IAEAInternational Atomic Energy Agency

External Beam Radiotherapy

Day 7 – Lecture 4

Radiation Sources in Radiotherapy

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Objective

To become familiar with the radiation sources, devices and ancillary equipment used in external beam radiotherapy.

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Contents

• Treatment planning systems;

• Radiotherapy simulators;

• Superficial / orthovoltage units;

• Cobalt-60 units including Gamma-knife;

• Linear accelerators;

• Computed Tomography (CT) scanners for radiotherapy;

• Multileaf Collimators (MLC).

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To deliver a dose and dose distribution that is adequate for tumor control but which also minimizes complications in normal tissue.

Clinical Objectives

Note: It is not the role of the Regulatory Body to evaluate the clinical decisions of medical practitioners authorized to prescribe radiotherapy treatments.

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Prescription

Planning

Treatment

Very important for optimization of

protection in medical exposures

Treatment Planning

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• About 1/3 of problems are directly related to treatment planning;

Treatment Planning

IAEA Safety Report Series 17; 2000“Lessons learned from accidental exposures in radiotherapy”

• Problems may affect an individual patient or cohort of patients.

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Radiotherapy Simulator

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Therapeutic x-ray equipment operates in the range of:• 10 kVp - 150 kVp (superficial);• 150 kVp - 400 kVp (orthovoltage / deep);

Radioactive sources ( γ ray equipment).• Cobalt 60 & Caesium 137

Megavoltage electron accelerators for X and electron therapy• Linear accelerator

External Beam Equipment

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• Source activity may be around 400 TBq (~10,000 Ci);

Typical Radiation Levels

Cobalt-60 teletherapy

• Average radiation leakage (beam off) should not exceed 0.02 mGy/h at 1 m i.e. it would take 50 hours exposure for 1 mSv;

• In general, minimize the time spent in the treatment room.

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Linear accelerator turned off• There is no useful radiation beam when turned off;

Typical radiation levels (cont)

• However, immediately after higher energy beams (> 10 MeV) are turned off there may be induced radioactivity but typically with very short half lives (seconds to minutes);

• It is suggested that room entry be briefly delayed, especially after long exposures.

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Superficial• 40 kVp to 120 kVp

Orthovoltage (“deep”)• 150 kVp to 400 kVp

Superficial and Orthovoltage x-ray equipment

• treat skin lesions, bone metastases to a depth of ~ 20 cm

• use applicators or diaphragm

• SSD 30 to 60 cm

• beam quality (HVL) typically 0.2 to 5 mm Cu

• treat small skin lesions to a depth of ~ 5 cm

• maximum applicator size typically < 7 cm diameter

• typical SSD < 30 cm

• beam quality (HVL) typically 0.5 to 8 mm Al

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• Interlocks prevent inappropriate combinations of kVp and filtration.

Superficial x-ray equipment

• Electron contamination from the applicator can be significant.

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Superficial x-ray equipment (cont)

• Dose is highly dependent on source-skin distance, filtration and applicator area.

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Superficial x-ray equipment (cont)

Provides a range of kVp, mA and filtration

Filters are used to absorb low energy photons which otherwise may unnecessarily increase skin dose.

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• Short focus to skin distance (FSD) and hence high output and large influence of inverse square law

• Calibration difficult due to strong dose gradient i.e. dose fall off and electron contamination

Issues with Superficial radiotherapy

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Issues with superficial therapy

• Dose determined by a timer • on/off effects must be

considered• Photon beams may be

contaminated with electrons scattered from the applicator

Control panel

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Orthovoltage (deep) x-ray equipment

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Deep X-ray therapy (Orthovoltage)

• Uses conventional X-ray tube• Energy range 150- 400 kV X-rays• Mostly used around 250 - 300 kVp

• Treatment depths of around 20 mm

• Applicators are used in superficial therapyX-ray tube

Applicator

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Deep X-ray therapy (Orthovoltage)

• Penetration sufficient for palliative treatment of bone lesions relatively close to the surface (ribs, spinal cord)

• Largely replaced by megavoltage treatment modalities for treatment of other lesions

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Disadvantages of deep x-ray

• Higher dose to bone - photoelectric absorption• Maximum dose on the surface hence higher skin

dose• Treatment to a depth of only a few centimeters

possible• Low energy, hence high scattered radiation and

larger penumbra

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Gamma ray equipment

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Gamma ray equipment (cont)

Source head and a typical source transfer mechanism

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Why prefer Cobalt unit over Orthovoltage?

Othovoltage unit Telecobalt unit 150-400 KV x-rays 1.25 MeV Photon Maximum dose on the skin Maximum dose at depth of 5 mm Higher absorption by bone Relatively uniform dose absorption Treatment to a depth of few centimeters Higher penetration deep seated tumours non uniform dose distribution Relatively uniform distribution Higher side scatter hence larger penumbra More of forward scatter, lesser penumbra Vertical unit Mostly isocentric unit

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GAMMA KNIFE

• The gamma knife device contains 201 cobalt-60 sources of approximately 30 curies each

• It is placed in a circular array in a heavily shielded assembly.

• The device aims gamma radiation through a target point in the patient's brain.

• The patient wears a specialized helmet that is surgically fixed to their skull so that the brain tumor remains stationary at target point of the gamma rays.

• Therefore it is also known as the stereotactic surgery.

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Gamma Knife

The Gamma Knife:

Patient positioning collimator

• uses numerous high activity 60Co sources positioned in a device so that the radiation beams converge at the specified point of treatment;

• is used to treat head tumors.

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Linear Accelerator

Modern accelerators have a number of treatment options e.g.

• X-rays or electrons (dual mode);

• 2 X-ray energies;

• 5 or more electron energies.

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Concept

Linear Accelerator (cont)

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• is controlled by two independent integrating transmission ionization chamber systems;

Linear Accelerator (cont)

Radiation exposure:

• one of these is designated as the primary system and should terminate the exposure at the correct number of monitor units;

• these also steer the beam.

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• the other system is termed the secondary system and is usually set to terminate the exposure after an additional 0.4 Gy;

Linear Accelerator (cont)

• most modern accelerators also have a timer which will terminate the exposure if both ionization chamber systems fail.

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Complex head structure to handle multiple energies and multiple modalities.

Linear Accelerator (cont)

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Complex control system

Linear Accelerator (cont)

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Verification systems

All accelerator manufacturers produce computer controlled verification systems which provide an additional check that the settings on the accelerator console:

Linear Accelerator (cont)

• are correct for proper accelerator function; and

• correspond exactly with the parameters determined for the individual patient during the treatment planning process

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Rectangular (conventional) • The transmission through the collimators should be less

than 2% of the primary (treatment) beam.

Linear Accelerator (cont)

X-ray Collimators

Multi-leaf collimators (MLC)

• the transmission through the collimators should be less than 2% of the primary (treatment) beam.

• The transmission between the leaves should be checked to ensure that it is less than the manufacturer’s specification.

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• open sided for modern accelerators using double scattering foils or scanned beams;

• enclosed for older accelerators using single scattering foils.

Linear Accelerator (cont)

Electron applicators may be:

Both types should be checked for leakage:

• adjacent to the open beam;

• on the sides of the applicators.

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• should be considered if the x-ray energy is greater than 10 MV

Linear Accelerator (cont)

Neutrons:-

Issues which need to be considered when neutrons are presents include:

• neutron activation• shielding problems

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A comparison: Cobalt unit Vs Linac

Telecobalt unit Linear Accelerator 1.25 MeV Photon 4 to 21 MV photon beams Maximum dose at depth of 5 mm

Maximum dose at higher depth with energy

Source to be changed every 4 to 5 years

No radioactive source

Leakage radiation present even while the beam is off

Radiation only when the source is switched is ON

1-2 cm source diameter 1mm source – nearly point source Larger penumbra Smaller penumbra Relatively uniform dose absorption

Uniform dose absorption

Higher penetration deep seated tumors

Higher energy than Cobalt possible. Also possible to select higher energy depending on patient thinkness.

Photon only Electron beam of various energies possible

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• Clear indication shall be provided at the control console and in the treatment room to show when the equipment is in operation.

General Safety Requirements

• Dual interlocks shall be provided on all doors to the treatment room such that opening a door will interrupt the treatment. It should only be possible to resume treatment from the control console.

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Warning Signals and Signs

General Safety Requirements

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There shall be at least two independent “fail safe” systems for terminating the irradiation. These could be:

“Fail safe” systems

General Safety Requirements

Each system shall be capable of terminating the exposure.

• two independent integrating in-beam dosemeters;

• two independent timers;

• an integrating dose meter and timer.

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The exposure shall be limited to the area being examined or treated by the use of collimating devices aligned with the radiation beam.

Collimation

General Safety Requirements

Exposure rates outside the examination or treatment area due to leakage or scatter shall be kept as low as reasonably achievable.