Effective Point of Measurement (EPOM) of Some Ionization Chambers for High Energy Photon Beam Dosimetry used in Radiotherapy for the Treatment of Cancer Patient

Effective of Some Ionization Chambers for High Energy Photon Beam Dosimetry used in Radiotherapy for the Treatment of Abstract The volumetric effect occupied by the air cavity for the dosimetry of high energy photon beam is impossible to ignore using standard ionization chambers. Hence, the dose measurement should be corrected with a displacement perturbation correction factor (P dis ) or using an Effective Point of Measurement (EPOM). The aim of this study was to calculate the EPOM of some ionization chambers and evaluation of the shift of EPOM that recommended by various international protocols under both reference and non-reference condition. The work was performed with Percentage Depth Dose (PDD) curves by placing chambers (PTW 30013, FC 65G and Semiflex 31010) at the geometrical centers for field size(s) of 5cm × 5cm to 30cm × 30cm at 100cm Source to Surface Distance (SSD) for photon energy 6, 10 and 15MV respectively. The shift of the cylindrical chambers also estimated from PDD values in comparison with reference PDD values by Parallel Plate Chamber (PPC 40 and Murkus 23343) of 100%, 80% and 50% depth in the water. The present study shows that the effective shift is not only varies with chamber materials but also with photon energy. On the other hand the periodical calibration factor of some ionization chambers at standard procedures were compared with manufacturer values also varies with time which is an important issues for the precisional dosimetry in radiotherapy. The details of the EPOM and chamber calibration factor is discussed.


Introduction
Cancer is a serious health issue and be a principle determinant of the human and economic wealth in a country. It is a malignant tumor or an overgrowth of abnormal cells. Its two main characteristics are uncontrolled growth of the cells in the human body and the ability of these cells to migrate from the original site and invade or spread to distant sites or other parts of the body. The motive of illuminating malignant tumor is to deliver a sufficient radiation dose to the target volume. However, it is impossible to illuminate only tumor cells. If we deliver low dose, the treatment will be failed while a high dose will damage the normal cells. In radiotherapy various quality assurance (QA) and quality control (QC) parameters are indispensable for the success of tumor eradication. The realistic goal of radiation therapy is to maximize the dose to abnormal cells while minimizing exposure to normal cells and spring the surrounding healthy tissues as much as possible. According to the formulation of the cavity theory, the walls of ionization chambers should be water equivalent [1]. However, even if the chamber wall is medium equivalent, the volume of water is replaced with the cavity of a cylindrical ionization chamber when the reference point of the chamber at the chamber axis is placed at the depth of measurement.
This effect is taken into account by the displacement correction factor, P dis or by placing the Effective Point of Measurement (EPOM) of the chamber at the measuring depth. Generally, the EPOM depends on the chamber design including the cavity height and radius, the mass density of the wall material of central electrode, and some other parameters. For instance, the factor, P dis is used in the international code of practice IAEA TRS-398 (IAEA 2000) [2] and the EPOM for instance in the German dosimetry protocol DIN 6800-2 (DIN 2008) [3].
For reference dosimetry in clinical photon beams, most dosimetry protocols recommend the application of a perturbation correction P dis but only the German protocol DIN 6800-2 applies the EPOM concept for all types of measurements. The distance between the central axis and the EPOM is termed as the EPOM shift. The EPOM shift is obtained by a shift factor multiplied by the inner chamber radius. According to chamber radius for different energies, the dosimetry protocols have expressed effective point value recently. In IAEA TRS-398, the shift equal to 0.6r (r being the radius of the chamber) mean before recommended for 60 Co γ-rays and all high energy photon beams whereas 0.5r for electron beams [4].
However, it has been shown that, the experimental evidence and Monte Carlo simulations on the magnitude of the shift 0.6r is not always correct. The EPOM of cylindrical ionization chamber in megavoltage photon beams has already been evaluated using Monte Carlo simulation with the EGSnrc system [5]. But it is too large for thimble ion chambers in high energy photon beams [6] and can also design for a thimble ionization chamber with zero EPOM by adjusting the wall thickness of chamber [7]. Moreover, the systematic dependence on chamber characteristics provides evidence that a universal parameterization in terms of a few design parameters is conceivable and has implication for the calculation of chamber correction factors.
Therefore, there is some possibility that the experimental results and Monte Carlo derived values do not always same. That's why there is an argument on which correction factor should be given preference although the uncertainties involved in this.
Because the uncertainties do not give any decision on this, it only reduces the range for the correct value. The application of the proposed effective point of measurement will increase the accuracy of calculating depth dose data from measured depth ionization curves, especially for depth beyond the reference depth. So, the main objectives of this study are quality control for ionization chambers based on absorbed dose measurements by determining the EPOM of some ionization chambers and evaluation of the shift of EPOM that recommended by various international protocol.

Ionization Chamber
In modern radiation therapy clinics, ionization chambers are the detectors of choice for calibrating the output of radiation therapy treatment machines. An ionization chamber is used to measure the rate of radiation exposure (how much radiation exposure is being received in a specified period of time).In this study nine ionization chambers are used; six for calibration (chamber model -  College and Hospital, Sirajgonj). When calibrating in terms of absorbed dose to water, the chamber protected by a PMMA sleeve, was positioned in a 30cm × 30cm × 30cm water phantom so that its reference point was on the central axis of the beam. The chamber axis was perpendicular to the central axis of the beam and the distance from the source to the reference point of the chamber was 80cm. The reference point of the chamber was at 5cm water depth and the size of the radiation field (50% isodose level) at the reference plane was 10cm × 10cm.

Calibration of Ionization Chamber
The absorbed dose to water (D w ) was calculated by the following Where, m raw is uncorrected electrometer reading and k tp , k elec , k pol and k s are correction factors for temperature and pressure, the electrometer, polarity and ion recombination respectively.  Various international protocol recommended for various constants value of shifts such as IAEA TRS protocol TRS 277 recommends Δr = 0.75r for high energy X-ray beam [9], whereas TG-51 (AAPM protocol) and DIN-6800 protocol recommends for the shifts of 0.5r [10,3]. On the other hand, TRS-398 (IAEA) protocol recommends the shift of 0.6r for non-reference condition [4].

Results and Discussion
In our research, six ionization chambers have been calibrated at radiation qualities ( 60 Co) standard procedure set-forth by SSDL of Bangladesh Atomic Energy Commission. The chambers are calibrated against reference standard chamber NE2571-1205 as well as traceable to NE2581-537, IAEA standard. The detail of the calibration factors obtained from this study is given in ( Table   1). The calibration factor obtained from the present experiment is compared with the values of quoted by the manufacturer. It is seen that the deviation between manufacturer values and present measurement lies within 0% to 1.44% with a maximum uncertainty of ±1.7%. It is mentioned here that the manufacturer uncertainties were quoted as ±1.1 to ±2.2%.

Effective Point of Measurement (EPOM) of Ionization Chambers
In order to derive EPOM, the PDD curve was measured with reference plane parallel plate and cylindrical chamber for various depths (100%, 80% and 50%) are summarized in (Tables 2-4) respectively. PDD data are taken at energies 6, 15 and 10MV in various field sizes (5cm × 5cm to maximum 30cm × 30cm). The variation of PDD as a function of depth in water (mm) for the 6MV, 15MV and 10MV photon beams for 10cm × 10cm field size of different chamber are shown in Figures 1-3

. From the figures it is
seen that build-up region increases with photon energies.

Determination of Effective Shift
The distance between the central axis and the EPOM is termed as effective shift or EPOM shift. It is generally assumed that the displacement effect is almost constant beyond the maximum.
Therefore, a fixed EPOM is used throughout the depth profile. This assumption allows normalizing depth profiles to the maximum and using PDDs to determine the EPOM. Here the PPC 40 and Murkus 23343 whose effective points are known were used as a reference.
The shift between the reference point (geometrical point) of the chambers and the EPOM was determined at different depth at different field size (5cm × 5cm to maximum 30cm × 30cm) for 6MV,  Note: *r is the radius of the chamber  Note: *r is the radius of the chamber From (Tables 5-7), it is seen that in case of reference condition (10cm × 10cm field size, 100cm SSD, 10cm depth in water) the shift of the effective point of the chambers PTW 30013, FC 65G and Semiflex 31010 are found 0.64r, 0.65r, and 0.73r respectively for 6MV photon beam. Whereas at 10MV, the the shift of Semiflex 31010 chambers is found 0. The shift of effective point increases with depth in water. Some exception also found in this study that might be due to the uncertainty in the measurement system.
It must be noted that for photon beam, the total uncertainty in the measurement of absorbed dose to water for chambers are lies between ±1.1% to ±1.7%, whereas, the total uncertainty for effective point of measurement is found from ±0.6% to ±0.7%. It is also seen that for FC 65G chamber, the shift of effective point is 0 at higher energy 15MV which might be due the chamber wall material (graphite which is water equivalent).

Conclusion
For accurate dosimetry planning and to reduce the uncertainty with maximum uncertainty of ±1.7% (coverage factor k=1) lies within the acceptance limit set-forth by IAEA (±1.5%). So, it is not reasonable to rely on the stability of calibration coefficient; rather it is mandatory to calibrate the ionization chamber once a year to be used in dosimetry and QA purposes in radiotherapy.