A. Coverage: All Radiation Oncology/Medical Physics personnel
B. Reviewed/Revised: June 2013
C. Policy Summary: It is the policy at ECH Radiation Oncology to provide safe operation and control of radioactive materials used with the high-dose-rate (HDR) brachytherapy
afterloader and to provide proper response to emergency and abnormal situations. This procedure is applicable to calibration of Iridium-192 sealed sources for the HDR afterloader. 1. Definitions:
a. Authorized User: A physician licensed by the California State Board of Medical Examiners who meets the applicable requirements of CA Code Title 17 specified on the facility Radioactive Materials License.
b. Radiation Safety Officer: An individual who has a knowledge of and the authority and responsibility to apply appropriate radiation protection rules, standards, and practices, who must be specifically authorized on a radioactive material license, and who is the primary contact with the agency. The roles and the responsibilities of the position are defined in CA Code Title 17.
c. Radiation Safety Committee: A committee to advise the administration an all matters relating to the procurement, preparation, use, storage, and disposal of radioisotopes and/or radiation producing devices that are used for the diagnosis or treatment of disease. The duties and responsibilities of the RSC are defined by CA Code Title 17. D. Policy and Procedure:
1. Procedure:
a. Calibration Frequency:
1) The HDR source must be calibrated immediately after installation and prior to the treatment of any patients.
2) Full calibration measurements and surveys of adjacent restricted and
unrestricted areas shall be performed on the HDR afterloader by the authorized licensed medical physicists with a specialty in therapeutic radiological physics before first medical use, following replacement of the sealed source, and following repair of the unit that involves the sealed source or components related to the exposure assembly.
2. Instrumentation: a. Well Chamber:
1) Standard Imaging HDR 1000 Plus well-type ion chamber, 2) Sun Nuclear Well Chamber
b. Electrometer: 1) Max-4000
c. Portable Survey Meter:
1) Fluke Victoreen 451P-RYR
d. Instruments used in calibration of the HDR unit must be calibrated, or cross calibrated with units calibrated by an Accredited Dosimetry Calibration Laboratory (ADCL) within the 24 months prior to the HDR calibration. The usual ADCL is at the University of Wisconsin.
e. Portable survey meters must be calibrated annually by a licensed vendor. 3. Full Calibration Tests:
a. Source activity will be verified within 5 %. b. Source positioning will be verified to within 1 mm.
c. Source retraction with backup battery power will be verified by switching off normal power.
d. Length of transfer tubes will be measured for consistency.
e. Timer accuracy will be verified at typically short and long dwell times to assure linearity using an independent timer.
f. Source travel time error will be measured using a two-position two-dwell time comparison method.
g. Lengths of all non-rigid applicators will be verified at the time of full calibration or within six months prior to use.
4. Source Activity Calibration Guidelines:
a. Place the well chamber on the ‘tennis racket’ of the linear accelerator couch. This minimizes in-scatter from surrounding objects and allows use of a room scatter factor Proom of 1.000. Place the HDR insert in the well chamber. Place a thermometer next to the chamber.
b. Connect the well chamber to the electrometer using the triax BNC connector, passing through the wall to the HDR control console. Set the electrometer to +100% bias. Verify that the bias voltage is at least 300 V DC. Set the electrometer to measure nA.
c. Connect a Nucleotron quick connect to a 100 cm afterloading catheter. Place the tip of the catheter in the well chamber insert, push to make sure the tip is at the bottom, and tighten the insert. Verify the catheter is held firmly. Connect to channel 1 of the HDR indexer.
d. Allow 15 minutes for the ion chamber to equilibrate to room temperature. Record the temperature of the ion chamber (oC); record the atmospheric pressure
(mmHg).
1) Calculate PTP = [(273.15 + T) x 760}/(295.15*P).
e. Exit the room and prepare the HDR console for exposures.
f. It is necessary to find the “sweet spot” of the ion chamber. This is about 5 cm from the bottom of the chamber. Program an exposure from 99 to 91 cm, stepping the source in 0.5 cm steps and using a dwell time of 10 sec at each location. Record the nA at each dwell position.
g. Find the position of the peak reading and make a second exposure with 0.2 cm steps centered on the peak position. Record the peak nA reading from this exposure as I100.
h. Switch the bias to + 150, allow a few minutes to equilibrate, and repeat the exposure. Record the nA reading at the same position that was located in the previous exposure. Record this reading as I50.
i. Calculate Aion = 4/3 – 1/3 * (I100/I50). Calculate Pion = 1/Aion.
j. Calculate Sk (air-kerma strength) in units of cGy m2 h-1 with the formula:
k. Sk = I100 * Pelec * P T-P * Pion * Proom * N(Sk)
l. Where Pelec is the electrometer calibration factor (dimensionless) and N(Sk) is the
air kerma strength calibration factor for the ion chamber (Gy m2 h-1 A-1).
1) Convert the Sk reading to curies (Ci) by multiplying by 2.4789 Ci/cGym2h- 1
). This value should be less than 10.
m. Decay the activity for the source calibration certificate (T1/2 = 73.83 d) and
compare to the measured activity. Calculate the difference. If the discrepancy is greater than 5.0%, contact the manufacture immediately. Do not give any patient treatments until the discrepancy is resolved.
n. Following calibration, input the activity of the new source into the planning system and afterloader control console.
5. Source Positioning Verification Guidelines:
a. Following calibration, a source should be verified.
b. Remove the catheter from the well chamber. Tape the end to a ready-pack type V film.
c. Program an exposure at a series of dwell points starting at 99 cm and spaced at 3.0 cm. Use times of 16, 8, 4, 2, 1, 0.5, and 0.3 sec.
d. Develop the film and examine to verify exposure uniformity. Purpose: This policy provides guidelines for the safe operation and emergency procedures for brachytherapy with the high-dose-rate afterloader.
6. Linearity Guidelines:
a. Following activity calibration, the linearity of the HDR unit must be checked. The standard is 1.0%
b. Set the electrometer to record nC. Remember to set the bias back to +100%. c. Make exposures at the “sweet spot” position for times of 15, 30, 60, 90, and
120 sec. Make two exposures for each time, average, and record as Q15, Q30,
etc.
d. Calculate Q60ss1 = Q90 – Q30 and Q60ss2 = Q120 – Q60. Average these to get
Q60ss.
e. Calculate t = 60 * (Q60 – Q60ss)/Q60ss. This is the source transit time error.
Expect a value between 0.3 to 0.6 sec. Tracking this value may be helpful in diagnosing a problem.
f. Set up a table to check linearity. Column 1 is time, column 2 is Q, column 3 is Q/(t + t) in nC/sec, column 4 is the column 3 value relative to 60 sec. g. Average column 4 and find the deviation for each exposure time. Expected
values are 0.9900 – 1.0000. 7. Spot Checks:
a. Periodic spot checks will be performed at the beginning of each treatment day and after source exchange. The spot checks shall include verification of electrical interlocks, exposure indicator lights, viewing and intercom systems, emergency response equipment, radiation monitors, timer accuracy, clock date and time, source position and indicators to within 1 mm, condition of the transfer tubes, and sealed source strength.
b. Output shall be correct to within 1% of physical decay strength.
c. If a system malfunction is found, the system shall be locked in the off position until corrective action is performed.
d. The Microselectron daily QA checklist and the microselectron patient QA checklist will be used to document the spot checks.