Radiation Therapy: Tissue Phantom Ratio and PDD

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.