·
Description of RPC’s Measurement and Calculation Procedures
a.
General
b.
Photon
c.
Electron
e.
Definitions of
Parameters in TG-51
f.
References
· Measured Parameters and Acceptance Criteria
·
Records
needed prior to a visit
RPC Equipment for photon and electron beams
The dosimeter used to measure external beams is a 0.6 cc Farmer-type ionization chamber, the charge is integrated in a modified Keithley model 602 electrometer. The collecting potential to the chamber is usually 300 V, and occasionally 600 V for special situations such as scanned beams. Measurements are made at two potentials in order to determine the ion chamber collection efficiency correction (Pion). The absorbed dose to water calibration factors for the chambers are obtained from the ADCL and verified against an intercomparison with a transfer quality chamber whose factor traceable to NIST. The RPC correction factor (coulomb/rdg) for the electrometer is obtained from the M.D. Anderson ADCL and verified against a precision high voltage power supply and a reference standard capacitor.
The institution is customarily requested to perform a spot check of the machine output immediately prior to the RPC's measurements. A correction can then be applied to the RPC measured dose rates to account for minor variations from the nominal clinical dose rates. Thus, the RPC/Institution ratios stated in an RPC Report are intended to be representative of the institution's system and not reflect a small output aberration that may have existed on the day of the visit.
Monitor end effect, Î, (also known as timer or shutter error) is determined by the RPC using the technique described by Orton and Seibert (Refs. 1 and 5). A positive timer error indicates that net irradiation time is greater than that set on the machine. It should be noted that this method gives a sign opposite to that obtained when end effect is measured by the linear regression and extrapolation method.
Congruence of light and radiation fields is determined by irradiating a film (usually Kodak X-omat V) to approximately 0.5 Gy in a plane perpendicular to the central axis of the beam under conditions of adequate buildup. Relative doses in the radiation fields are determined from densitometric measurements using a film characteristic curve developed with the films. The edge of the radiation field is defined at the position of 50% of the central ray dose. Additional information concerning beam symmetry can also be obtained from these films.
1. In-air measurements
The RPC measures off-axis factors and verifies beam symmetry by ion chamber measurements in air (with sufficient buildup) at the nominal treatment or source-axis distance.
Horizontal-to-vertical ratios are obtained by rotating the gantry. This screens for output variations with gantry angle and provides a correction factor if the RPC and institution calibrate with different gantry orientations.
2. Output
The RPC measures ionization for photon beams with the chamber 10 cm deep in a 30 cm x 30 cm x 45 cm water phantom for all energies. A 1.0 mm acrylic cap is used to protect the chamber in water or a waterproof ion chamber is used.
The RPC calculates absorbed dose rate to muscle at the reference depth in a water phantom from the ionization data using the following equation based on the AAPM "TG-51" Protocol (Ref. 6):
See Section III for definition of terms.
The last term of the equation is
described in Section III but not found on the TG‑51 calibration
worksheet. Other correction factors may be applied as needed to determine
agreement with the institution. These may include horizontal-to-vertical
output ratios (H/V), peak scatter factors (PSF), isotope decay corrections
(DF), clinical-to-measured output ratios 
,
etc. The depth dose factor (ddf) is that used clinically by the
institution. The last term of the equation is not used if the institution
calibrates to water. See Section I.C. above.
3. Wedge and Tray Transmission Factors and Field Size Dependence
The RPC measures field size dependence (FSD), and tray or wedge factors at the RPC's depth of calibration. Wedges and trays are usually measured with a 10 x 10 cm field and FSD is measured at selected field sizes from 6 x 6 to 30 x 30 cm. Wedges are measured in two or four orientations with 180 collimator and wedge rotations (independently) as conditions allow. Care is taken to ensure that the chamber is centered on the axis of the collimator.
4. Depth Dose Data
Central axis depth dose data (percentage or fractional depth-dose, TMR, TAR, etc.) are measured in water. The effective depth is taken to be 2 mm shallower than the geometric center of a 0.6 cc Farmer-type chamber. These factors are derived by normalizing the RPC's measurements to the institution's value at the RPC's depth of calibration. On occasion the RPC will attempt to verify this value by searching for dmax or by making measurements at the reference depth.
1. Output
The RPC measures ionization at the reference depth using a 0.6 cc Farmer-type chamber in water. The thimble is given negative polarity. The usual collecting potential is 300 V. Occasionally measurements are taken with a 600 V collecting potential to minimize variations in ion collection efficiency, which can be a problem with scanned beams and depth dose measurements. The RPC searches for the depth of maximum ionization and dose for all electron beams.
The depth of 50% ionization, used in the calculation of R50 and dref, is estimated from the RPC's depth-ionization readings described in the next section.
Dose rate is calculated to muscle using this equation based on TG-51:
The last term is not found in the TG-51 worksheet but is described in Section III of this Appendix. The monitor end effect, Î, is usually not used for electron calibrations, but may be measured. A clinical-to-measured output correction may also be applied, if needed. The depth dose factor (ddf) is that used clinically by the institution. The last term is not used if the institution calibrates to water. See Section I.C. of this appendix.
2. Percentage Depth Dose
Check of depth-dose data for electrons is done by measuring ionization at the institution's stated depths of 100%, 80%, and 50%. The dosimeter readings (M) are multiplied by L/ρ and Prepl for the appropriate depths and chamber diameter to convert to relative dose. The effective depth of measurement is taken to be 2 mm shallower than the geometric center of the chamber when a 0.6 cc Farmer-type chamber is used. The depths of 80% and 50% are then estimated by interpolation/extrapolation of the calculated relative dose values. These estimated depths are then compared to the institution's values. The results are presented as a difference in the depth of the given isodose line.
3. Cone Ratios
Selected field sizes (for machines with trimmers) or selected cones are measured for output ratios relative to the standard field. Usually, the most commonly used electron energy is selected. If necessary, the RPC will again search for the depth of maximum ionization. This ionization is corrected to dose, per section C.2 above, and compared with the calibration cone dose to form the cone ratio.
Measurements of brachytherapy sources are made by the RPC with a well-type ionization chamber made of air-equivalent plastic, having an ion collecting volume of about 1 liter and a collecting potential of 300 volts. Sources are centered in the active volume through the use of spacers. Calibration factors for Ra-226, Cs-137, Ir‑192 and I-125 sources are obtained from the measured chamber responses to sources calibrated by the NIST. These factors include the 1.1, 0.8, and 0.7% reduction in exposure rates for Co-60, Cs-137, and Ir-192, respectively, that NBS (now NIST) instituted on January 1, 1986.
For isotopes other than iodine and radium, the unit "mg-eq (Ra, 0.5 mm Pt)" means that the exposure rate, measured at one meter from the source in the equatorial plane about the source, is the same as from that mass of radium encased in 0.5 mm Pt. This exposure rate is 0.825 Rm2h-1(mg-eq)-1. I-125 source strength is usually specified in "apparent mCi", which is defined such that an "apparent mCi source" has an exposure rate of 1.45 Rcm2h-1mCi-1 measured in the equatorial plane.
Brachytherapy source strength (Ref. 7) is determined from the equation:
Source Strength = M • N
where M is the corrected dosimeter response and N is the chamber calibration factor for that isotope. Additional corrections may be applied to account for geometrical variations of chamber response with source length and position and for dose-rate anisotropy arising from source configuration and/or cladding differences.
Because mg-eq and apparent mCi are defined by their exposure rate at a distance, we can speak of an exposure calibration factor NX in these units for our brachytherapy chamber. The AAPM (Ref. 7) and others are recommending that sources be characterized by the air kerma rate at a distance. We will therefore also speak of a kerma calibration factor, NK.
E. Definitions of Parameters in TG-51
M/U = Meter reading, corrected to 22 degrees Celsius and 760 mm Hg, per monitor unit (or minute), uncorrected for timer error or monitor end effect.
Î = The monitor end effect, timer or shutter error. See Refs. 1 and 4. It is usually applied to photon beams, but not to electron beams as the latter are commonly calibrated with the clinically-used number of monitor units. The RPC uses the symbol Î for this term so it is not confused with the symbol "a" in TG-21 Figs. 1 and 7. Refs. 1 and 4 use "a".
NX = Exposure calibration factor in R per scale division (SD) or reading (rdg.) (includes electrometer correction factor).
NK = Air kerma calibration factor in Gy per scale division. This term is also used as the brachytherapy dosimeter calibration factor. See Section II E of this Appendix for values of NK/NX.
ND,W = Absorbed dose to water calibration factor in cGy per reading (rdg.) (includes electrometer correction factors)
KQ = Quality conversion factors for photons
KR50,Kecal = Quality conversion factors for electrons
= Mean restricted collision mass stopping power
ratio, from TG-21, Fig. 2, using ionization ratio (TMR),
not NAP, as the independent variable.
Ppol = Polarity correction factor
Pion = Ion recombination correction factor, from TG-21, Fig. 4.
Prepl = Gradient correction factor for photon beams and fluence correction for electron beams, from TG-21, Fig. 5 or Table VIII.
= Gradient correction factors for electrons.
ddf = The depth-dose factor (fractional depth dose, tissue-air ratio, etc.) used to convert dose at the calibration depth to dose at the reference depth (usually dmax).
=
Ratio of mean mass energy absorption coefficients, used with photons to
convert from dose-to-water to dose-to-muscle. The RPC uses a value of
0.99 for all of the megavoltage beams it calibrates. See
Ref. 10
and Ref. 1,
eq. 39.
= Ratio of mean unrestricted collision mass stopping
power coefficients, used with electron beams to convert dose-to-water to
dose-to-muscle. The RPC uses a value of 0.99 for all the beams it
calibrates. See Ref. 11 or 12.
1. NCRP Report
#69, "Dosimetry of X-Ray and Gamma-Ray Beams for Radiation Therapy in the
2. AAPM Report
#12, "Physical Aspects of Quality Assurance in Radiation Therapy"
(1984). AAPM Reports are available from the AAPM,
3. Task Group 40, Radiation Therapy Committee, American Association of Physicists in Medicine: "Comprehensive QA for radiation oncology", Med. Phys. 21:581-618 (1994).
4. AAPM Task Group 10, “Code of Practice for X-ray Therapy Linear Accelerators.” Med. Phys. 2:110-121 (1975).
5. Orton, C.G. and Seibert, J.F.: "The Measurement of Teletherapy Unit Timer Errors." Phys. Med. Biol. 17:198-205 (1972). This method minimizes the need for dosimeter linearity, a requirement of the linear regression-and-extrapolation method.
6. Task Group 51, Radiation Therapy Committee, American Association of Physicists in Medicine: “Protocol for clinical reference dosimetry of high-energy photon and electron beams”, Med. Phys. 26: 1847-1870 (1999)
7. AAPM Report #21, "Specifications of Brachytherapy Source Strength" (1987). See Ref. 2 for availability.
8. Gastorf, R.J., et al, "Cylindrical Chamber Dimensions and the Corresponding Values of Awall and (Ngas/NxAion)." Med. Phys. 12:751-754 (1986).
9. Hanson, W.F. and Tinoco, J.A.D.: "Effects of Plastic Protective Caps on the Calibration of Therapy Beams in Water." Med. Phys. 12:243-248 (1985).
10. Hubbel, J.H.: "Photon Mass Attenuation and Energy-Absorption Coefficients from 1 keV to 20 MeV." Int. J. Appl. Radiat. Isotopes 33:1269 (1982).
11. Berger, J.J., and
Seltzer, S.M.: Stopping Powers and Ranges of Electrons and Positrons, 2nd
ed.,
12. ICRU Report #35, "Radiation Dosimetry: Electron Beams with Energies Between 1 and 50 MeV" (1984). See Ref. 1 for availability.
Absorbed Dose: External Beams (photons and electrons)
|
a. Tumor dose Delivery b. Beam Calibration c. Relative measurements (E.g., tray, wedge, depth dose factors, electron cone rations, etc.) d. Depth for stated percentage depth-dose for electrons |
±5% ±3% ±2%
±3 mm |
Absorbed Dose: Brachytherapy
|
a. Dose Delivery b. Source Calibration |
±15% ±5% |
Mechanical Checks
|
a. Congruence of light field and radiation field b. Agreement between light field and field size indicators c. Agreement between treatment distance indicators |
±3 mm on a side ±3 mm ±3 mm |
|
Reference Cases |
±5% |
These are links to forms in either Microsoft Word of Acrobat (PDF) that we might need from the institution prior to our visit.
· RPC Visit Questionnaires
o Cobalt-60 machine data (Word Doc)
o Photon machine data (Word Doc)
o Electron machine data (Word Doc)
o Brachytherapy information (Word Doc)
· Quality Assurance Documentation and Procedures (Word Doc)
· Treatment Planning Data Needed (PDF)
· Reference Cases
o RPC Reference Case #1 (Pelvis)
o RPC Reference Case #2 (Lung -Mantel Field)
o RPC Reference Case #3 (Breast)