Original article
HDR and PDR 192Ir source activity control procedures, as the part of the quality assurance system at Brachytherapy Department of Greater Poland Cancer Centre

Purpose
Data published in IAEA report no 17 [1] showed that incorrect source strength specification in brachytherapy planning system or treatment delivery console of HDR or PDR afterloaders is the main reason of serious radiation accidents. Other human errors and incorrect usage of quantities and units could also induct errors relevant to patient safety and treatment results. For the quality assurance programme realised in brachytherapy department – calibration of the used sources is one of the most essential components [2, 3]. Main aim of this procedure is to ensure that the values provided by source vendor certificate agree with measured source strength within the predefined tolerance. Obtained values are used by the treatment planning systems and also by treatment console software to recalculate step times according to sources decay. Measurements performed between source exchanges are used to assuring that the source decay is properly represented in the software an its properly taken into account during the calculation of the steep time pattern. Proper calibration of the sources also assuring the traceability to international standards – for simple comparisons between national and international reports of the treatment results [4, 5].
The main aim of this study was to summarize and compare the results of the three years of HDR and PDR source activity control procedure realized by using two methods of measurements – according to recommended standards (redundancy and local).


Material and methods
The measurements of the source activity were performed for HDR and PDR afterloaders. Every changed source was checked three times. First measurement was performed at the day of source exchange, second – after 30 days and third – after 60 days. All measurements was performed twice: first using redundancy standard equipment (thimble chamber, PMMA phantom) and afterwards with the local standard equipment (well-type chamber). For phantom measurements air kerma strength of the source (Sk) was recalculated to activity, using air kerma rate constant for 192Ir (4.082 cGy cm²/mCi h). For all measurements Unidos E electrometer was used.

Redundancy standard
Main reason for using redundancy standard is possibility to check the HDR and PDR with independent equipment. Measurements (of the doses) are done in solid media (PMMA) and thus this conditions are much closer to the clinical conditions, while by using the well type chamber the measured quantity is the kerma in gas (air). It’s relevant that the redundancy standard cannot be used to determine reference values [6-8]. For the redundancy standard cylindrical PMMA phantom was used. The catheter was fixed inside custom made insert and placed in the central hole of the phantom. Measurements were performed using Farmer type chamber inserted inside one of the peripheral holes. Setup used for this type of measurements is shown on Fig. 1. For all the measurements PTW 30013 Farmer type chamber was used, and Unidos E electrometer.
The air kerma strength Sk [mGy/h m²] can be determined from a dose measurement made by an ionization chamber calibrated in absorbed dose to water for Co-60 radiation:
M: instrument reading [digit]
KD: air density correction factor
where T is the phantom temperature [°C] and p is the air pressure [hPa]
kt: time correction factor (60/t), where t [min] is the measurement time
Nw: calibration factor for dose absorbed to water for Co-60 radiation [mGy/digit]
kl: correction factor for Ir-192
This value was calculated from the instrument specifications. Assuming the instrument response is 1.00 for Co-60, the correction factor is equal to the interpolated response between Co-60 and the highest X-ray available assuming a mean Ir-192 energy of 380 keV.
kzp: geometry correction factor for the cylindrical phantom used
This factor contains the volume correction for the used chamber 30013 (PTW) at distance 8 cm.
kr: inverse square law correction factor (kr = (8/100)2 = 0.0064)
kw-p: perturbation factor from water to PMMA environment (kw-p = 1)
(µen/r)w and (µen/r)a: mass energy absorption coefficients for air and water respectively, (µen/r)w/(µen/r)a = 0.899
gw: relative energy lost by bremsstrahlung (gw = 0.001)

Local standard
The local standard is established as well-type chamber. This chamber type is open to the atmosphere, pressurized chambers are not appropriate instrument due to some serious recombination problems due to high activity of HDR/PDR sources. The recommended calibration factor is the air kerma strength (cGy m² h–1).
Measurements of the activity for PDR and HDR sources were done using PTW well chamber with vented sensitive volume of 200 cm³. Dedicated adapter was used for assuring repeatable position of the source during measurements. Unidos E (PTW) was used as electrometer due to sensitivity and wide dynamic range. In the both checked sources (HDR and PDR) a source was driven into insert adapter to a reference depth of 61 mm above the chamber bottom.
The Source strength was calculated from the measurements reading with the following formula:

Sk = M × Ni × Pion × KD (2).

Si: source strength of the 192Ir source
Depending on the selected calibration factor Ni, the output Si can be calculated in:
• Air – Kerma Strength in cGy × m² × h–1
• (Apparent) Activity in GBq or Ci
• “Exposure strength” in R × m² × h–1
M: measurement reading in nA
Ni: calibration factor
• for Air – Kerma Strength, Ni is in cGy × m² ×h–1 × A–1
• for (apparent) Activity Ni is in GBq × nA–1 or Ni is in Ci × nA–1
• “Exposure strength” in R × m² × h–1 ×A–1
Pion: the reciprocal of Ion collection efficiency factor Aion.
When chamber is employed with a collection potential of 300 V, Aion is greater than 0.996 and in practice, for Pion a value of 1 can be used.
When the source dosimetry system (chamber and electrometer) is employed with a different collecting potential, the ion collection efficiency factor Aion is calculated as follows:
(3).
Q1 and Q2 are the charge (or current) reading at nominal (300 V) and half (150 V) potential, respectively. KD: environmental correction factor
The chamber is vented to the atmosphere, the currents are normalised to 20°C and 1013 hPa. Use of the chamber at other pressures and tempera tures requires correction of the currents to these conditions. The multiplicative correction factor KD is calculated from the following expression:
(4).
T: room temperature in °C
p: atmospheric pressure in hPa.


Results
192Ir source activity measurements (14 source exchanges) for HDR afterloader are presented in the Table 1. Every first and every third value represents the source activity at the day of the exchange. Values between them represents activity after 30 and 60 days from source exchange respectively. Measurements were performed using two methods SHDR,WELL value is for local standard, SHDR,PMMA value is for redundancy standard, SHDR,SYS is the activity calculated by treatment console, SHDR,WELL vs. SHDR,SYS and SHDR,PMMA vs. SHDR,SYS represents the percentage differences between measured and calculated values respectively. Graphical representation of obtained results is shown on Figs. 2 and 3.
192Ir source activity measurements (14 source exchanges) for PDR afterloader are presented in the Table 2. Every first and every third value represents the source activity at the day of the exchange. Values between them represents activity after 30 and 60 days from source exchange respectively. Measurements were performed using two methods SPDR,WELL value is for local standard, SPDR,PMMA value is for redundancy standard, SPDR,SYS is the activity calculated by treatment console, SPDR,WELL vs. SPDR,SYS and SPDR,PMMA vs. SPDR,SYS represents the percentage differences between measured and calculated values respectively. Graphical representation of obtained results is shown on Figs. 4 and 5.
Wilcoxon test was used for statistical evaluation of obtained results – there were no statistically significant differences observed between activity values measured using two dosimetry standards and values calculated by treatment console for both (HDR and PDR) sources (Table 3).


Discussion
For absolute calibration of the 192Ir sources the recommended method is using the primary standard. This is realised by measuring the air kerma rate at relatively large to the source dimensions distances (it’s defined at 1 m) [9]. In this conditions when small charges or currents are measured chamber positioning errors can induct large uncertainties. For the best results it’s needed to interpolate the values by measuring kerma at several different distances [10, 11]. Increasing of the measuring distance decreases positioning inaccuracies but also reduces the signal to noise ratio and increasing scatter contribution. Chamber size effects, and current leaks also contribute to the overall uncertainties when in-air calibration is going to be performed. Proper realisation of calibration using primary standard could be time consuming and difficult to realise in hospital conditions. In practice – calibration and quality control actions for the sources used in brachytherapy could be done in well chambers or solid phantoms. Setup for measurements is repeatable and whole procedure is not time consuming.
After three years of using both methods of HDR and PDR source activity checking there were no statistically significant differences observed between values measured using well-chamber and thimble chamber and solid PMMA phantom. For reducing equipment inducted errors exact the same electrometer (Unidos E) was used for both measurements conditions. The most important question in this part of quality control procedures it’s “reaction level” and “reaction type”. The reaction level was set (and newer reached) at 5% for both machines, according to recommendation and previous experiences [12, 13].
In realised measurements the maximum observed percentage differences between values from the system and measured activity were 3.49% for HDR source and 4.05% for PDR. As in other aspects of quality control reaching of reaction level should result in appropriate and previously planned actions. In general the irradiation equipment should not be used clinically if the levels are exceeded and always carefully taken into consideration by responsible medical physicist.

Conclusions
1. Checking of source parameters is one of the most important parts of quality control system in brachytherapy facilities. Elaborated procedure is essential to assure patient safety and reliable clinical results.
2. Well-chamber and thimble chamber based dosimetry systems both are fast and reliable tools for 192Ir source parameters checking in working brachytherapy department conditions.


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