Introduction:

The quantitative determination of energy deposition in a given medium by directly or indirectly ionizing radiation is called radiation dosimetry^{[1, 2]}. Radiation dosimetry has a vital role in the medical field because the irradiation of patients is totally based upon the values determined by it^[3]. The International Atomic Energy Agency (IAEA) and several other international organizations have published their dosimetry protocols for calibration of photon and electron beams during the last several decades. With the passage of time, these protocols have been updated for improvement in the accuracy in dosimetry of high-energy photons and electrons beams [4]. In connection with these advances in dosimetry, the two latest dosimetry protocols, those of Task Group 51 (TG-51) and TRS-398, have been implemented by the American Association of Medical Physicists (AAMP) and the IAEA, respectively ^{[5, 6]}. Both protocols are based on the calibration of an ionization chamber in terms of absorbed dose to water (ND_w,Q0) in a standard reference quality beam, Q₀, from which the dose to water in the user radiation beam is derived ^{[7, 8]}. In these protocols, different experimental approaches have been adopted, which results in variations in the patient dose.

Huq et. al. ^[7] have compared these two protocols by analyzing differences in the basic input data for photon and electron beam dosimetry. Experiments were performed using 6, 18, and 25 MV clinical photon beam energies and 6, 8, 10, 12, 15, and 18 MeV electron beams energies^[7]. In this study, two Farmar-type chambers and three plane-parallel chambers, calibrated by a US accredited dosimetry calibration laboratory (K & S Associates, Inc, Nashville, TN, USA), were used ^[7]. It should be mentioned here that, in Pakistan, there is only one secondary standard dosimetry laboratory (SSDL) which provides calibration of radiation instruments on the national level^{[2, 9]}. The SSDL follows the IAEA TRS-398 dosimetry protocols for the calibration of radiation instruments, whereas physicists in some medical centers either follow AAPM TG-51 or IAEA TRS-398 for the radiation beam output measurement of different machines.

Calibration factors are provided under standard environmental conditions of temperature, pressure, and relative humidity according to the recommendations of followed dosimetry protocol. In hospitals, measurement conditions are normally different from the standard laboratory conditions recommended by each protocol. The difference in the environmental conditions affects the response of the ionization chamber. Therefore, correction factors must be applied for converting the cavity air mass to the reference conditions. Our main objective in the present study was to find out the effect of environmental conditions on the patient dose.

Materials and Methods:

In this study, dosimetry was performed for two therapeutic units, namely a Co-60 unit (THERATRON PHOENIX) and a high-energy Varian linear accelerator (CLINAC DMX 2100C) with 6 and 15 MV photon and 6, 9, 12, and 15 MeV electron beams. The dosimetry system consisted of a PTW stationary water phantom (T41014) with a cylindrical chamber (PTW-30001) connected to an electrometer (PTW UNIDOS E), which was used for the absolute dosimetry of the THERATRON PHOENIX. A Farmer-type ionization chamber (IBA-FC65-G) and a plane-parallel chamber (IBA PPC-05) were used for absolute dosimetry of the two photon and four electron beams that were generated by the CLINAC DMX 2100C, respectively. The chambers were connected to PTW UNIDOS E, attached to Blue water Phantom, and each chamber type combined with PTW UNIDOS E was calibrated in a Co-60 radiation beam with its reference point at a measuring depth of 5 cm in water at the SSDL at PINSTECH, Pakistan. The calibration was performed under the standard conditions of the IAEA TRS-398 protocol.

The clinical dosimetry was performed following the recommendations and reference conditions of the AAPM TG- 51 and IAEA TRS-398 protocols ^{[10, 11]}. The depth dose curves were obtained experimentally with the help of a Blue Phantom scanner system loaded with the software Scanditronix-Wellhöfer OmniPro-Accept for all photon and electron beams, as shown in Figs. 1a and 1b^{[12, 13, 14]}. The relative measurements were evaluated with the software Scanditronix-Wellhöfer OmniPro-Accept. The tissue phantom ratio (TPR_{20, 10}) and percentage depth dose (%dd(10)) were determined from the curves given in Fig. 1a, as these values are required for all of the photon beams under study, as per the requirements of the TRS-398 and TG-51 protocols. From the measured TPR_20,10 and %dd(10), beam quality factors were determined. For each electron beam under study, the reference depth (z_ref) and R₅₀ (i.e., "depth in water in a 10 x 10 cm² or larger beam of electrons at an SSD of 100 cm at which the absorbed dose falls to maximum and 50% of the dose maximum, respectively" ^[5]) were calculated from the depth dose curves given in Fig. 1b as per the requirements of TRS-398 & TG-51. Similarly, a polarity correction factor and an ion recombination correction factor were calculated according to the guidelines provided in the TG-51 and TRS-398 protocols for each beam generated by the CLINAC DMX 2100C. Multiple measurements were performed in each session, and were used as average values to minimize the error in the experimental values. Finally, the dose calculations were performed according to the recommendation of the AAPM TG-51 and IAEA TRS-398 protocols ^{[5, 6]}.

Fig. 1Full-screen View

(Color online) (a) 6 and 15 MV photon depth dose curves. (b) Electrons depth dose ionization curves of 6, 9, 12, and 15 MeV

Results and Discussion:

Photon Dosimetry:

Table 1 shows beam quality specifiers, namely, TPR_20,10 and %dd(10) _X, for 6 and 15 MV photon beams which have been calculated from Fig. 1a. With these values, a beam quality correction factor, K_q, for both photon energies (6 and 15 MV) was determined against the provided values of either TPR_20,10 or %dd(10) _X ^{[5, 6]}, summarized in Table 1. In the case of TG-51, the K_q value is smaller than that of TRS-398. The percentage difference in K_q is - 0.1% and - 0.3% for 6 and 15 MV photon beams, respectively.

Table 1:Full-screen View

Comparison of K_q, K_ion, K_pol, and absorbed doses at Z_max (D_W(Z_max)) values calculated based on the recommendations of AAPM TG-51 and IAEA TRS-398 protocols for 6, 15 MV and Co-60 photons beams

Energy	AAPM TG-51						IAEA TRS-398
	KTP	%dd(10)x	Kq*	Kion	Kpol	DW(Zmax)	KTP	TPR20,10	Kq**	Kion	Kpol	DW(Zmax)
6 MV	1.16055	66.7	0.991	1.00734	1.00054	0.9970	1.16055	0.668	0.992	1.00714	1.00054	0.9975
15 MV	1.16055	77.63	0.970	1.01135	1.00129	0.9967	1.16055	0.762	0.973	1.01109	1.00129	0.9995
Co-60	1.16055	NA	1.0	1.0	1.0	1.6489	1.16055	NA	1.0	1.0	1.0	1.6489

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* depends upon the type of ionization chamber and beam energy. For 6 MV beam, %dd(10) _X =%dd(10) while for 15 MV, %dd(10) _X =1.267%dd(10)-20.0 was used. K_q for the ionization chamber was determined from the tabulated values of K_q and corresponds to %dd(10) _X for the ionization chamber ^[5]. The interpolation techniques was used while using these tabulated values.

The beam quality, type of chamber, and some other measurement conditions, such as cable length and position etc., may affect the polarity for a particular ionization chamber. Therefore, a polarity correction factor, K_pol, must be applied for beams of different qualities. Furthermore, due to the Lack of ion collection on the corresponding electrodes of the ion chamber, some of the ions produced may not contribute toward the actual signal. Therefore, recombination of ions (i.e., ion recombination correction factor, K_ion) has to be applied. K_ion depends upon the dose rate, the chamber geometry, and the applied polarizing voltage. Although the method (two-voltage method) for both protocols under study is the same, their measurement approaches are different. Similarly, the temperature and pressure correction factor, K_T,P, also must be applied, as the clinical conditions are always different from the standard conditions under which the calibration of the dosimetry systems is performed. The K_T,P, K_pol, and K_ion were measured for both protocols according to the recommendations and procedures. These values are summarized in Table 1.

The values of K_T,P for both dosimetry protocols in the case of both 6 and 15 MV-photon beams were same as the calibration for the dosimetry systems performed under the standard conditions of the IAEA TRS-398 protocol. Figs. 2a and 2b show a comparison of the K_ion and K_q values for both protocols, whereas Fig. 2 (c) shows polarity correction factors for 6 and 15 MV-photon beams for both protocols. As both the protocols follow the same approach for measurement of the polarity effect, contribution of polarity correction factor in the dosimetryis similar.

Fig. 2Full-screen View

(a) Comparison of K_ion of TG-51 and TRS-398 protocol. (b) Comparison of K_q in both protocols. (c) Calculated value of K_pol vs energy.

By comparing the values of K_ion and K_q, calculated based on recommendations of both protocol for corresponding beams, a measurable difference was observed for the studied photon beams (see, Fig. 2-a, -b). In the case of the TG-51 protocol, the K_ion had a relatively greater value for both photon beams, as shown in Fig. 2a, while, K_q had a relatively smaller value for both photon beams, as shown in Fig.2b. The results also showed that the polarity correction factor depends upon the photon beam energy. The polarity correction factor increases with an increase in the energy of the photon beam (see Fig. 2c).

The absorbed doses (D_W(Z_max)) were calculated and normalized to Z_max for the Co-60 beam and 6 and 15 MV photon beams (Table 1) by using the %depth doses (%DD) that correspond to each beam quaility. Fig. 3 shows a comparison of the D_W(Z_max) ratio (i.e., D_W(Z_max)_TG-51/ D_W(Z_max)_TRS-398) of Co-60 for the 6 and 15 MV photon beams for TG-51 and TRS-398 protocols. The same values of D_W(Z_max) were observed for both protocols in the case of the Co-60 tele-therapy unit, as there was no difference between the measured parameters. For the 6 and 15 MV photon beams, the percentage difference in absorbed dose was -0.05% and -0.3% respectively. The TRS-398 gives relatively higher doses as compared to that of the TG-51 protocol. Moreover, the percentage difference in the calculated dose increased with an increase in the energy.

Fig. 3Full-screen View

Ratio of the absorbed doses at Z_max (i.e., D_W(Z_max)_TG-51/ D_W(Z_max)_TRS-398) for Co-60, for 6 MV and 15 MV photons, by comparing the two protocols (TG-51 and TRS-398).

Electron Dosimetry:

Following the recommendations and procedures adopted in both TG-51 and TRS-398 protocols, K_q, K_T,P, K_pol, and K_ion for 6, 9, 12, and 15 MeV-electron beams were measured. These values are summarized in Table 2. The K_T,P were the same for both protocols, as explained earlier in the section on photon dosimetry. The calculated values of R₅₀ and the corresponding values of the reference depth, Z_ref, for electron energies of 6, 9, 12, and 15 MeV are also summarized in Table 2. According to the TRS-398 protocol, the K_q value is directly provided against R₅₀, whereas, for TG-51 protocol, K_q was calculated (see Table 2) from R₅₀ for each electron energy ^[5]. The calculated absorbed dose normalized to Z_max for the 6, 9, 12, and 15 MeV electron beams for the TG-51 and TRS-398 protocols is also listed in Table 2.

Table 2:Full-screen View

Comparison of K_q, K_ion, K_pol, and absorbed doses at Z_max (D_W(Z_max)) values calculated on the basis of the recommendations of the AAPM TG-51 and IAEA TRS-398 protocols for 6, 9, 12, and 15 MeV electron beams.

Energy (MeV)	AAPM TG-51							IAEA TRS-398
	KTP	Zref*(cm)	R50**(cm)	Kq	Kion	Kpol	DW(Zmax)	KTP	Zmax (cm)	R50***(cm)	Kq	Kion	Kpol	DW(Zmax)
6	1.17	1.19	2.15	0.94	1.023	1.013	1.026	1.17	1.08	2.15	0.92	1.023	1.013	1.013
9	1.17	1.98	3.46	0.92	1.010	1.005	1.006	1.17	1.99	3.42	0.91	1.005	1.010	0.996
12	1.17	2.85	4.91	0.91	1.013	1.008	1.006	1.17	2.44	4.83	0.91	1.013	1.008	1.003
15	1.17	3.64	6.24	0.90	1.014	1.007	0.910	1.17	2.80	6.12	0.90	1.013	1.007	0.998

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* Z_ref for electrom beams is the depth in water in a 10 x 10 cm² or larger beam of electrons at an SSD of 100 cm at which the absorbed dose falls to 50% of its maximum ^[5]. It was determined from Fig. 1b

Figs. 4a and 4b show a comparison of the K_ion and K_q values for both protocols, and Fig. 4c shows K_pol for 6, 9, 12, and 15 MeV electron beams. There was no regular pattern recorded in the variation in K_ion with a change in the energy of the beam. Both the dosimetry protocols followed the same irregular pattern of variation in K_ion (see Fig. 4a). By inter-comparison of both protocols for the values of K_q, (i.e., by keeping the energy of the beam constant) higher values of K_q for the TG-51 protocol as compared to the TRS-398 protocol were observed for all the studied electron beams as shown in Fig. 4b. Furthermore, variation in K_q decreases as the electron beam energy increases. The percentage difference between the K_q values for 6, 9, 12, and 15 MeVelectron beams where 1.5%, 0.9%, 0.3%, and 0.0%, respectively, for both protocols. Fig. 4c shows a variation in K_pol with a change in the energy of the electron beam. K_pol decreases with an increase in the energy of the electron beam.

Fig. 4Full-screen View

(a) Comparison of the K_ion of TG-51 and TRS-398 protocols. (b) Comparison of the beam quality factor, K_q, for both protocols. (c) Calculated K_pol vs energy.

The ratio of the absorbed doses at Z_max corresponding to TG-51 and TRS-398 is illustrated in Fig. 5. The percentage differences in the measured dose for both protocols for 6, 9, 12, and 15 MeV electron beams were 1.3%, 0.9%, 0.3%, and 0.1%, respectively. In this case, the TG-51 gives relatively high doses as compared to the TRS-398 protocol, and it showed an inverse relationship with energy of the electron beams.

Fig. 5Full-screen View

Ratio of the absorbed doses at Z_max (i.e., D_W(Z_max)_TG-51/ D_W(Z_max)_TRS-398) for 6, 9, 12, 15 MeV electron beam by use of two protocols (TG-51 and TRS-398).

Conclusion:

To conclude, small differences in the absorbed doses to water at the Z_max ratio were found for the two protocols. The TRS-398 protocol gave higher doses as compared to the TG-51 protocol. The percentage difference in the measured absorbed dose increased as the energy increased in the case of the photon beams, however, for the electron beams, TG-51 gave relatively higher doses as compered to the TRS-398 protocol. Unlike photon beams, the percentage difference in the measured absorbed dose decreased with an increase in the energy. To reduce uncertainty in the patient dose, the clinically followed dosimetry protocol and dosimetry protocol followed for the calibration of a dosimetry system should be the same.