Radiation Oncology Dosimetry Quiz

Introduction

• Accurate dose delivery for external photon or electron beams relies on a chain of steps, including beam output calibration, dose measurement, equipment commissioning, quality assurance, treatment planning, and patient setup.

Basic Output

• Clinical beam output is usually described as a dose rate (Gy/min or Gy/MU) at a reference depth (often the depth of maximum dose, Z_max).
• Measurements are taken in a water phantom at a standard source-to-surface distance (SSD) or source-to-axis distance (SAD).
• Field size is commonly 10x10 cm².

Machine Basic Output

• Kilovoltage x-ray generators and teletherapy units express output in Gy/min.
• Linear accelerators use Gy/MU (monitor units), a measure of accelerator output for radiation therapy. Monitor units are measured by ionization chambers, which are built into the treatment head.
• Superficial and orthovoltage beams (and sometimes teletherapy beams) may use air kerma rate (Gy/min) at a specific distance from the source.

Basic Output Calibration

• Calibration of photon and electron beams is done using radiation dosimeters and specific dosimetry techniques.
• Dosimetry is the measurement and/or calculation of absorbed dose or other relevant quantities (e.g., air kerma, fluence, equivalent dose).

Radiation Dosimeter

• A radiation dosimeter is a device that measures the dose (D) deposited in its sensitive volume (V) by ionizing radiation.
• Dosimeters are categorized into absolute and relative/secondary types.
• An absolute dosimeter determines dose from first principles, independent of other dosimeters. Calorimeters are an example.
• Relative/secondary dosimeters require calibration against a primary standard. Examples include thimble chambers, plane-parallel ion chambers, TLDs, diodes, and films.
• Calibration often involves a calibration ion chamber, and corrections for energy dependence and other conditions are often needed.

Ionization Chamber Based Dosimetry

• Two types of ionization chambers are used for beam calibration: cylindrical (thimble) chambers for orthovoltage and megavoltage x-rays, and electron beams with energies of 10 MeV and above, and parallel-plate (end window or plane-parallel) chambers for superficial x-rays, electron beams with energies below 10 MeV, and photon beams in the buildup region and surface dose.

Reference Dosimetry

• Ionization chambers are practical and widely used in radiotherapy for accurate machine output measurement.

• They can be used as absolute or relative dosimeters.

• The sensitive volume is filled with ambient air, and measured quantities include charge (Q) for dose and current (I) for dose rate.

• Measured charge (Q) and sensitive air mass (m_air) are related to absorbed dose in air (D_air) by a formula involving W_air/e, the mean energy to produce an ion pair in air per unit charge.

• Sensitive air volume/mass in an ion chamber can be determined directly or indirectly via calibration against a known radiation field.

• The value of W_air/e is usually assumed constant for the entire photon and electron energy range in radiotherapy, but experimental support is limited to Co-60, Cs-137 gamma ray beams, and 2 MV x-ray beams.

• Various ionization chambers are used depending on the needed energies, including standard free-air chambers, cavity ionization chambers, and extrapolation chambers.

Clinical Beam Calibration

• Clinical photon and electron beams are usually calibrated with ionization chambers, which are often used as relative dosimeters.
• Calibration coefficients are determined in air or water and traceable to a national primary standard dosimetry laboratory (PSDL).

Traceability in Calibration

• Calibration traceability to a national PSDL indicates either direct calibration at a PSDL or calibration traced to a PSDL via an accredited dosimetry calibration laboratory (ADCL) or a secondary standards dosimetry laboratory (SSDL).
• A cross-calibration with another ionization chamber can also establish traceability.

Ionization Chamber Types

• Cylindrical (thimble) chambers are commonly used for relative dosimetry. The volume of these chambers typically ranges from 0.05-1.00 cm³, and they have a radius of ~27 mm, length of ~4-25 mm, and thin walls (~0.1 g/cm²).
• Parallel-plate chambers are used to measure electron beams below 10 MeV, depth dose, and build-up region of megavoltage photon beams.

Other Ionization Chambers

• Brachytherapy chambers, often well-type, are characterized by high sensitivity and large volumes (~250 cm³).
• Extrapolation chambers are parallel-plate chambers with a variable electrode separation, useful in absolute radiation dosimetry when embedded in a tissue-equivalent phantom, and for calibrated absorbed dose measurements.

Film Dosimetry

• Radiographic X-ray film is used in diagnostic radiology, radiation protection, and radiotherapy for qualitative and quantitative dose measurements, including electron beam dosimetry.
• Film dosimetry also plays a role in controlling radiotherapy machines and verifying treatment techniques applied to various phantoms.

Luminescence Dosimetry

• Luminescence refers to the process where energy is converted into visible/ultraviolet/infrared light.
• Two main types are fluorescence (10-¹⁰ to 10-⁸ seconds delay) and phosphorescence (delay exceeds 10-⁸ seconds).
• Thermoluminescent dosimeters (TLDs) are based on the use of heat, while optically stimulated luminescence dosimeters (OSLs) use light.
• Common TLDs include LiF:Mg,Ti, LiF:Mg,Cu,P, and Li2B4O7:Mn, while other high sensitivity TLDs include CaSO4:Dy, Al2O3:C, and CaF2:Mn.

Optically Stimulated Luminescence (OSL) Systems

• OSL systems utilize light and release trapped energy as luminescence. These systems are used in in vivo dosimetry.
• To achieve OSL, the chip is exposed to laser light via an optical fiber, the emitted luminescence is collected by the same fiber and measured, and OSL can be performed by using pulsed lasers (POSL).
• Promising materials for OSL are often Al2O3:C.

Calorimeters and Semiconductor Devices

• Calorimeters are absolute dosimeters that measure temperature changes based on absorbed radiation.
• Semiconductor dosimeters are used to measure radiation doses based on the ionization and interaction of radiation.

Phantoms

• Phantoms are materials that replace patients in radiation studies.
• Ideal phantom materials absorb and scatter radiation in the same way as tissue, have the same density, have the same electron count per gram as tissue, and have the same effective atomic number as tissue. Water is a common phantom material for photon and electron beams.
• Water equivalency for photon beams means the mass-energy absorption coefficient, mass stopping power and mass scattering power match.
• For electron beams, water equivalency implies a linear stopping power and linear scattering power match.
• Water is particularly suitable for megavoltage calibrations, though there may be specific testing depths for various beam types.
• Phantom construction should include a margin surrounding the nominal field size (at least 5 cm of water) and sufficient water (at least 10 cm) beyond the chamber for proper scattering conditions.