Abstract
Background/Aim: This study compared two types of parallel-plate ionization chamber to clarify the pitfalls of dosimetry in electron radiation therapy. Materials and Methods: The ion recombination correction factor and polarity effect correction factor, sensitivity, and percentage depth doses (PDDs) of PPC05 and PPC40 parallel-plate ionization chambers were compared in a small-field electron beam. The output ratios were measured for 4-20 MeV electron beams with field sizes of 10 cm × 10 cm, 6 cm × 6 cm, and 4 cm × 4 cm. Furthermore, the films were placed in water and positioned in the beam with their surface perpendicular to the beam axis, and lateral profiles were obtained for each beam energy and each field. Results: Regarding PDDs, at depths greater than the peak dose, the percentage depth dose for PPC40 was smaller than that for PPC05 in small fields and at beam energies greater than 12 MeV, which could be attributed to the lack of lateral electron equilibrium at small depths and multiple scattering events at large depths. The output ratio of PPC40 was approximately 0.025–0.038, which was lower than that of PPC05 in a 4 cm × 4 cm field. For large fields, the lateral profiles were similar, regardless of the beam energy, however, for small fields, the flatness of the lateral profile was beam energy dependent. Conclusion: The PPC05 chamber, which has a smaller ionization volume, is therefore more suitable than the PPC40 chamber for small-field electron dosimetry, in particular at high beam energies.
Small-field electron beams are routinely used to treat many diseases, including superficial skin cancer, breast cancer of the chest wall, and keloid scars (1-3). Dosimetry is performed frequently in conjunction with electron beam therapy. However, accurate measurement of small fields is difficult because of the lack of lateral electron equilibrium at small penetration depths, which shifts the maximum depth and decreases the stopping power ratio at larger depths (4, 5). Although film-based dosimetry measurements have been performed in ultra-small fields (6), these measurements are complicated and not compatible with the treatment of conditions requiring immediate attention, such as keloid scarring (7, 8). Some reports suggest that solid-state dosimeters with small volumes, such as diamond dosimeters or metal–oxide–semiconductor field-effect transistors, are useful (9-11); however, these dosimeters are expensive and difficult to install in general institutions. A parallel-plate ionization chamber that minimizes scattering perturbation effects is advantageous for percentage depth ionization (PDI) measurements (6, 12). Furthermore, the cavity correction factor (Pcav) for cylindrical ionization chambers depends only on the calibration depth (dc) (13). Therefore, PDI must be measured using a parallel-plate ionization chamber to determine dc.
Therefore, parallel-plate ionization chambers are in demand. Although various parallel-plate ionization chambers are available, a detailed comparison of their properties based on differences in the beam energy range and field size is yet to be presented, and a specific type of ionization chamber has not been recommended. In this study, parallel-plate ionization chambers, PPC05 and PPC40 (IBA Dosimetry GmbH, Schwarzenbruck, Germany), with field sizes of 10 cm × 10 cm, 6 cm × 6 cm, and 4 cm × 4 cm were compared. Specifically, the ion recombination correction factor (ks), polarity effect correction factor (kpol), sensitivity, percentage depth dose (PDD), and output ratio at the calibration depth were compared to clarify the pitfalls of dosimetry in electron radiation therapy.
Materials and Methods
Equipment. The specifications of the parallel-plate ionization chambers, PPC05 and PPC40, used in this study are listed in Table I. A RAMTEC Smart electrometer (TOYO MEDIC, Tokyo, Japan) and Novalis Tx linear accelerator (Varian Medical Systems, Palo Alto, CA, USA) were used. The electron beam energies were 4, 6, 9, 12, 16, and 20 MeV, and the dose rate was 1,000 monitor units (MU)/min. Profiles and factors were obtained using the Blue Phantom2 advanced three-dimensional water phantom system (IBA Dosimetry GmbH, Schwarzenbruck, Germany) or the GAFCHROMIC EBT3 dosimetry film (Ashland, Covington, GA, USA). The irradiated films were stored in a light-shielded area for at least 24 h (14) and then digitized at 72 dpi resolution in the red channel using an ES-10000G scanner (Seiko Epson, Nagano, Japan). ImageJ software (15) was used to analyze the film dose profiles.
Specifications of parallel-plate ionization chambers PPC05 and PPC40.
PDD. PDIs for both chambers in water were obtained at each beam energy in fields of 10 cm × 10 cm, 6 cm × 6 cm, and 4 cm × 4 cm. A CC13 cylindrical ionization chamber (IBA Dosimetry) was used as the reference chamber to monitor the output dose. The voltage was switched between ±300 and −150 V, and PDI was measured at a source-to-surface distance of 100 cm, three times at each voltage. A step-and-shoot method was used from the deepest point that fully contained the contaminated X-ray region to the shallowest point (16). After temperature and pressure correction, the PDD was calculated by multiplying each PDI by the calculated ks, kpol, and average restricted mass collision stopping power ratio at each depth. In addition, the half-value depth (R50) was calculated using the linear interpolation of the obtained PDD profile.
Cross-calibration. Cross-calibration of PPC05 and PPC40 was performed at 20 MeV in water using a 30013 Farmer Chamber (PTW, Freiburg, Germany) traceable to the primary standard dosimetry laboratory. The measurement was performed at depth dc, which was obtained from the value of R50 determined from the PDD profile with a field size of 10 cm × 10 cm. The calibration factors ND,W,20MeV were obtained from the average of five irradiations of 100 MU, using a field size of 10 cm × 10 cm. Measurements followed the Standard Dosimetry 12 method (13). The sensitivity was obtained from the reciprocal of ND,W,20MeV.
Correction factors. The ks and kpol values of PPC05 and PPC40 were measured in water at depth dc for each beam energy using a field size of 10 cm × 10 cm. The radiation doses were measured five times at 100 MU, and the average value was obtained. The value of ks was obtained using the two-voltage technique and applied voltages of −300 and −150 V (17). The value of kpol was obtained using applied voltages of −300 and +300 V. Temperature and pressure corrections were performed under all measurement conditions.
Output ratios. The output ratios at each beam energy were calculated for 6 cm × 6 cm and 4 cm × 4 cm to 10 cm × 10 cm fields, with dc calculated from the value of R50 for a 10 cm ×10 cm field at the same beam energy. The average output ratio was determined from five measurements using 100 MU irradiation and a 10 cm × 10 cm field as the standard value. Temperature and pressure corrections were performed under all measurement conditions. For comparison, the output ratios were obtained for cylindrical ionization chamber CC13.
Lateral profile measurement using film dosimetry. To compare PPC05 and PPC40 with different diameters in the lateral direction, the films were placed in water and positioned in the beam with their surface perpendicular to the beam axis, and lateral profiles were obtained for each beam energy and 10 cm × 10 cm, 6 cm × 6 cm, and 4 cm × 4 cm fields. The lateral profiles were obtained around the maximum depth (dmax), 90% dose depth (d90), 80% dose depth (d80), 50% dose depth (d50), and 20% dose depth (d20). The film was rotated symmetrically about the beam axis, and the resulting profiles were averaged to compensate for the symmetry and placement uncertainties.
Results
PDD measurement. The PDD profiles and R50 values derived from them are shown in Figure 1 and Table II, respectively. At all beam energies, the R50 values for PPC05 and PPC40 agreed well for the 10 cm × 10 cm field. For small field of 4 cm × 4 cm and beam energies greater than 12 MeV, the PDD for PPC40 was lower than that for PPC05 at depths greater than those corresponding to the peak PDD. The R50 value for PPC40 was 2 mm smaller than that for PPC05 at high beam energies and in a 4 cm × 4 cm field.
The percentage depth dose (PDD) for parallel-plate ionization chambers PPC05 and PPC40 in (a) 10 cm × 10 cm, (b) 6 cm × 6 cm, and (c) 4 cm × 4 cm irradiation fields. Black arrows in panel (c) indicate the regions where key differences in profiles were obtained.
Half-value depth (R50) of PPC05 and PPC40 obtained from PDD plots (Figure 1) and the difference between R50 values for PPC40 and PPC05 (Δ).
Cross calibration. Cross-calibration at 20 MeV and dc (48.6 mm) produced ND,W,20MeV values of 54.2 and 7.65 cGy/nC for PPC05 and PPC40, respectively, whose reciprocal values yielded sensitivities of 0.019 and 0.13 nC/cGy, respectively (Table I).
Correction factors ks and kpol. Figure 2 shows the values of ks and kpol for the PPC05, PPC40, and 30013 ionization chambers. The values of ks were highest for PPC05 at all beam energies. The values of kpol for 30013 and PPC40 exceeded 1.0, and those for PPC05 were less than 1.0.
Ion recombination (ks) and polarity effect (kpol) correction factors for PPC05, PPC40, and Farmer (30013) ionization chambers.
Output ratios. The output ratios relative to the 10 cm × 10 cm field at dc for PPC05, PPC40, and CC13 at each beam energy are shown in Figure 3. The ratios for PPC40 were smaller in the 4×4 cm field than those in the 6×6 cm field.
Output ratios for PPC05, PPC40, and cylindrical (CC13) ionization chambers.
Lateral dose profiles. The lateral profiles of the relative absorbed doses were measured at each depth using film dosimetry. Figure 4 shows representative results obtained using 6 MeV and 16 MeV irradiation. For large fields, the lateral profiles were similar, regardless of the beam energy. However, for small fields, the flatness of the lateral profile was beam energy dependent.
Lateral profiles of relative absorbed dose measured using film dosimetry. (a) 6 MeV, 10 cm × 10 cm field size. (b) 6 MeV, 6 cm × 6 cm field size. (c) 6 MeV, 4 cm × 4 cm field size. (d) 16 MeV, 10 cm × 10 cm field size. (e) 16 MeV, 6 cm × 6 cm field size. (f) 16 MeV, 4 cm × 4 cm field size. dmax, Maximum depth; d90, 90% dose depth; d80, 80% dose depth, d50, 50% dose depth; d20, 20% dose depth.
Discussion
In this study, we evaluated the parallel-plate ionization chambers PPC05 and PPC40, focusing on their characteristics under small-field electron irradiation. In the field of 4 cm × 4 cm, the PDD for PPC40 was lower than that for PPC05 at depths greater than those corresponding to the peak dose and at beam energies greater than 12 MeV. The R50 value for PPC40 was 2 mm smaller than that for PPC05 at high beam energies and in a 4 cm × 4 cm field (Table II). To obtain depth-sensitive electron profiles, a parallel-plate ionization chamber with a small chamber volume in the depth direction is recommended (13). A large-volume cylindrical ionization chamber is not available for PDI measurements because the value of Pcav is unknown except at the calibration depth derived from R50 (13). Moreover, the characteristics of the PPC05 and PPC40 parallel-plate ionization chambers with different volumes have not been compared until now.
Both chambers had comparable PDD profiles in the reference field of 10 cm × 10 cm at all beam energies, whereas the profiles were narrower in smaller irradiation fields because of the lack of equilibrium in lateral electron scattering for narrow electron beams (4, 5). In contrast, different PDD profiles and R50 values were obtained for PPC05 and PPC40 in small fields (Figure 1 and Table II), with PPC40 having a lower PDD than PPC05 at depths greater than those corresponding to the peak dose at high beam energies. This might be explained by the fact that while lateral scattering at small depths was greater at high beam energies than at low beam energies, the distribution was narrower at larger dose depths (Figure 4). This is the reason electron beams lose beam energy as they reach greater depths and the angle of multiple scattering increases in the lateral direction (18). PPC40 had a large volume in the lateral direction, along which a wide dose distribution was expected. Thus, compared with PPC05, PPC40 is expected to have a higher dose at the reference depth and a lower dose at greater depths, resulting in a lower PDD. The R50 value for PPC40 was up to 2 mm smaller than that for PPC05 at high beam energies in the 4 cm × 4 cm field and thus the calibration depth for PPC40, determined from dc (cm)=0.6R50 − 0.1 (13), was up to 1.1 mm smaller than that of PPC05.
According to nominal values, the sensitivities of PPC05 and PPC40 were 0.02 and 0.13 nC/cGy, respectively (Table I). Cross-calibration was performed at 20 MeV, a high beam energy level that minimizes the effect of Pcav on the cylindrical ionization chamber (13). When using the tabulated sensitivity value as a reference, it should be noted that its value is specific to the beam energy at which cross-calibration was performed. The difference between the polarity correction factors of PPC05 and PPC40 is slight. Whether this difference reflects the individual characteristics of each chamber or is caused by random measurement errors requires further investigation.
In this study, two parallel-plate ionization chambers and a cylindrical ionization chamber were compared at the same calibration depth, and the output ratio of the PPC40 chamber was found to be lower than that of the other chambers under small irradiation fields (Figure 3). This might be due to the non-equilibrium of lateral electron scattering in small fields (4, 5), as was evident from the PDD analysis. In the 10 cm × 10 cm reference field, the measurement area is flat, whereas in the smaller field, the area of poor flatness increases commensurately with the lateral volume of the parallel-plate ionization chamber, which can explain the relatively small output ratio for PPC40. In addition, although the cylindrical ionization chamber is provided with a value of dc but not Pcav (13), this was not a problem in this study because the measurements were made at a depth equal to dc. Furthermore, although the TG-25 report recommends that the output factor should be measured at dmax (12), dc was used to standardize the depth of each chamber in this study. Because the value of dmax depends on the field size, the output factor may be overestimated by up to 3% for small fields (4).
Conclusion
In this study, the PDD of PPC40 was smaller than that of PPC05 at a field size of 4 cm × 4 cm at beam energies greater than 12 MeV. Therefore, the PPC05 chamber, which has a smaller ionization volume than that of PPC40, is more suitable for small-field electron dosimetry in particular at high beam energies.
Acknowledgements
The Authors thank Edanz (https://jp.edanz.com/ac) for editing a draft of this manuscript.
Footnotes
Authors’ Contributions
Y Yanagi, H Monzen, and K Kubo were associated with concept and design. Y Yanagi, J Sugiyama, and K Noma took the measurements. Y Yanagi, J Sugiyama, K Noma, T Ito, Y Sakai, and K Nakamura analyzed the data. Y Yanagi, H Monzen, T Kida, H Doi, and Y Nishimura prepared the manuscript. All Authors read and approved the final manuscript.
Conflicts of Interest
All Authors declare that they have no conflicts of interest in relation to this study.
Funding
This work was supported in part by the Japan Society for the Promotion of Science (JSPS) KAKENHI grant number 20K08093.
- Received February 17, 2023.
- Revision received February 28, 2023.
- Accepted March 1, 2023.
- Copyright © 2023 International Institute of Anticancer Research (Dr. George J. Delinasios), All rights reserved.
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