ORIGINAL ARTICLE Year : 2017  Volume : 13  Issue : 6  Page : 968973 Assessment the accuracy of dose calculation in buildup region for two radiotherapy treatment planning systems Bagher Farhood^{1}, Mohammad Taghi Bahreyni Toossi^{2}, Mahdi Ghorbani^{2}, Elahe Salari^{3}, Courtney Knaup^{4}, ^{1} Department of Medical Physics, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran ^{2} Medical Physics Research Center, Mashhad University of Medical Sciences, Mashhad, Iran ^{3} Department of Medical Physics, Reza Radiation Oncology Center, Mashhad, Iran ^{4} Comprehensive Cancer Centers of Nevada, Las Vegas, Nevada, USA Correspondence Address: Aim: Our objective is to quantify dose calculation accuracy in the buildup region using TiGRT and Prowess Panther treatment planning systems (TPSs). Materials and Methods: Thermoluminescent dosimeter100 chips were used in a phantom for dose measurement. TiGRT Version 1.2 (LinaTech, Sunnyvale, CA, USA) and Prowess Panther version 5.1 (Prowess Inc., Concord, CA, USA) TPSs were also used for dose calculations. Finally, the confidence limit values obtained to quantify dose calculation accuracy of the TPSs at buildup region for different field sizes and various gantry angles. Results: For 8 cm × 10 cm, 10 cm × 10 cm, and 15 cm × 10 cm field sizes, the confidence limit values for TiGRT TPS were 16.64, 16.56, and 25.85; for Prowess TPS with fast photon effective (FPE) algorithm were 15.17, 14.22, and 9.73; and for Prowess TPS with collapsed cone convolution superposition (CCCS) algorithm were 10.53, 9.97, and 9.76, respectively. For wedged field with gantry angles of 15°, 30°, and 60°, the confidence limit values for TiGRT TPS were 12.11, 12.96, and 22.69 and for Prowess TPS with FPE algorithm were 24.50, 22.07, and 7.82, respectively. Conclusions: It is concluded that for open field sizes without gantry angulation, dose calculation accuracy in Prowess TPS with CCCS algorithm is better than TiGRT and Prowess TPSs with FPE algorithm. Furthermore, it is concluded that for wedged field with large gantry angle, dose calculation accuracy of Prowess TPS with FPE algorithm is better than TiGRT TPS while, for medium and small gantry angles, dose calculation accuracy of TiGRT TPS is better than Prowess TPS with FPE algorithm.
Introduction When the energy of ionizing radiation increases, the penetration strength of primary and secondary charged particles increases leading to a deeper position of the maximum dose point. While the maximum dose is obtained at a larger depth, the dose close the patient skin is not negligible and needs to be considered.[1] Therefore, in radiotherapy, the accurate calculation of dose in the buildup region is important, in particular when the target volume has superficial extension near the skin.[2] The dose of buildup region consists of a primary component such as electrons, photons, and electron contamination from the treatment head and the air between the patient and treatment head. For the megavoltage photons used in radiotherapy, dose distribution in the buildup region usually depends on the field size, beam spectrum, angle of beam incidence, and electron contamination.[3],[4],[5] For any arbitrary field shape and a given system, the dose distribution in the buildup region at various depth can be determined accurately through an empirical formula that models the dose of buildup region in terms of a primary and electron contamination components.[6] Another method for determination of dose in the buildup region includes direct measurement, which is appropriate in situ ations where the skin dose deposited per radiation therapy fraction is required.[7] In radiotherapy, the accuracy and precision of treatment planning and dose delivery are important to achieve tumor control and spare normal tissue from unnecessary radiation dose.[8] The actual dose to the target volume needs to be as near as possible to the prescribed dose to maximize the possibility of cure without serious complications to other normal organs.[9] Several investigators have reported values for the accuracy required in radiation therapy to limit damage to normal tissue while maintaining enough tumor control in the population.[10],[11] The maximum suggested values for uncertainty in the dose have various level ranging from 5%[11] to 3.5% (1 standard deviation [SD]).[10] Accurate calculation of the dose distribution in the buildup region remains a challenge for most of the photon dose calculation algorithms. This is mainly due to difficulties in modeling the contribution of doses from contaminated electrons emanated from collimator assembly, flattening filter, and to an insignificant extent, secondary scatter photons from the treatment machine head.[6],[12],[13],[14],[15] The accuracy of dose modeling in the buildup region mainly depends on the dose computation algorithm used in a specific treatment planning system (TPS).[16] Several studies have been carried out to investigate dose calculation in buildup region for different algorithms and TPSs. Chung et al.[17] evaluated surface and buildup region doses for Pinnacle and Corvus TPSs in intensitymodulated radiation therapy (IMRT). They found that both TPSs overestimated for these regions. The amount of overestimation ranged from 7.4% to 18.5%. Akino et al.[18]evaluated superficial dosimetry by eclipse TPS and measurement data. They concluded TPS even with advanced algorithm does not provide accurate dose values in buildup region. Panettieri et al.[19] showed that absorbed dose in the surface buildup region might considerably change depending on the type of algorithm used. Their results showed that in the first 2 mm of depth, the analytical anisotropic algorithm (AAA) tends to provide more accurate results in comparison to the pencil beam convolution (PBC) algorithm due to its improved electron contamination source model, so AAA is a better choice for calculation of absorbed dose in the skin. Oinam and Singh [16] verified IMRT dose calculations in buildup regions using AAA and PBC algorithms. They concluded that AAA calculated the dose more accurately than PBC at 4 mm and 6 mm depths. To the best of our knowledge, the dose calculation accuracy in buildup region for TiGRT and Prowess Panther TPSs has not been investigated. In this study, we investigated the accuracy of dose calculation in buildup region for Prowess Panther and TiGRT TPSs using thermoluminescent dosimeter (TLD) as well as the effect of different field sizes and various gantry angles on dose calculation accuracy of the TPSs. Materials and Methods Phantom irradiation The phantom used for irradiation included Perspex plates which were irradiated with 6 MV Xrays emitted from Siemens Primus Accelerator (Siemens AG, Erlangen, Germany) in Reza Radiation Oncology Center (Mashhad, Iran). For the work, 16 Perspex plates with the thickness of 1 mm were used. The plates were milled along the central axis, inline and crossline for insertion of TLD100 chips. The depths of 2–16 mm were evaluated (in other words the effective points of measurements were at the depths of 2–16 mm), and their distances from each other were 2 cm and doses were measured with the insertion of TLDs in each hole. These plates were drilled every other one; as the first plate was not drilled and the second plate was drilled. The arrangement of the Perspex plates placement were repeated for plates 3–16 and dose values were obtained in prespecified points. Furthermore, 20 cm of Perspex plates were placed below the first 16 Perspex plates to create full scattering conditions. Finally, computed tomography scan was taken from all Perspex plates to create a standard treatment plan as well as for dose calculation in the TLD points and to compare them with dose measurement. [Figure 1] shows the Perspex phantom and their holes for insertion of TLD100 chips.{Figure 1} Treatment planning system TiGRT version 1.2 (LinaTech, Sunnyvale, CA, USA) and Prowess Panther version 5.1 (Prowess Inc., Concord, CA, USA) TPSs were used for dose calculation in different points. TiGRT TPS uses a threedimensional (3D) photon dose calculation algorithm based on full scatter convolution (FSC). Prowess Panther TPS has two algorithms; one is fast photon effective (FPE) model that calculates dose based on measured data where all the tissue is assumed to be water with no tissue heterogeneity or effective path length through tissue is taken into. The other model is collapsed cone convolution superposition (CCCS) that calculates the dose based on full heterogeneity correction and is a full 3D dose calculation. It is noted that TiGRT TPS has an algorithm (FSC) for open and wedge fields while Prowess Panther TPS has two algorithms (FPE and CCCS) for open fields and an algorithm (CCCS) for wedged fields. Open fields of 8 cm × 10 cm, 10 cm × 10 cm, and 15 cm × 10 cm as well as 10 cm × 10 cm wedge field (wedge angle of 30°) with gantry angles of 15°, 30°, and 60° were used to evaluate the accuracy of dose calculation in the buildup region by TiGRT and Prowess Panther TPSs. The source axis distance (SAD) technique was used for dose delivery to the selected point and SAD = 100 cm was used for all the mentioned fields. The prescribed dose to the maximum dose point was 50 cGy for all the fields. Finally, doses in the selected points were calculated by the TPSs and compared with measured doses by TLD100. [Figure 2]a and [Figure 2]b show the Perspex phantom plotted by the TPSs for open and wedged fields, respectively.{Figure 2} Calibration of applied dosimeters and dosimetric method TLD readout and analysis was carried out at the medical physics research center (Mashhad, Iran), which has a special protocol for TLD analysis. The TLD100 chips used in this research were produced by Harshaw Company and were made from LiF: Mg, Ti with the 3 mm × 3 mm size, and thickness of 0.9 mm. This type of TLDs has a reproducibility of approximately ± 1.5% (1 SD). One hundred fortyfour dosimeters were used in the different positions of Perspex plates, and 6 dosimeters were used to measure the background radiation. Fifteen TLD chips were placed in a Perspex holder that was located on a 20 cm water equivalent phantom, and 1.5 cm water equivalent slab was placed on the holder to create a buildup region. They were irradiated to determine their individual efficiency correction coefficient (ECC), and then they were irradiated with 50 cGy and readout by a reader (Harshaw, Arizona, USA) to determine reader calibration factor. Finally, all of the TLD chips were irradiated with 50 cGy and their individual ECC were determined. To have high accuracy of dosimetry results, three TLD chips were irradiated in each of the evaluated depths, and the average absorbed dose per depth was obtained by three TLD chips. Analysis of results For analysis of the results, TECDOC 1540[20] and TRS 430[21] protocols were used. These protocols include detailed information on quality assurance (QA) of TPSs. According to these protocols, the difference between the measured and the calculated dose is defined the following: [INLINE:1] where Dcalc and Dmeas are the calculated dose by TPS and the measured dose by TLD100, respectively. Therefore, the confidence limit is defined as the following: [INLINE:2] The confidence limit is obtained by calculation of the average deviation between the calculated and the measured dose values for several data points in comparable positions, and the SD of the differences (1 SD of the average). Finally, the confidence limit was obtained for each field size and each gantry angle and was compared to the tolerance limit suggested in TECDOC 1540 and TRS 430 protocols. Results In this study, for the first stage, dose values were measured at selected points inside Perspex plates for open fields of 8 cm × 10 cm, 10 cm × 10 cm, and 15 cm × 10 cm as well as 10 cm × 10 cm wedged field (wedge angle of 30°) with gantry angles of 15°, 30°, and 60°. For the second stage, dose values were calculated in same selected points using TiGRT and Prowess Panther TPSs. Finally, differences between calculated and measured doses were obtained. These results are shown in the following tables. Furthermore, for better understanding, the results are also shown in histograms. The mean dose differences (%) between the measured doses by TLD100 and the calculated doses by TiGRT, Prowess with FPE algorithm, and Prowess with CCCS algorithm TPSs in open field sizes of 8 cm × 10 cm, 10 cm × 10 cm, and 15 cm × 10 cm are listed in [Table 1].{Table 1} [Figure 3] illustrates the mean dose differences (%) between the measured doses by TLD100 and the calculated doses using TiGRT (a), Prowess with FPE algorithm (b), and Prowess with CCCS algorithm (c), TPSs in open field sizes of 8 cm × 10 cm, 10 cm × 10 cm, and 15 cm × 10 cm.{Figure 3} The mean dose differences (%) between the measured doses by TLD100 and the calculated doses by TiGRT, Prowess with FPE algorithm, and Prowess with CCCS algorithm TPSs in wedged field (wedge angle of 30°) with gantry angles of 15°, 30°, and 60° are listed in [Table 2].{Table 2} [Figure 4] illustrates the mean dose differences (%) between the measured doses by TLD100 and the calculated doses using TiGRT (a) and Prowess with FPE algorithm (b) TPSs in wedged field (wedge angle of 30°) with gantry angles of 15°, 30°, and 60°.{Figure 4} Finally, the confidence limit values were obtained in different field sizes (8 cm × 10 cm, 10 cm × 10 cm, and 15 cm × 10 cm) and various gantry angles (15°, 30°, and 60°) for various TPSs (TiGRT and Prowess Panther) and were listed in [Table 3]. Note that to achieve the confidence limit values, the data for depths from 4 to 16 mm were applied.{Table 3} Discussion In this study, we evaluated the accuracy of dose calculation in the buildup region for TiGRT (version 1.2) and Prowess Panther (version 5.1) TPSs using TLD100. Furthermore, the effect of different field sizes and various gantry angles on dose calculation accuracy of the TPSs was investigated within the buildup region. Overall, TiGRT TPS, compared to Prowess Panther TPS, showed a greater difference for depth of 2 mm from the surface. Hence, data related to this depth were not applied to achieve the confidence limit values for different field sizes and various gantry angles. The data obtained at depths of 4–16 mm were applied to achieve the confidence limit values. For each of the three open field sizes (8 cm × 10 cm, 10 cm × 10 cm, and 15 cm × 10 cm), TiGRT TPS overestimates the dose in comparison to TLD100 dose values while both algorithms of Prowess Panther did not show constant trends. The confidence limit values for the Prowess Panther TPS with CCCS algorithm were within the tolerance limit (tolerance limit is 10 for simple fields) while the confidence limit values for TiGRT TPS and Prowess Panther TPS with FPE algorithm were not within the tolerance limit. As a function of depth (mm) from the surface, TiGRT TPS showed decreasing trend in the difference between the calculated and measured doses. Both algorithms of Prowess Panther TPS did not show constant trends. In assessment of the effect of field size on dose calculation accuracy of the TPSs in buildup region, it has been shown that [Figure 3] there was not a constant trend of increasing or decreasing with the variation of field size. For each of the three gantry angles (15°, 30°, and 60°), TiGRT TPS overestimated dose compared to TLD100 measured dose for most of the depths while Prowess Panther TPS with FPT algorithm underestimated the dose for most of the depths. The confidence limit value for the Prowess Panther TPS with FPE algorithm in gantry angle of 60° was within the tolerance limit while the two other gantry angles (15° and 45°) were not within the tolerance limit (tolerance limit is 15 for the complex fields). Furthermore, the confidence limit value for TiGRT TPS in gantry angles of 15° and 45° were within the tolerance limit while for gantry angle of 60° were not within the tolerance limit; therefore, it is concluded [Table 3] that for large gantry angles, the dose calculation accuracy of Prowess Panther TPS with FPE algorithm is better compared to TiGRT TPS while for medium and small gantry angles, the dose calculation accuracy of TiGRT TPS is better compared to Prowess Panther TPS. As a function of depth from the surface, TiGRT TPS and Prowess Panther TPS with FPE algorithm did not show constant trends (decreasing or increasing) in the differences between the calculated and measured dose with increasing depth from the surface. To assess the effect of gantry angle on dose calculation accuracy of the two TPSs in the buildup region, it has been shown that [Figure 4] there was not a constant trend of increasing or decreasing in dose calculation accuracy of the TPSs with variation gantry angle. Differences between the calculated and measured dose values may be due to the related uncertainties in the measurements, approximations in calculations of the algorithms used, etc. It is notable that application of TLD is accepted as a tool for QA in radiotherapy.[22] Furthermore, using TLD for measurement of dose in the regions where there is not electron equilibrium (such as buildup region) has a long history.[23] TLDs have a number of relevant advantages such as small physical size, high sensitivity, and tissue equivalence.[22] Some studies have applied TLDs for QA of TPSs in the buildup region.[16],[24],[25] In addition, Budanec et al.[26] showed that there is an agreement of within 1.5% between percentage depth doses measured by TLD and calculated by Monte Carlo method in buildup region. Monte Carlo simulation can be applied for evaluation of the accuracy of dose calculation of TPSs in the buildup region. As a subject for future study, evaluation of the accuracy of dose calculation by Prowess Panther and TiGRT TPSs in buildup region using Monte Carlo simulation will be interesting. Conclusions For depth of 2 mm from the surface, the TiGRT TPS, compared to Prowess Panther TPS, showed a greater difference that it is concluded that for shallow depths, dose calculation accuracy of TiGRT TPS is not adequate. For each of the three open field sizes, TiGRT TPS overestimates the dose in comparison to TLD100 dose values while both algorithms of Prowess Panther did not show constant trends. For each of the three gantry angles, TiGRT TPS overestimated dose compared to TLD100 measured dose for most of the depths while Prowess Panther TPS with FPT algorithm underestimated the dose for most of the depths. In the assessment of the effect of various field sizes and gantry angles on dose calculation accuracy of the two TPSs in the buildup region, it has been shown that [Figure 3] and [Figure 4] there was not a constant trend of increasing or decreasing in dose calculation accuracy of the TPSs with variation of field size and gantry angle. Acknowledgment The author would like to thank the student research committee of Mashhad University of Medical Sciences (MUMS) for approval of this research work, we also would like to thank the office of vicepresident for research affairs of MUMS for financial support of this study. We are also thankful to Reza Radiation Oncology Center for allowing us to use their systems and their sincere cooperation. Financial support and sponsorship Mashhad University of Medical Sciences (Mashhad, Iran) has financially supported the work. Conflicts of interest There are no conflicts of interest. References


