Radiation Therapy: Tissue Phantom Ratio and PDD

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Questions and Answers

What is the primary advantage of using TPR over PDD in megavoltage relative dosimetry?

TPR is measured at the phantom surface

TPR is only used for isocentric treatments

TPR is dependent on SSD

TPR avoids dependence on SSD (correct)

Why do PDDs become inconvenient for isocentric treatments?

Due to the dependence on SSD (correct)

Due to the decrease in SSD

Due to the increase in SSD

Due to the increase in beam energy

What is the purpose of combining central axis depth dose with off-axis data?

To determine the SSD

To calculate TPR

To measure beam profiles

To deliver an accurate dose description inside the patient (correct)

What is the definition of Off-axis Ratio (OAR)?

The ratio of dose at an off-axis point to the dose on the beam central axis Signup and view all the answers

What is the effect of increasing field size on the radiation output of a linac?

It increases due to an increase in scattered radiation Signup and view all the answers

What is the Tissue Maximum Ratio (TMR) referred to?

When zref is equal to zmax Signup and view all the answers

What is the purpose of measuring beam profiles?

To provide 2-D and 3-D information on the dose distribution in the phantom/patient Signup and view all the answers

What is the relationship between PDDs and TPRs/TMRs?

PDDs and TPRs/TMRs are related and can be calculated using each other Signup and view all the answers

What is the purpose of measuring output factors under standard conditions?

To accurately estimate dose in patient output Signup and view all the answers

What type of beams are typically used to treat deep-seated tumors?

A combination of two or more megavoltage photon beams Signup and view all the answers

What is the primary function of waterproof ionisation chambers in scanning water phantoms?

To scan the beam in real time Signup and view all the answers

What type of radiotherapy can use as many as 12 beams with small field sizes?

Stereotactic Ablative Body Radiotherapy (SABR/SBRT) Signup and view all the answers

What is obtained from measuring relative dose data in scanning water phantoms?

PDDs, TPR/TMRs, beam profiles, output factors, and isodoses Signup and view all the answers

What type of treatment involves the rotation of the radiation beams around the patient?

Intensity Modulated Arc Therapy (IMAT/VMAT) Signup and view all the answers

What is the purpose of using multiple radiation beams in external beam radiotherapy?

To achieve a uniform dose distribution inside the target volume and to minimize the dose in healthy tissues Signup and view all the answers

What is the effect of increasing the field size on the percentage depth dose?

It increases the percentage depth dose due to increased scatter contribution Signup and view all the answers

What is the effect of increasing the source-to-surface distance on the percentage depth dose?

It decreases the percentage depth dose due to increased inverse square factor Signup and view all the answers

What is the depth of dose maximum dependent on?

Only the beam energy Signup and view all the answers

What is the tissue phantom ratio related to?

The dose at arbitrary points inside the patient Signup and view all the answers

What is the purpose of calibrating the linac under reference conditions?

To link the dose at the reference point to the dose at arbitrary points Signup and view all the answers

What is the effect of increasing the beam energy on the surface dose?

It decreases the surface dose Signup and view all the answers

What is the central axis percentage depth dose dependent on?

The field size, source-to-surface distance, and photon beam energy Signup and view all the answers

What is the purpose of using a water phantom in calibration?

To simulate the human body for calibration purposes Signup and view all the answers

What is the effect of increasing the source-to-surface distance on the beam attenuation?

It decreases the beam attenuation Signup and view all the answers

Study Notes

Beam Penetrating Power

PDD (Percentage Depth Dose) represents the dose distribution within a medium as a function of the field size.
SSD (Source-to-Skin Distance) affects PDDs, making them less convenient in isocentric treatments.

Tissue Phantom Ratio (TPR)

TPR is crucial for megavoltage relative dosimetry and is defined as TPR(z, AQ, h) = DQ / DQref.
TPR is independent of SSD, providing a reliable measurement even when SSD varies.

Measurement of TPR

Measurement point Q is fixed at a constant distance from the source for accurate readings of DQ and DQref.
TPR is influenced by field size at the SAD (Source Axis Distance), not at the phantom surface.
If zref equals zmax, TPR is termed Tissue Maximum Ratio (TMR).

TPR/TMR Relationship

TPRs/TMRs are functionally similar to PDDs but do not depend on SSD.
One can relate TMR to PDD using mathematical formula for accurate dose distribution.

Off-axis Ratios and Beam Profiles

OAR (Off-axis Ratio) measures dose at an off-axis point relative to the dose on the central axis at the same depth.
Combining central axis depth dose with off-axis data produces a 2-D and 3-D dose distribution matrix.

Isodose Distributions in Water Phantoms

Isodose curves represent varied doses at fixed intervals, depicting dose distribution from a single beam.

Output Factors

Linac radiation output increases with field size due to increased scattered radiation.
Accurate dose estimation requires measured output factors under standard conditions.

Measurement of Relative Dose Data

Waterproof ionisation chambers enable real-time beam scanning for PDDs, TPR/TMRs, and dose profiles.
Collected data is integrated into Treatment Planning Systems like Philips Pinnacle3.

Isodose Distributions in Patients

Single photon beam treatments are primarily for superficial tumors.
Deep-seated tumors often require multiple megavoltage photon beams for effective treatment.

Beam Production

Main sources include X-ray tubes, Linear Accelerators (linacs), and rarely Co-60 machines.
Linacs provide the most common clinically available radiation source for EBRT.

Radiation Beam Field Size

Available field sizes include square, rectangular, circular shapes, and irregular fields created by multi-leaf collimators.
Equivalent square or circular field size can be determined from arbitrary field shapes for dosimetry purposes.

Relative Dose Functions

EBRT relies on linacs calibrated under specific conditions: 100cm SSD, 10cm depth, and 10x10cm² field size.
Relative dose functions such as PDD and TPR link reference dose to doses at arbitrary points within patients.

Central Axis Percentage Depth Doses in Water

PDD distributions are normalized to Dmax (100%) at the depth of maximum dose.
Key factors affecting PDD include depth, field size, SSD, and photon beam energy.

PDD Characteristics

PDD first increases to zmax and then decreases with depth increase, influenced by beam energy.
Higher beam energy results in greater depth of dose maximum and lower surface doses.
Increasing field size enhances PDD due to increased scatter contributions at central axis points.

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Description

This quiz covers the concepts of Tissue Phantom Ratio (TPR) and Percentage Depth Dose (PDD) in radiation therapy, including their relationships with field size and source-to-surface distance (SSD).