Radiation Therapy Dosimetry Basics

Introduction

• External photon or electron beam dose delivery accuracy relies on a sequence of steps.
• These steps include: output calibration for the beam, relative dose measurement procedures, equipment commissioning/quality assurance, treatment plan development, and patient positioning on the treatment machine.

Specific Beam Output

• Output for clinical beams is often expressed as dose rate at a particular point (P).
• Units include Gray per minute (Gy/min) or Gray per monitor unit (Gy/MU).
• Measurements happen at a defined reference depth (often the depth of maximum dose, Zmax).
• Measurements occur in a water phantom, with either a source to surface distance (SSD) or a source to axis distance (SAD).
• A standard field size is often used for reference, typically 10x10 cm².

Machine Basic Output

• Kilovoltage X-ray generators and teletherapy units often use Gy/min as a basic output value.
• Clinical linear accelerators typically use monitor units (MU) which correlate to a given dose delivery(Gy/MU), as measured by ionization chambers built into the treatment head.
• For superficial and orthovoltage beams, air kerma rate (in Gy/min) is another possible way to express output at a specific distance from the source.

Basic Output Calibration

• Calibration of photon and electron beams uses radiation dosimeters and specific dosimetry techniques.
• Radiation dosimetry aims to determine absorbed dose or other relevant physical quantities (such as air kerma, fluence, or equivalent dose).

Radiation Dosimeters

• A radiation dosimeter is a device that measures the radiation dose (D) given to a determined volume (V).
• Types of dosimeters include absolute and relative dosimeters.
• Absolute dosimeters, like a calorimeter, work without comparing to other dosimeters.
• Relative dosimeters, like thimble chambers or ion chambers, require calibration against a primary standard.

Ionization Chambers

• Ionization chambers are a key type of dosimeter for beam calibration.
• They are practical and widely used in radiotherapy to accurately measure machine output.
• Ionization chambers can serve as absolute or relative dosimeters.
• The sensitive volume of the chamber is typically filled with ambient air, and measurements are of charge (Q) and current (I) produced in this volume by radiation.
• Absorbed dose relationship uses the equation: D= , where Q is the measured charge and mair is the sensitive air mass.
• Wair/e represents mean energy required to produce ion pairs in air.
• Sensitive air volume or mass can be determined directly or indirectly (through calibration in known radiation fields).
• The value of Wair/e is assumed constant for the range of photon and electron energies in radiotherapy.

Types of Chambers

• Three types of chambers include: standard free air ionization chambers (for orthovoltage X-rays), cavity ionization chambers, and extrapolation chambers, each applied for different energies and beam types.

Clinical Beam Calibration

• Clinical photon and electron beams are calibrated with ionization chambers, typically as relative dosimeters.
• Calibration coefficients are determined in air or water, and must be traceable to national primary standards dosimetry laboratories(PSDL).
• Calibration coefficients essentially remove the need to precisely determine the chamber's sensitive volume.

Traceability

• Traceability of a calibration coefficient to a national PSDL indicates that the chamber calibration is done either directly at the PSDL (in terms of air kerma in air or absorbed dose in water) or through an accredited or secondary standards laboratory, where the calibration procedure traces back to a PSDL.

Types of Ionization Chambers

• Cylindrical (thimble) chambers, popular for beam directions independent measurements.
• Parallel-plate chambers, employed for superficial and low-energy electron beams.
• Other specialized chambers for specific applications, such as brachytherapy or high sensitivity measurements.

Characteristics

• Ionization chambers function as capacitors, where leakage current relates to radiation beam actions.
• Electric current created is measured by a sensitive electrometer.

Ionization Chamber Dosimetry

• Ion chambers are cavities filled with gas, surrounded by conductive walls, and have a central electrode.
• A high quality insulator keeps the chamber walls from contacting the collecting electrode.
• Guard electrodes in ionization chambers increase the field’s uniformity, reducing the leakage current. This improves charge collection

Film Dosimetry

• Radiographic X-ray film has applications in diagnostic radiology, radiation protection, quality control for radiotherapy machines and treatment technique verification.
• Quantitative dosimetry is also possible through this method, including measurements from electron beams

Luminescence Dosimetry

• Luminescence is the emission of light following energy absorption.
• Two types: Fluorescence and phosphorescence, distinguished by their time delay.
• Thermoluminescent dosimeters (TLDs) and Optically stimulated luminescence (OSL) are used for radiation measurement..

Phantom

• Phantoms (simulated patient models) substitute for actual patients in radiation interaction studies.
• They must closely resemble tissue absorption, scattering and electron density.
• Water-equivalent materials are often used for photon and electron dosimetry measurements, making corrections redundant.
• Phantoms need specific dimensions for effective calibration conditions, including depth of calibration, field margins and extended water around the measurement chamber for scattering.

Other Important Aspects (not all are detailed)

• Mention of specific dosimeter models (like the Farmer chamber in ionization chambers or the well type of ionization chambers used in brachytherapy.)
• There may be discussions of the details of the calibration procedures, including equipment, process steps and needed instruments.