AAPM TG-51 Protocol (Med Phys 26:1847-1870, 1999) A protocol for clinical reference dosimetry of high-energy photon and electron beams 亞東紀念醫院 放射腫瘤科 蕭安成

For clinical reference dosimetry of external beam radiation therapy AAPM TG-51 Protocol For clinical reference dosimetry of external beam radiation therapy photon beams with nominal energies between 60Co and 50 MV electron beams with nominal energies between 4 and 50 MeV. The protocol uses ion chambers with absorbed-dose-to-water calibration factors, traceable to national primary standards absorbed dose to water The formalism is simpler than the TG-21, but it is applicable in water only.

For photon beams, with beam of quality Q, General Formalism For photon beams, with beam of quality Q, M is corrected For 60Co, kQ = 1, only cylindrical chamber allowed at present Converts beam quality (energy) from Co-60 to Q.

For electron beams, with beam of quality Q (R50), General Formalism For electron beams, with beam of quality Q (R50), , only for cylindrical chambers, to correct for gradient effects. , photon-electron conversion factor, chamber dependent, , Converts electron energy from ecal to R50

Obtaining An Absorbed-dose To Water Calibration Factor 60Co source d = 5 cm D d = 5 cm M (corrected)  ( Gy/C or Gy/rdg ), traceable to national primary standards

Obtaining An Absorbed-dose To Water Calibration Factor

Obtaining An Absorbed-dose To Water Calibration Factor

Obtaining An Absorbed-dose To Water Calibration Factor Chamber waterproofing equivalent waterproofing techniques must be used for measurements in the user’s beam and in the calibration laboratory. inherently waterproof For nonwaterproof chambers a waterproofing sleeve which minimizes air gaps near the chamber wall (< 0.2 mm) and it should be made of polymethylmethacrylate (PMMA) < 1 mm thick.

Measurement Phantoms Charge Measurement Water phantom with dimensions of at least 303030 cm3. Charge Measurement fully corrected charge reading, M M = PionPTPPelecPpolMraw (C or rdg), Polarity corrections , should be  0.997 or 1.003

Electrometer correction factor It is common practice in the US to calibrate ion chambers and electrometers separately. If the electrometer is calibrated separately from the ion chamber, Pelec , corrects the electrometer reading to true coulombs. Pelec = 1.00 if the electrometer and ion chamber are calibrated as a unit.

Corrections for ion-chamber collection inefficiency Corrected for lack of complete collection efficiency. Pion , must be  1.05 Voltages should not be increased above normal operating voltages just to reduce Pion For continuous (i.e., 60Co) For pulsed or pulsed-swept beams (i.e., Linac)

Beam Quality Specification For cylindrical and spherical chambers the shift is taken 0.6rcav for photon beams 0.5rcav for electron beams rcav : radius of the air cavity in a cylindrical ion chamber.

Point of Measurement and Effective Point of Measurement  r point of measurement rcav cylindrical parallel plate Photon: r = 0.6 rcav electron: r = 0.5 rcav

Beam Quality Specification For photon beams the depth-ionization curve is treated as a depth-dose curve For electron beams, the depth-ionization curve must be further corrected for the significant change in the stopping-power ratio with depth to determine depth-dose curves

Beam Quality Specification For photon beams, curve I : raw data, curve II: shifted data. For electron beams, curve I : raw data, curve II: shifted data. curve II must be further corrected to obtain the percentage depth-dose curve.

Beam Quality Specification For measurements of absolute dose at the reference depth in both electron and photon beams, a cylindrical chamber’s point of measurement (center of the chamber) The gradient effects are included implicitly in the beam quality conversion factor kQ for photons and explicitly by the term for electrons.

Beam-quality specification for photon beams %dd(10)x : PDD at 10 cm depth in water, FS = 1010 cm2, SSD = 100 cm, includes photon component only. %dd(10) : PDD at 10 cm depth in water, FS = 1010 cm2, SSD = 100 cm, measured photon beam includes the effects of electron contamination in the beam. %dd(10)pb : Same as %dd(10) except that a 1 mm lead foil is in place below the accelerator at about 50 cm from the phantom surface ( or 30 cm if 50 cm clearance is not available)

Beam-quality specification for photon beams 1010 cm2 100 cm d =10 cm dmax %dd(10) 1010 cm2 100 cm d =10 cm dmax %dd(10)pb 1 mm pb 50 cm %dd(10)x = %dd(10), E < 10MV %dd(10)x = [0.8905+0.00150%dd(10)pb] %dd(10)pb, E ≥ 10MV, foil at 50 cm

Beam quality specification for electron beams Beam quality in electron beams is specified by R50 Field size on the phantom surface 1010 cm2 (  2020 cm2 for R50  8.5 cm, i.e., E  20 MeV) . (cm) 2  I50  10 cm I50  10 cm I50 , ionization curve falls to 50% of its maximum

Either an SSD or an SAD setup can be used Photon Beam Dosimetry Reference conditions 1010 cm2 100 cm d =10 cm SAD setup 1010 cm2 100 cm d =10 cm SSD setup Either an SSD or an SAD setup can be used

Photon Beam Dosimetry Absorbed dose to water in clinical photon beams

Photon Beam Dosimetry Absorbed dose to water in clinical photon beams

Electron Beam Dosimetry Reference conditions open beam reference depth dref = 0.6R50 - 0.1 (cm) Field size ≥ 1010 cm2, R50  8.5 cm ( E  20 MeV ) ≥ 2020 cm2, higher-energy FS SSD=100 cm dref SSD setup

Electron Beam Dosimetry Absorbed dose to water in clinical electron beams

Electron Beam Dosimetry Absorbed dose to water in clinical electron beams

Electron Beam Dosimetry Absorbed dose to water in clinical electron beams

Electron Beam Dosimetry Absorbed dose to water in clinical electron beams For Farmer-like cylindrical chambers, 2  R50  9 cm . error  0.2%:

Electron Beam Dosimetry Absorbed dose to water in clinical electron beams For well-guarded plane-parallel chambers, 2  R50  20 cm .

Electron Beam Dosimetry Absorbed dose to water in clinical electron beams gradient effects, , for plane-parallel chambers (for cylindrical chambers)

Electron Beam Dosimetry Absorbed dose to water in clinical electron beams Use of plane-parallel chambers SSD SSD FS FS dref dref MCyl Mpp

Summary - photons get a traceable measure %dd(10)Pb with lead foil (shift depth if necessary) deduce %dd(10)x for open beam from %dd(10)Pb measure Mraw at 10 cm depth in water (no depth shift !!!) M = PionPTPPelecPpol Mraw lookup kQ for your chamber

Summary - electrons get a traceable measure I50 to give R50 (shift depth if necessary) deduce dref = 0.6 R50 -0.1 cm (approx. at dmax) measure Mraw at dref (no depth shift !!!) M = PionPTPPelecPpol Mraw lookup kecal for your chamber determine (fig, formula) establish (Mraw 2 depths)