Question

explaine the theory of the procedures that was used for reference dosimetry protocol TG 51 ?

Answer 1

Protocol for Clinical Reference Dosimetry of Protocol for Clinical Reference Dosimetry of High-Energy Photon and Electron Beams. Energy Photon and Electron Beams

Based on an Absorbed Dose to Water (in Water) Standard (60Co):

• Conversion to Absorbed dose at other energies and modalities (electrons) is by correction factors, kecal, k’R50, and Pgr (electrons).

• Based on Bragg Gray cavity theory.

• All of the chamber correction factors introduced in TG-21 are incorporated into Kecal, and k’R50.

• Many chamber specific corrections are averaged into “classes” of chambers, so only a few k’R50 values are needed.

Requires Calibration in Water ( at least annually): ¾Plastics may be used for monthly output checks, but must be referenced to the water calibration by a correction factor.

• Electrons Energy characterized by depth of 50% of max dose, d50 Reference measurement depth, dref,is related to d50.

Chamber Calibration Factor Chamber Calibration Factor

• Obtain from ADCL

• Chamber waterproofing material: – 1 mm Acrylic (PMMA) wall – Provided by ADCL – Waterproof chambers

Calibrate P-P Chamber -P Chamber P Chamber

• ADCL’s: – TG-39, in water – 60Co – Evidence of problems with kecal

• User: (Recommended) – TG-39 in water/plastic – Cylindrical Chamber is NIST traceable – TG-51 In water – high energy electron R50 near 7.5 cm – compare with cylindrical chamber.

TG-51 Measurements

• Look up kecal for your chamber.

• Search for Imax and I50 (use 0.5 rcav shift)

• Determine R50

• Determine dref and k’R50 • Move chamber to physical dref (no shift)

• Measure Ppol and Pion • Move chamber to dref + 0.5 rcav

• Calculate the gradient correction, Pgr

Protocol for Clinical Reference Dosimetry of Protocol for Clinical Reference Dosimetry of High-Energy Photon and Electron Beams. Energy Photon and Electron Beams

Based on an Absorbed Dose to Water (in Water) Standard (60Co):

• Conversion to Absorbed dose at other energies and modalities (electrons) is by correction factors, kecal, k’R50, and Pgr (electrons).

• Based on Bragg Gray cavity theory.

• All of the chamber correction factors introduced in TG-21 are incorporated into Kecal, and k’R50.

• Many chamber specific corrections are averaged into “classes” of chambers, so only a few k’R50 values are needed.

Requires Calibration in Water ( at least annually): ¾Plastics may be used for monthly output checks, but must be referenced to the water calibration by a correction factor.

• Electrons Energy characterized by depth of 50% of max dose, d50 Reference measurement depth, dref,is related to d50.

Chamber Calibration Factor Chamber Calibration Factor

• Obtain from ADCL

• Chamber waterproofing material: – 1 mm Acrylic (PMMA) wall – Provided by ADCL – Waterproof chambers

Calibrate P-P Chamber -P Chamber P Chamber

• ADCL’s: – TG-39, in water – 60Co – Evidence of problems with kecal

• User: (Recommended) – TG-39 in water/plastic – Cylindrical Chamber is NIST traceable – TG-51 In water – high energy electron R50 near 7.5 cm – compare with cylindrical chamber.

TG-51 Measurements

• Look up kecal for your chamber.

• Search for Imax and I50 (use 0.5 rcav shift)

• Determine R50

• Determine dref and k’R50 • Move chamber to physical dref (no shift)

• Measure Ppol and Pion • Move chamber to dref + 0.5 rcav

• Calculate the gradient correction, Pgr