TRS 398 - Absorbed Dose Measurement for Photon

Summary

This video script explains the process of measuring absorbed dose for high-energy photon and electron beams, emphasizing the TRS 398 protocol. It covers essential steps such as calibration using Cobalt-60, applying correction factors (polarity, pressure-temperature, recombination), and calculating dose to water at reference depths. The script details how to convert ionization chamber readings to dose, considering beam quality and depth dose parameters. The process is vital for accurate radiation therapy treatment planning, ensuring precise dose delivery to patients using high-energy beams.

Takeaways

• 😀 Accurate absorbed dose measurement requires using a water phantom and calibrated ionization chamber.
• 😀 The ionization chamber must be calibrated to a primary standards dosimetry laboratory (PSDL) reference.
• 😀 Measurements must be performed under reference conditions, including proper chamber positioning and SSD (source-to-surface distance) settings.
• 😀 The depth of measurement is typically 10 cm for high-energy photon beams, with a reference depth for Cobalt-60 of 5 cm.
• 😀 Temperature and pressure corrections are essential for accurate dose readings, requiring real-time monitoring during the procedure.
• 😀 Polarity effects need to be corrected by applying a polarity correction factor (K) based on voltage measurements (±300V).
• 😀 Ion recombination corrections are required when using pulsed beams, with adjustments based on reduced voltage values.
• 😀 The correction factors for pressure, temperature, polarity, and ion recombination must be applied systematically to ensure accurate meter readings.
• 😀 After applying corrections, the corrected meter reading is used to calculate the dose at the reference depth (10 cm for high-energy photons).
• 😀 Calibration factors for cobalt-60 beams should be adjusted using KQ values specific to photon energy, ensuring the readings are applicable to the beam type being measured.
• 😀 The final dose calculation may include additional adjustments such as the percentage depth dose for 6 MV or tissue maximum ratio (TMR) when using SAD setups.

Q & A

What is the significance of the ionization chamber in measuring absorbed dose for high-energy photon and electron beams?

The ionization chamber is a critical tool for measuring the amount of ionization produced by radiation in a water phantom. It provides a direct measurement of the radiation dose, which is then corrected for various factors like temperature, pressure, polarity, and recombination. For high-energy photon and electron beams, the ionization chamber's calibration must be traceable to a primary standard dosimetry laboratory (PSDL).

Why is the water phantom used in the measurement of absorbed dose for radiation therapy?

The water phantom is used because it simulates the human body's tissue properties for radiation dose measurements. It provides a homogeneous medium that mimics the dose distribution of radiation in human tissues, allowing for accurate dosimetric calculations for both photon and electron beams.

What are the reference conditions for measuring absorbed dose with high-energy photon beams?

For high-energy photon beams (like 6 MV), the reference conditions for dose measurement typically involve using a cylindrical ionization chamber in a water phantom at a depth of 10 cm. The source-surface distance (SSD) is typically set at 100 cm, and the field size should be 10 cm x 10 cm.

How does the beam quality index (TPR) influence absorbed dose measurements for high-energy photon beams?

The beam quality index, such as Tissue Phantom Ratio (TPR), is important because it accounts for differences in the energy of the radiation beam. Variations in the energy of high-energy photon beams can affect the dose distribution, so using TPR values at depths like 10 cm and 20 cm helps ensure accurate dose measurements by accounting for energy attenuation in the tissue.

What correction factors are applied to the ionization chamber measurements, and why are they necessary?

Several correction factors are applied to ionization chamber measurements, including polarity correction, recombination correction, temperature and pressure corrections, and electrometer calibration. These are necessary because various factors like temperature, pressure, and chamber characteristics can influence the raw reading, and applying these corrections ensures that the measured dose is accurate and representative of the true dose delivered to the tissue.

What role does the KQ factor play in absorbed dose measurements for high-energy photon beams?

The KQ factor is a calibration factor that corrects for differences in beam quality between Cobalt-60 and high-energy photon beams. Since ionization chambers are calibrated using Cobalt-60, the KQ factor is applied to adjust the readings to the energy-specific conditions of the photon beam, ensuring accurate dosimetry for high-energy photon beams.

How is the recombination correction factor determined, and what is its significance?

The recombination correction factor is determined by taking measurements at different voltages (e.g., +300V and +100V) and using specific equations based on pulse beam characteristics. This correction accounts for the phenomenon where ion pairs recombine before being collected by the ionization chamber, especially in pulsed beams. It ensures that the measured dose reflects the true ionization produced by the radiation.

Why is the temperature and pressure correction applied to the ionization chamber reading?

The temperature and pressure correction is applied because the ionization chamber's response is affected by changes in environmental conditions. Variations in temperature and pressure can alter the volume and density of the gas inside the chamber, impacting the accuracy of the dose measurement. By correcting for these factors, the measurement is standardized to reference conditions.

What is the process for calculating the absorbed dose at the reference depth after correcting the ionization chamber reading?

To calculate the absorbed dose at the reference depth, you first correct the raw ionization chamber reading by applying factors like polarity, recombination, temperature, pressure, and electrometer calibration. After this, you use the calibration factor (NDW) for Cobalt-60 to convert the corrected reading into dose units. For high-energy photon beams, a further adjustment using the KQ factor may be required. Finally, for treatment planning systems, the dose may be adjusted for the percentage depth dose or tissue maximum ratio, depending on the setup.

How are absorbed dose measurements adjusted for high-energy electron beams?

For high-energy electron beams, the absorbed dose is measured similarly to photon beams but with adjustments for electron-specific factors like electron range and dose fall-off. The ionization chamber is typically calibrated for Cobalt-60, and the KQ factor is used for energy-specific corrections. The depth dose may also be adjusted using the electron-specific percentage depth dose or tissue maximum ratio.