boost fields

Some body areas are unintentionally shielded by other body sections or

inadequately exposed due to limitations of beam geometry


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Profiles

Left: 6E 10x10 surface

Right: 6E HDTSE (no cone, 10x10 XY jaws)


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Room Requirements

Space

In order to provide good dose uniformity large distances are needed

Typically 2-7m from scatterer and patient (depending on technique)

Ventilation

TSET involves significant ozone production from ionizing large

volumes of air in the treatment room

Frequent exchange of air is essential for confining ozone exposure

Shielding

Typically MV X-ray shielding is adequate, but should measure Electron Ranges

Max track length range of ~0.5 g/cm2 per MeV (~4 m/MeV) in air

Range is typically not in direction of beam, most

stop far short of this

Bremsstrahlung


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Techniques

Why do we need special techniques, are large AP/PA fields

not sufficient?

No, they are not sufficient:

Patients body shape is not flat

Dose uniformity impossible with AP/PA


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Techniques

Flat = good

Round = bad


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Techniques

How do you treat curved surfaces? Multiple fields

Electron arcs


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Techniques

Prior to use of LINACs

Beta Particles

Beta-particle beams used Sr/Y-90 with Emax=2.18 MeV

Use 10% isodose line and deliver 2Gy/fx in 15 min fx by scannins over a

patient surface 60 cm x 180 cm

Narrow rectangular beams

Use Van de Graaf accelerators in fixed positions with vertically

downward beams

Patient translated horizontally on a motor driven couch under the 1.5 to

4.5 MeV beam


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Techniques

Used with LINACs

Scattered single beam

6.5 MeV beam with 0.15mm thick titanium scattering foil placed 10

cm from accelerator window

Shaped polystryrene beam-flattening filter mounted on front of

treatment head

Producing a flattened 4 MeV beam at the Tx plane

Pair of parallel beams

2 horizontal parallel beams with axes contained in a vertical plane

(axes seperation=150cm) with Tx distance of 2m

8MeV linac with carbon energy degraders just beyond exit window

Adjusting thicknesses adjusts depth of penetration in patient

X-ray background=2%

Pair of angled beams

Stanford Technique


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Stanford Technique

Most commonly used technique

Utilizes 6 dual fields

Each dual field is comprised of two angled beams

Typically 18 to 20 from 270 or 90 gantry position

The 6 fields correspond to 6 different patient treatmentpositions

Provides acceptable dose uniformity

3 sets of dual fields treated per day


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Stanford Technique


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Stanford Technique


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Stanford Technique


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Mayo Clinic Jacksonville TSET Technique

IMPAC Rx and Tx fields

Each day 6 fields (upper and lower of 3 fields),

e.g. 1U,1D,2U,2D,3U,3D


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Mayo Clinic Jacksonville TSET Technique


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University of Iowa TSET Technique


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Tray allows 40 x 40cm field size


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EORTC: R80 > 4mm, R20 < 2cm


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Techniques

Used with LINACs

Pendulum-arc

8 MeV beam with the gantry rotated continuously during treatment

in a 50 arc (six fields)

Arc starts from an initial angle with CAX above the head and ends

with CAX below the feet

Degrader: large 1 cm thick plexiglass sheet 5cm from the patient

Provides large angle electron scattering near the patient

Patient Rotation

Use single horizontal 6 MeV beam (3.5 MeV at Tx plane) with a

scatterer near the exit window and a 7 m treatment distance

X-ray background = 2.2%

Reduced setup and Tx times as well as simplified beam matching

Self shielding by limbs is unavoidable


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Pendulum Arc


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Patient Rotation


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LINAC operating perameters

Stable, Repeatable operating energy is essential

Energy changes can shift large SSD fields laterally and change dose

calibration and uniformity

High average beam current (100 times normal electron beam) for highdose rate (>1Gy/min) at Tx plane

Beam scatterer-energy degraders

Scatterers are thin materials used to spread out the beam

Energy degraders are thick materials used to reduce beam energy at

the Tx plane

Can be placed internally in the Tx head or externally

Location and Materials used are important in determining dose rate

X-ray background is least when scatterer-

degrader is close to patient (~15mm from pt)


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Beam Monitoring

Typically monitor electron fluence rate or dose rate at Dmax

Monitor response should be directly proportional to parameter of

interest

Acceptable monitors include: Built-in transmission ion chambers, secondary electron emission

monitors, and electromagnetic induction monitors

Should be placed where beam exits accelerator

A common combination for TSET monitoring:

a full-beam transmission ionization chamber at or within the treatmenthead

a sampling chamber or electron collector placed at or near the patient Tx

plane but not in line with the patient


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Methods

Acceptable detectors:

ion chambers, film, TLD, Fricke dosimeters, electron collectors, and

faraday cups

Ion chambers (small thimble or small volume/thin window p-p) are

recommended for scanning in a water tank

If film is used with solid water, no air gaps should be present

Might be difficult if film is in a package

Phantoms

Square water phantoms are recommended for depth dose data

Otherwise, layered, flat phantoms made of conducting plastics or thin

laminae of polystyrene with conductive graphite coatings are

recommended for depth dose and buildup data

Elliptical, oval or cylindrical phantoms are

recommended for simulating a patients body


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Dosimetry and

Instrumentation

Measurements

Energy

Determined from depth ionization curve and the range-energy

relationship given in section 2.1 of TG-30:

Fluence

Evacuated Faraday cup with collimator placed over the aperture

Electron fluence is determined from the charge collected and the area of

the collimator

This fluence can be used to estimate entrance surface dose

Depth dose

Using a parallel-plate chamber overlayed with varying thicknesses of

polystyrene

EORTC: R80 > 4mm, R20 < 2cm


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Calibration Point Dose Measurements

TG-30 recommends that TSET absorbed dose be evaluated at a point

(0,0,0) see figure

The TG-21 protocol should be followed for calibration using data for

electron energy of0 determined from R50 ( )

A p-p chamber with known Ngas is recommended

The chamber surface should be placed at dmax using polystyrene

The rest of the chamber should also be surrounded with polystyrene

1 cm posteriorly and 5 cm radially

Expose the chamber to a single dual-field


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Treatment Skin Dose Measurements

Treatment skin dose is defined as the dose along a circle at or near the

surface of a cylindrical polystyrene phantom 30 cm in diameter and

30 cm high which has been irradiated as a hypothetical patient with

all 6 dual fields.

During irradiation the phantom is outfitted with appropriate dosimeters

These dosimeters are calibrated with a single dual field

Therefore calibration point dose is related to treatment skin dose by

a factor B (typical values between 2.5 and 3.1)


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Positioning

Patient should be positioned to minimize self shielding

Support devices

Patients may have trouble standing

Should be prepared with multiple alternativepositioning methods

Shielding

Lenses of the eyes can be shielded by placing high Z material either

over or under the eye lids Finger and toe nails can be shielded with thin sheets of lead cut to size


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Shielding


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University of Iowa TSET Shielding


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Boost Fields

Typically the Soles of the feet, the perineal area, the dorsal surface of

the penis, peri-anal skin, and the inframammary region of large

breasted women

Areas requiring a boost are determined by in-vivo dosimetry

In-vivo dose measurements

Important for 2 reasons:

1. determination of the distribution of dose to the patient's skin

2. verifying that the prescribed dose to the patient's skin is correct

In-vivo dosimeters include:

Ion chambers, diodes, film, TLDs, OSLs, MOSFETs

Ion chambers and diodes are impractical due to the number required (at

least 40)

TG-30 recommends TLDs for In-vivo

dosimetry (could argue for OSL or MOSFET too)


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Boost Fields


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Commissioning

LINAC must be capable

Room must be large enough

Determine a Beam monitoring approach

Decide on a technique

Build or purchase patient support system

Create a written procedure for changing from conventional

modalities to TSET and back to conventional

Determine an In-vivo dosimetry protocol


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TSET at MDACCO

IX Vault

Patient Support System

Stanford Technique

In-Vivo Dosimetry system


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In-Vivo Dosimetry system

Landauer OSL system (InLight microStar)

Carbon-doped aluminum oxide (Al2O3:C)

Similar to TLD except use LED light for stimulation instead of heat

The light used is of a specific wavelength

The stimulation process doesnt anneal the OSL OSL can be read multiple times and or stored as record of delivered dose

Independent of energy for 6 and 18 MV beams

Response increases linearly with dose rate

Time resolution = 0.1s, Spatial resolution


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Review

What is TSET e- beam energy? Nominal? 9 MeV

At pt surface? 4 MeV

What are differences between clinical e- beam and TSET

beam? Higher dose rates; Larger Field Sizes; No Cones What are common Tx indications for TSET?

T-cell Lymphoma (micosis fungoides)

Doses? 36Gy in 9 fx

Describe one technique for delivering TSET. Stanford Technique

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