Calibration Techniques

Calibration

verifies that the machine output accurately corresponds to the treatment plan.

Accurate dose delivery

minimizes unnecessary radiation to normal tissues

Overdose

increases toxicity and complications, while underdose may reduce tumor control

licensure and quality assurance

facilities must follow established calibration standards to maintain

clinically significant dose deviation

Even a 2–3% error repeated over 25–30 treatments can result in

International Measurement System (IMS) for Radiation Metrology

Provides a mechanism for consistency in radiation dosimetry by disseminating to users' calibrations of radiation instruments, which are traceable to primary standards.

Primary Standards Dosimetry Laboratory (PSDL)

a national standardizing laboratory designated for the purpose of developing, maintaining & improving primary standards in radiation dosimetry.

primary standard

is an instrument of highest metrological quality that permits determination of the unit of a quantity from its definition

Secondary Standards Dosimetry Laboratory (PSDL)

a national standardizing laboratory designated by competent authorities to provide calibration services which equipped with at least one secondary standards that has been calibrated against a primary standard

PNRI & DOHSSDL

are the highest accuracy in the metrological pyramid in the country

RA9236:National Metrology Act 2003

All measures & equipment shall be internationally traceable to the national metrology laboratory

Ionization Chamber

is the simplest gas-filled radiation detector, designed to measure the intensity of X-rays, gamma rays, and charged particles.

Ionization Chamber

It consists of electrodes in a gas-filled volume that collect ions produced by radiation, generating a small electric current proportional to the dose rate.

Ionization Chamber

They are essential for medical, industrial, and safety dosimetry

Free-Air

Thimble (Farmer)

Parallel-Plate

Well-Type

Types of Ionization Chambers (4)

Free-Air

Types of Ionization Chambers

Shape: Open-air

Common Use: Primary Standard

Clinical Setting: National labs

Thimble (Farmer)

Types of Ionization Chambers

Shape: Cylindrical

Common Use: LINAC Calibration

Clinical Setting: Radiotherapy

Parallel-Plate

Types of Ionization Chambers

Shape: Flat

Common Use: Electron Beams

Clinical Setting: Radiotherapy

Well-Type

Types of Ionization Chambers

Shape: Well-shaped

Common Use: Electron Beams

Clinical Setting: Radiotherapy

Air temperature

Pressure

____and____ affects the reading onion chamber during therapy beam calibration.

decreases

increases

Density or mass of air increase as temperature___, and as pressure _____

decreases

increases

exposure increases as temperature _____, and as pressure___.

22^oC

760mmHg

Ionization chambers were standardized to ___ temperature and ____ pressure during calibration.

ambient temperature

pressure

As such readings obtained from ion chambers must be corrected for _____ and ___ during therapy beam calibration.

273

^oC is converted to degrees Kelvin by adding ___ to Celsius temperature.

Roentgen (R)

is the unit of exposure.

Roentgen (R)

It is the measure of ionization produced in air by photons.

Gray(Gy)/rad

Energy absorption per unit mass

Roentgen (R)

It is a quantity of charge released during the ionization of air by passage of radiation under strictly defined conditions.

1 R

represents an amount of radiation that liberates 1.6 x 10¹² ion pairs per gram of air.

coulomb per kilogram (C/kg)

SI unit for exposure is

orbital electrons

Absorption; primary electrons

The deposits of energy in matter occur in two steps:

• Transfer of photon energy to ___.

• ___, by the medium of energy released by the _____ as they ionize and excite atoms along their paths

absorbed dose (Gy)

In therapy, we focus on

f_tissue

is related to mass energy absorption coefficient of the medium relative to air

Medium Composition

Photon energy

f_tissue is related to mass energy absorption coefficient of the medium relative to air which is affected by : (2)

American Association of Physicists in Medicine

AAPM

1. Select desired beam energy.

2.Set SSD to 100 cm, and the depth of the phantom appropriate to the beam energy. Example, the depth 5 cm for 6 MV beam

3.Set field size to 10x10 cm2 at the surface

4.Employ chamber (calibrated) without the buid up cap

5.Position ion chamber with its center in the central axis of the beam at 5 cm

6.Take a series of electrometer reading for several exposures with 100 MU set on the controls and average the readings.

Steps in Calibrating Megavoltage Beam (Based on AAPM Protocol)

Ionization Chamber

provide stable, reproducible, and traceable radiation measurements. They are the standard instrument for absolute dose calibration in radiotherapy

Roentgen(R)

a measure of how much you are exposed

rad or Gray(Gy)

how much you absorb

rem or Sievert (Sv)

how much damage it does

Water

closely mimics human soft tissue in radiation absorption and scattering.

water phantom

Calibrating in a _____ allows us to measure absorbed dose to tissue-equivalent material, making it clinically relevant for patient treatment.