External Beam Radiotherapy Radiation Sources in Radiotherapy External Beam Radiotherapy Day 7 – Lecture 4

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

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).

Clinical Objectives To deliver a dose and dose distribution that is adequate for tumor control but which also minimizes complications in normal tissue. Note: It is not the role of the Regulatory Body to evaluate the clinical decisions of medical practitioners authorized to prescribe radiotherapy treatments. MODIFIED

Very important for optimization of protection in medical exposures Treatment Planning Prescription Very important for optimization of protection in medical exposures Planning Planning is the most important link between clinical intention and the practical treatment. Treatment

Treatment Planning About 1/3 of problems are directly related to treatment planning; Problems may affect an individual patient or cohort of patients. IAEA Safety Report Series 17; 2000 “Lessons learned from accidental exposures in radiotherapy”

Radiotherapy Simulator

External Beam Equipment 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

Typical Radiation Levels Cobalt-60 teletherapy Source activity may be around 400 TBq (~10,000 Ci); 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.

Typical radiation levels (cont) Linear accelerator turned off There is no useful radiation beam when turned off; 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.

Superficial and Orthovoltage x-ray equipment 40 kVp to 120 kVp Orthovoltage (“deep”) 150 kVp to 400 kVp 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 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

Superficial x-ray equipment Interlocks prevent inappropriate combinations of kVp and filtration. Electron contamination from the applicator can be significant.

Superficial x-ray equipment (cont) Dose is highly dependent on source-skin distance, filtration and applicator area.

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.

Issues with Superficial radiotherapy 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 The lecturer may particularly emphasize the first point. The strong dose gradients are due to FSDs of 10 to 25cm in practice. Shorter the treatment distance higher the influence of inverse square law and hence to ensure accurate dose delivery the treatment distance should be accurate Calibration of beams is typically done at surface, therefore electron contamination from lead glass cones can be of concern.

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

Orthovoltage (deep) x-ray equipment

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 therapy In order to reach deep seated tumours higher energy x-rays of 150-400 kVp were used. As it could reach deeper tissue than superficial x-rays were termed deep x-ray therapy. The usual depth of treatment with this is about 20 mm. Applicators / cones are used in this type of treatment also. X-ray tube Applicator

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 Deep X-ray therapy was used for treatment close to the surface such as ribs and spinal cord. However once the mega-voltage therapy was started, this was withdrawn from use.

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 The lecturer may point out the drawbacks of using low energy x-rays for the treatment of deep seated tumours.

Gamma ray equipment

Gamma ray equipment (cont) Source head and a typical source transfer mechanism

Why prefer Cobalt unit over Orthovoltage? The above table gives a comparison of Cobalt with an orthovoltage unit (Deep X-ray therapy unit. The lecturer may use this slide to further explain why Telecobalt units are preferred over deep x-ray therapy units.

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.

Patient positioning collimator Gamma Knife The Gamma Knife: 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. Patient positioning collimator

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.

Linear Accelerator (cont) Concept

Linear Accelerator (cont) Radiation exposure: is controlled by two independent integrating transmission ionization chamber systems; 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.

Linear Accelerator (cont) the other system is termed the secondary system and is usually set to terminate the exposure after an additional 0.4 Gy; most modern accelerators also have a timer which will terminate the exposure if both ionization chamber systems fail.

Linear Accelerator (cont) Complex head structure to handle multiple energies and multiple modalities.

Linear Accelerator (cont) Complex control system

Linear Accelerator (cont) Verification systems All accelerator manufacturers produce computer controlled verification systems which provide an additional check that the settings on the accelerator console: are correct for proper accelerator function; and correspond exactly with the parameters determined for the individual patient during the treatment planning process

Linear Accelerator (cont) X-ray Collimators Rectangular (conventional) The transmission through the collimators should be less than 2% of the primary (treatment) beam. 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.

Linear Accelerator (cont) Electron applicators may be: open sided for modern accelerators using double scattering foils or scanned beams; enclosed for older accelerators using single scattering foils. Both types should be checked for leakage: adjacent to the open beam; on the sides of the applicators.

Linear Accelerator (cont) Neutrons:- should be considered if the x-ray energy is greater than 10 MV Issues which need to be considered when neutrons are presents include: neutron activation shielding problems

A comparison: Cobalt unit Vs Linac The above table gives a comparison of the Telecobalt unit and Linear accelerator.

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

General Safety Requirements Warning Signals and Signs

General Safety Requirements “Fail safe” systems There shall be at least two independent “fail safe” systems for terminating the irradiation. These could be: two independent integrating in-beam dosemeters; two independent timers; an integrating dose meter and timer. Each system shall be capable of terminating the exposure.

General Safety Requirements Collimation The exposure shall be limited to the area being examined or treated by the use of collimating devices aligned with the radiation beam. Exposure rates outside the examination or treatment area due to leakage or scatter shall be kept as low as reasonably achievable.