Total Skin Electron Therapy

What common locations need to be shielded during TSET?

- Eyes (internal eye shields) - Fingernails and toenails (shielded always) - Hands and Feet (shielded after Rx dose)

What dosimeter options do you have for measurement of TSET?

- Ion chambers - film - TLD/OSLD - Fricke dosimeters - electron collectors - Faraday cups

What affects need to be accounted for when using ionization chambers for TSET measurement?

- Recombination and polarity effects - Stem leakage (just shield the cord)

What dosimeter is best for beam scanning?

- Small volume thimble ionization chamber for profiles - Parallel plate chamber for PDD measurement

How would a PDD measurement be performed for dual angled fields?

- Take two separate PDD curves and sum them - Use integrating dosimeters (film, TLD/OSLD)

What dosimeter needs to be used for absolute dose calibration?

- Traceable ionization chamber

Why are in vivo measurements performed?

- Verify correct dose to patient's skin - Determine any boost or shielding areas

What phantoms should be used for dosimetry measurements?

- Water - Solid water - Anthropomorphic

What dose uniformity should be achieved in TSET?

-6% or better in the horizontal plane -4% in the vertical plane

What minimum dose rate should be used for TSET?

0.25 Gy to several Gy per minute at a minimum

What should x-ray background dose be limited to?

1% or less ex: 36 Gy prescription should only have 0.36 Gy of x-ray contamination dose

What considerations for beam requirements need to be made when starting a TSET program?

1. Field Size 2. Penetration depth 3. Energy 4. Dose 5. Dose Rate 6. Field Flatness at the treatment plane 7. X-ray background 8. Boost fields

How can x-ray dose be minimized?

1. Scatterer placed near the patient to increase electron use efficiency 2. Angle beam above and below the patient so the forward scattering photons miss the patient surface altogether

What are two major considerations when evaluating a treatment vault for TSET?

1. Space - need at least 3 meters of space to set up patient 2. Air circulation - electron ionization of the air produces hazardous ozone (O3), and the air should be circulated to eliminate this ***Most shielding is adequate already but a survey should still be performed

What field flatness should be achieved across the treatment field?

10% or better is a goal but most places do better

What field size needs to be achieved to treat the largest TSET patients?

200 cm in height 80 cm in width

What is the incidence of mycosis fungoides?

3 per million people

What electron energies are used in TSET?

4 - 10 MeV at the accelerator window but degrades to 3 - 7 MeV at the patient surface

What can be added to the treatment setup to improve dose uniformity?

A 1 cm thick lucite scatterer can be placed ~25 cm away from the patient to increase the divergence of the beam. It does reduce the penetration and depth dose falloff

What is mycosis fungoides?

A cutaneous T-cell lymphoma that appears as blotchy red spots across the body surface

What is x-ray contamination if TSET?

Added does from photon production by electron interactions. They only add background dose to the patient.

What arrangement of monitor chambers should be utilized for added safety measures?

An ionization chamber at the exit window and after all attenuators to take a measurement of the radiation produced, and the amount that actually reaches the patient

Where should the absolute dose be calibrated?

At Dmax at the midline of the patient near the abdomen (umbilicus)

Where should dosimetry measurements be taken for TSET?

At the treatment plane

What is a common value for B?

Between 2.5 and 3.1

How should absolute dose calibration be performed for dual fields?

Both fields should be shot and taken as 1 cumulative field, with the calibration being valid for the 2 fields combined

What are the pros/cons of placing the scatterer close to the patient or close to the linac?

Close to patient: Larger angular distribution due to the lower energy when scattered, decreases depth dose max and penetration depth due to larger obliquity Close to Linac: Smaller angular distribution, possibly some increased photon production due to lower electron efficiency

What other dosimeter can be used for beam scanning?

Diode when proven reliable through verification with an ion chamber

What is the "Treatment Skin Dose"

Dose to the skin surface, often used to compare treatment outcomes

What is the energy-range relationship equation for mean energy?

E_mean = 2.33*R_50 E_mean = mean energy R_50 = depth at which electrons have deposited 50% of their dose

What is the energy-range relationship equation for most probable energy?

E_p = 1.95*R_p + 0.48 E_p = probable energy R_p = practical range determined from PDD curve extrapolation

What is the energy-range relationship equation for mean energy at any depth?

E_z = E_mean (1 - z/R_p)

Why should shields not be mounted to the gantry as in TBI?

Electron scattering in the air will make the shields obsolete

How should In-Vivo dosimetry be performed?

First fractions with TLDs/OSLDs placed in verification locations (mid section) and in known hotspot/coldspot locations

What properties do an electron scatterer need to have?

High Z thin

What is the advantage of using 6 fields with a rotating patient position for TSET?

Improves dose falloff distally and shifts the dose up to the skin surface, which is where we want the dose deposited

What irradiation techniques (i.e. beam arrangement and patient orientation) have been used for TSET?

In order of most uniform to least - Patient on a rotation platform - 8 field technique - 6 field technique - 4 obliques - 2 fields (AP/PA) The 6 field technique is the most common as its uniformity is sufficient and is easiest/fastest to implement and treat

How are boost prescriptions determined?

In-vivo dosimetry for the first treatment session gives relative dose measurements for different parts of the body compared to the prescription dose. The deficit can be delivered with boosts

What happens to the PDD curve of an electron as you increase the distance from isocenter?

Increased scattering degrades the mean beam energy, causing the PDD curve to change accordingly to represent a lower E. i.e. Dmax shifts closer to the surface and surface dose decreases

For the 6 field technique, what are the patient positions?

LAO, RAO, POST LPO, RPO, ANT This gives a dose repetition every 60 degrees around the patient for dose uniformity

What effect does beam obliquity have on the PDD curve?

More of the dose is deposited in the lateral direction than in the forward. As a result, the depth of maximum dose shifts closer to the surface

What disease is TSET used to treat?

Mycosis fungoides

For the 6 field technique, how are the beams oriented?

Pair of angled beams aimed above and below the patient to limit x-ray contamination

Rule of thumb for calculating R_80?

R_80 = E/2.8

Rule of thumb for calculating R_90?

R_90 = E/3.2

Rule of thumb for practical range?

R_p = E/2

Why is a 6 MeV beam exiting the linac not a 6 MeV beam when it reaches the patient plane?

Scatter material, whether intentionally placed or immovable, and distance (air scatter and divergence), causes the electrons to lose energy, causing a change in spectrum at the treatment plane

For the 6 field technique, what is the dose uniformity at the skin surface, and at 3 mm depth?

Skin surface = 10% 3 mm depth = 5%

When are boost fields needed?

Some areas are self shielded by the patient's extremities. Consideration for boosts to areas such as the medial arms and legs should be made

What task group reports are useful for TSET?

TG23 (TSET) TG25 (electron dosimetry)

How can a "Treatment Skin Dose" be prescribed based off of the calibration of 1 cGy/MU at Dmax

The constant of proportionality, B, relating the TSD to the calibration dose from the phantom measurements is used. TSD = B*D_cal so TSD/B = D_cal then MU = Rx dose (cGy) / D_cal (cGy/MU)

Why should dose rates by increased?

The longer treatment distances reduces the dose rate compared to normal geometry. To keep treatment times short, especially since most patients are elderly, the dose rate has been increased 10x for this treatment modality.

How is electron energy defined?

The most probable energy based on the energy fluence spectrum

How are over-exposed areas protected?

2 mm of lead shielding is typically sufficient. Shielded areas are also determined through in vivo dosimetry, and shielded for the remainder of fractions once they receive the prescription dose

What is the most common treatment geometry and patient position used for TSET?

6 pairs of angled beams with the patient standing in a different position for each pair

What safety feature should be used with scatterer and degraders?

Interlock of some kind - Absence of the material will cause a serious overdose

What is the necessary penetration depth for electrons?

Treatment depths of 5mm - 15mm at the 50% isodose surface will encompass most lesions.

How far away does a TSET patient stand?

Varies depending on technique but 3 meters is a common distance.

Is detuning an option to adjust the electron energy?

Yes, but you will sacrifice beam symmetry. It is better to add low Z degraders to modify the beam

What properties do an electron degrader need to have?

low Z Thick

What is an example prescription for TSET?

- 36 Gy over 9 weeks - 4 days/wk at 1 Gy/day - Deliver all fields for the first fraction for in vivo, then 3 fields alternating every other treatment

How is "Treatment Skin Dose" determined?

- An anthropomorphic phantom is irradiated in all 6 treatment positions with TLDs/OSLDs placed on the midline. The average of the readings is taken as the TSD. This value can then be related to the dose at the calibration depth through a constant of proportionality, B.

How will a change in beam energy affect a TSET treatment?

- Change depth dose - Change field size due to different scattering angle

What dosimetry measurements need to be taken for TSET?

- Energy (from PDD) - Fluence - PDD - Profiles - X-ray contamination

What are some methods of electron beam delivery used in TSET?

1. Scanning a beta emitter across the patient surface (Strontium-Yttrium 90) 2. Narrow rectangular beams and scanning the patient through 3. Scattered single beam 4. Pair of parallel beams to treat high and low 5. Pair of angle beams 6. 50 degree linac arc delivery 7. Rotating patient with single scattered beam