Absolute Dose Measurement

TG-51 : Photons DQ,water is the absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions for a clinical beam with beam quality Q.

TG-51 : Photons D,wNCo is the absorbed-dose-to-water calibration factor traceable to national primary standards for the ion chamber used.

Photon Beam Quality (Q) The beam quality Q is specified by the %dd(10)x. %dd – Percent Depth Dose (10)x – Photon component at a 10-cm depth for a field size of 10x10 cm2 on the surface of a phantom at SSD of 100 cm. Photon energies < 10 MeV, %dd(10)x is the fractional depth dose at 10 cm depth, %dd(10). Depth ionization curve can be treated as depth dose. Be sure to shift the depth dose upstream by 0.6rcav. rcav is the radius of the air cavity of the ion chamber.

Photon Beam Quality (Q)

Photon Beam Quality (Q) Photon energies > 10 MeV, %dd(10)x is obtained by taking depth-dose measurements using a 1-mm lead foil placed 30 or 50 cm from the phantom surface and applying appropriate equation in the protocol. %dd(10)x = [0.8905 + 0.00150 . %dd(10)Pb] . %dd(10)Pb Foil at 50 cm, %dd(10)Pb ≥ 73% %dd(10)x = [0.8116 + 0.00264 . %dd(10)Pb] . %dd(10)Pb Foil at 50 cm, %dd(10)Pb ≥ 71% If %dd(10)Pb is less than thresholds, %dd(10)x = %dd(10)Pb

Photon Beam Quality (Q) No lead foil available? %dd(10)x = 1.267%dd(10) – 20.0 For 75% < %dd(10) ≤ 89% %dd(10)x = %dd(10) For %dd(10) ≤ 75% Not recommended, but acceptable if unavoidable.

Ion Chamber Reading (M) Point of measurement – the point at which absorbed dose is measured. Cylindrical ion chambers – on the central axis of the cavity at the center of the active volume of the cavity. Plane-parallel chamber – at the front (upstream side) of the air cavity at the center of the collecting region. Mraw is the uncorrected ion chamber reading with the point of measurement at a depth d in water, for a given number of monitor units.

Ion Chamber Reading (M) M : the fully corrected ion chamber reading. M = Pion . PTP . Pelec . PPol . Mraw Pion : recombination correction factor PTP : temperature/pressure correction factor Pelec : electrometer correction factor Calibrated at ADCL for a specific scale PPol : polarity correction factor

Recombination (Pion) Pion : recombination correction factor Pion is dependent on chamber design, pulse rate, and bias voltage. Usually pulse rate is set for a particular dose rate, but measurements should be redone if pulse rate is changed for that dose rate. Ion chambers with Pion > 1.05 should not be used because the uncertainty in the Pion value. Bias voltage should not be above 300 V, again because theory starts to breaks down.

Recombination (Pion) For continuous beams (i.e., 60Co) For pulsed or scanned beams

Temperature/Pressure (PTP) Standard conditions Temperature: 22° C Pressure 101.33 kPa (760 mmHg) Wait 5-10 minutes after readings stabilize to assume equilibrium with environment. Humidity in range of 20% to 80% not a factor.

Polarity (PPol) Polarity effects vary with beam quality and other conditions such as cable position. Measured with reference voltage of opposing bias.

Quality Conversion Factor (kQ) kQ is the quality conversion factor which converts the calibration factor for a 60Co beam to that for a photon beam of quality Q. kQ has been determined for numerous commercial ion chambers and varies with beam quality. kQ = 1.000 for 60Co by definition. kQ have not been determined for plane-parallel chambers because of insufficient information about wall correction factors in photon beams other than 60Co.

Quality Conversion Factor (kQ)

Quality Conversion Factor (kQ)

TG-51 Electrons M – raw measured ionization reading kQ – beam quality conversion factor D,wNCo60 – absorbed-dose to water calibration factor

Beam Quality Conversion Factor grPQ – gradient correction factor – the component of kQ that is dependent on the ionization gradient at the POM. k50 – component of kQ in an electron beam that is independent of the ionization gradient at the POM (point of measurement). R50k' – electron quality conversion factor kecal – photon-electron conversion factor

Depth of 50% dose (R50) First measure ionization curve Remember to shift depth dose upstream by 0.5rcav when using a cylindrical ion chamber. Locate depth of 50% ionization (I50)

Detectors for TG-51 Electrons R50 ≤ 2.6 cm (6 MeV or less) Must use plane-parallel chamber R50 ≤ 4.3 cm (10 MeV or less) Plane-parallel chamber is preferred R50 > 4.3 cm Cylindrical or plane-parallel chamber can be used

Reference Condition Reference depth dref = 0.6R50 – 0.1 cm POM of chamber is placed at dref SSD = 100 cm For R50 ≤ 8.5 cm, use a 10x10-cm2 field For R50 > 8.5 cm, use a 20x20-cm2 field

Gradient Correction Factor grPQ grPQ = 1.00 for plane-parallel chambers Close to unity for cylindrical chambers in an electron beam of 10 MeV or less. For cylindrical chambers:

R50k' – plane parallel chambers Figure 6 and 8 gives r50k' as a function of R50 for various plane-parallel chambers. Figure 6. Low-energy electron beams Figure 8. High-energy electron beams

R50k' – plane parallel chambers Analytical expression of curves in Figs. 6 and 8.

R50k' – cylindrical chambers Figure 7 plots r50k' vs. R50 for various cylindrical chambers.

R50k' – cylindrical chambers Analytical expression of curves in Figs. 6 and 8.

Photon-electron conversion factor kecal determined for commercial chambers using beam quality R50 = 7.5 cm. Cylindrical chambers Table III lists kecal value for cylindrical chambers. Plane-parallel chambers Table II lists kecal value for plane-parallel chambers It is recommended to cross-calibrate plane-parallel chambers with cylindrical chambers in a high-energy electron beam using equation 21 (item 8 on worksheet).

Absorbed dose at dmax If dref does not occur at dmax on the depth dose curve, the reference output dose must be corrected to dmax. Will happen for high-energy beams. Example: dref = 95%, D(dmax) = D(dref)/0.95 Use the depth-dose curve, not the depth-ionization curve. Will require TG-25 protocol to determine stopping power ratios and Prepl as a function of depth if measured with ion chamber. Can be measured directly with diode detector.