Farmer and Well-Chamber

Calibration of Brachytherapy Sources: Farmer and Well-Chamber

Become acquainted with the usage of a farmer chamber in a full-scatter phantom, as well as the well chamber procedures for Brachytherapy source calibration.

Apparatus:

Farmer and Well type ion chambers, electrometer thermometer, barometer HDR unit

Introduction:

The well-type chamber is required for the source strength measurement of radioactive afterloading sources, according to AAPM TG-56. The air kerma strength (cGy m2/h) is the suggested calibration factor. The PTW calibration certificate covers apparent activity (GBq or Ci) and exposure strength (R m2 /h) variables.

For commercial standard afterloading systems, suitable applicator adapters and calibrations are available.For measurements, the chamber is connected to a sensitive PTW electrometer (UNIDOS, UNIDOS E, MULTIDOS, TANDEM). The well-type chamber is suitable for the calibration of high dose rate (HDR) and pulsed dose rate (PDR) sources such as Ir-192.

Experiment:

Exercise I – Source activity verification with Well chamber:

Figure 1

Place the chamber on a horizontal surface near the brachytherapy unit.

Connect the 3.2mm Tube Catheter to channel 1 of the indexer and place the insert in the chamber.

Switch the dosimeter on, a stabilization period of at least fifteen minutes must be allowed for the experiment.

Set Bias to +400V.

Switch the dosimeter into electrical units and set it to the pA range

Current (A) = Charge (C) per second.

Stepsize = 1cm; Channel No# = 1; Dwell Position 1; Origin = 48mm

Nominal Time (s) = 10/(factor/100); to give 10 seconds of real time

Run the plan three times to get readings

Exercise II – Source activity verification with Farmer in full scatter phantom:

Figure 2

The Afterloading Calibration Phantom is an acrylic cylinder with a diameter of 20 cm and a height of 12 cm. It is a practical tool for afterloading source strength measurement in a solid-state phantom. In addition, it is used for calibration of semiconductor probes for afterloading dosimetry.

For both calibration purposes, the radioactive afterloading source is positioned into the afterloading applicator in the center hole of the phantom by remote control after the reference chamber has been placed into one of the peripheral holes. On a circle with a radius of 8 cm, there are four holes situated 2 cm from the rim of the phantom for detector positioning at 0°, 90°, 180° and 270° by using appropriate adapters.

Place the chamber in perspex insert and at position 1

Connect the SGT and 3mm Steel Applicator to channel 1 of the indexer and place in insert at position 0 (i.e., the centre of the phantom)

Switch the dosimeter on make measurements in electrical units

Perform the measurement in nC and set the integration interval on the dosimeter to 60 seconds

Stepsize = 1cm; Channel No# = 1; Dwell Position 1; Origin = 0mm

Nominal Dwell Time large enough so that 3 x 60 seconds actual time measurements can be performed

Run the plan 4 times with the chamber in the 4 different positions to get all readings

Results:-

The source specifications:

Reference Air Kerma Rate. 47.64 mGy -1 +/ - 5 % at 1 m (2)

11.25 CET (1)

Apparent activity: 433.06 in GBq (11.70 Ci) at the date of measurement(3,4)

10.0 Ci at 09-01-2017at 12.00 CET (1)

Source type: IR -192 GAMMED PLUS HDR 0,9MM

Capsule dimensions: 0.90mm diameter. 4.52 mm length

Source pellet dimensions: 0.60mm diameter, 3.5mm length

Source pellet form: solid Indium

Radionuclide : 1r192

Encapsulation: single

Capsule material: Stainless steel, AISI 316L

ISO classification: ISO/80/C53211

Special form certificate number: USA/0723/S-96(REV.3)

The half-life is 74 days and there 57 days after the test of measurement.

1-IR-192 source calibration using a well-type ionization chamber:-

Temperature=21, Pressure=1019.2 Hpa, ptf= 0.9966

Reading1 (nA)

Reading2 (nA)

Reading3 (nA)

Average reading (nA)

29.34

29.34

29.36

2.9347

Table 1

Sk ( air kerma strength)= Mraw x Ctp x Nsk x Pion= 28.08 cGym2/h

Air Kerma Strength = Average Reading (A) ×NK × Ptf× Pion cGym2h-1=

28.07 cGym2/h

Diff to certificate Air Kerma = 0.03%

192Ir source activity on 18-Feb-2017:-

Activity= 11.700 Ci

Apparent Activity (Ci) = Avg Reading (A) ×Nc (CiA) ×Pkm× Ptf = 6.89 Ci

Difference to Certificate Activity = 51.7 %

2-IR-192 source calibration using a farmer-type ionization chamber & full scatter phantom:-

Temperature= 20.6 degrees C, Pressure= 1019.4 Hpa, Ktp= 0.9966

Measurement position

Reading 1(nc)

Reading 2(nc)

Reading 3(nc)

Average(nc)

1

1.270

1.270

1.270

3

1.271

1.271

1.271

Average

1.271

Table 2

Sk (Air Kerma Strength)= Mraw x Nk x Ctp x Ek x R x P x ISQ x Kt x Kap x Kph =

2.7136 cGym2/h

Air Kerma Strength = Average Reading × Nk × Ptf × Pol × Recom ×Kap ×Kph× KT×ISQ = 2.7160 cGym2/h

Diff to Certificate Air Kerma = 0.08 %

Activity = 11.700 Ci

Apparent Activity (Ci) = Avg Reading (A) × Nc (CiA-1) ×Pm× ptf =16.557 Ci

Diff to Certificate Activity = 34.37%

Discussion

In the above experiments, chamber polarity refers to how negative or how positive the chamber is in terms of ionized charges. When the potential is reversed, current from an ionization chamber is exposed to constant radiation intensity changes in magnitude. This is known as the polarity effect of the ionization chamber (Davidovits, 2013). The possible reason that induces the polarity effect in the chamber is the field distortion caused by a potential difference between the collector and the guard electrode. We calculated the field distortions caused by the potential difference between the collector and the guard electrode for the two ionization chambers. The polarity effects of the two ionization chambers were measured, and they were consistent with the field calculation.

Recombination occurs between ions formed at different points in the gas when the ions drift or diffuse toward the opposite electrodes randomly. Recombination occurs in any ionization chamber however its importance depends on the density of the ionizing particles and conditions of ion collection (Gerald, 1956).

In the first exercise, we used the well-type chamber to verify the source of activity. Readings were recorded on different dates as the decay process took time. Pure iridium metal cylindrical core was used as the radiation source surrounded by stainless steel AISI 316L. The air kerma strength obtained for our radionuclide source was 28.07 cGym2/h. This value was compared with the established value, 28.08 cGym2/h and the difference was 0.03%. The differences are attributed to different forms of calculation and small differences in geometry source.

In the second exercise, a Farmer-type ionization chamber & full scatter phantom was used to calibrate the source. Comparing the results of our method with the others, all air kerma strengths measured from the same source were traced back to the same time point, according to the half-life of Ir-192. The percentage difference was defined as the air kerma strength measured with our method, the 7-distance technique minus the air kerma strength measured with the well type ion chamber (or data provided by the manufacturer) divided by the air kerma strength measured with the well type ion chamber (manufacturer) and then multiplied by 100 (Baltas, Zamboglou, & Sakelliou, 2007). From the above table, using the Farmer-type ion chamber for the first two sources, where D was 0.3 m. When compared to the calibration results of the SNC well type ion chamber and manufacturer provided data, the differences were 0.08% with our technique.

Conclusion

We could conclude that the polarity effect is mostly induced from the field distortion due to the potential difference between the guard electrode and the collector in our experiment and it depends significantly on the design of the electrodes. The calibration of sources using therapy ionization chambers of small sizes show great accuracy compared to the manufacturer's given source strength of the ionization chamber. Users should get an energy-response curve of the cylindrical ionization chamber, including for equivalent energy of 192 Ir radiation source. The procedure to be followed during calibration using cylindrical chamber should be used with a simple designed jig. The right buildup cap ought to be used with the ionization chamber containing the charged particle equilibrium in other cases; correction should be made for attenuation of photons or excess ionization from the wall of the chamber.

References

Baltas, D., Zamboglou, N., & Sakelliou, L. (2007). The physics of modern brachytherapy for oncology (1st ed.). Boca Raton: Taylor & Francis.

Davidovits, P. (2013). Physics in biology and medicine (1st ed.). London: Academic Press.

Gerald, J. H. (1956). Radiation Dosimetry (1st ed.). (L. B. Gordon, Ed.) Boston: Academic Press Inc.