Study of dosimetric properties of flattened and unflattened megavoltage x ray beam on high Z implant materials

Abstract Purpose Addition of high Z implants in the treatment vicinity or beam path is unavoidable in certain clinical situation. In this work, we study the properties of radiation interaction parameters such as mass attenuation coefficient (MAC), x ray beam transmission factor (indirect beam attenuation), interface effects like backscatter dose perturbation factor (BSDF) and forward dose perturbation factor (FDPF) for flattened (FF) and unflattened (UF) x ray beams. Methods MAC for stainless steel and titanium alloy was measured using CC13 chamber with appropriate buildup in narrow beam geometry. The x ray beam transmission factors were measured for stainless steel and titanium alloy for different field size, off‐axis, and depths. Profile analysis was performed using a radiation field analyzer (RFA) as a function of field size and depth to study the influence of phantom scattering and spectral variation in the beam. In addition, interface effects such as BSDF and FDPF were measured with gafchromic films at maximum BSDF peak position calculated using Acuros XB algorithm and with PPC40 chamber measured at exit side of high Z material, respectively. Results The MAC in both cases decreases with increase in energy for stainless steel (SS) and titanium (Ti) alloy. The MAC increases with the change in x ray beam type from flattened to UF beam because of relatively lower mean energy. The x ray beam transmission factor increases with the increase in energy, field size, and depth owing to increase in penetration power phantom scatter, respectively. The measured BSDF and FDPF were found to be in good agreement with AXB algorithm. Conclusion The dosimetric properties of x ray photon beam were studied comprehensively in the presence of high Z material like stainless steel and titanium alloy using both flattened and UF beams to understand and incorporate the findings of various parameters in clinical condition due to the variation in energy spectrum from FF to UF x ray beam.

Z) elements. Materials with an effective atomic number (Z eff ) greater than that of cortical bone ranging from 6 to 16 (AAPM-TG65) 6  Many users try to avoid using beams through high-Z material even if it results in additional dose to adjacent critical organs.
Some users try to account for the presence of high-Z material using computerized treatment planning system which uses correction for attenuation of the material. In the recent times, unflattened x ray beam is being used more frequently at treatment sites like prostate with hip prosthesis, spine mets with spinal cord fixation device and less frequently in head and neck treatment with tooth implants. [8][9][10][11][12][13][14][15][16] The impact of flattened and unflattened beams in the presence of high-Z implant needs to be studied as both energy types have different energy spectrum, mean energy, and varied fluence. In our study, high-Z materials of stainless steel and titanium alloy are considered as they are commonly used in implants. 17,18 The dosimetric characteristics of flattened or UF beam in the presence of high-Z material have been studied and understood thoroughly.

| MATERIALS AND METHODS
The experiment was carried out in TrueBeam 2.0 (Varian Medical System Inc., Palo Alto, CA, USA) linear accelerator capable of delivering flattened and UF x ray beams of 6 MV (6FF), 10 MV (10FF), 15 MV (15FF) and 6 MV-FFF (6UF), 10 MV-FFF (10UF). Even though this study expresses the impact of high-Z material on flattened and unflattened beams of respective energies, we have included additional flattened 15 MV to examine the impact of high-Z material. In this study, we used stainless steel (SS316) and titanium alloy (Grade 5) high-Z materials (Table 1) which are austenitic grades (nonmagnetic). These two high-Z materials were studied to imitate the biocompatible generally used in implants. The effective atomic number (Z eff ) of stainless steel (SS316) and titanium alloy (Grade 5) are 29.23 and 22.15 and average mass number (A) of 56.32u, 46.7u, respectively. The composition of stainless steel (SS316) and titanium (Grade 5) material are as follows. 19 Breadth, width, and thickness of stainless steel and titanium alloy dimensions are 3 × 3×2.5 cm 3 . A special RW3 slab of about 2.5 cm thickness was prepared to accommodate these high-Z material inserts.

2.A | Mass attenuation coefficient
The penetration ability of the beam, that is, mass attenuation coefficient μ/ρ (cm 2 /g) for flattened and unflattened x ray beams for high-Z material were calculated for narrow beam geometry. 20 The measurement was carried out in air using CC13 ionization chamber with appropriate buildup to avoid electronic disequilibrium and the chamber was positioned at isocenter and high-Z material (SS316 and Ti alloy Grade 5) was placed 10 cm above the chamber level exactly equally shadowing around the chamber. A field size of 3 × 3 cm 2 was opened so that the filed border was inside the high-Z material. The gap in between chamber and high-Z material was good enough to avoid any scattering electron reaching chamber to overestimate the result. Measurements were carried out both along the central axis and off-axis of about 15 cm from central axis longitudinally to quantitate the variation in mean energy at off-axis that affects mass attenuation coefficient. It was impractical to do measurements at 15 cm along lateral direction because of over traveling of X jaws on either side beyond −2 cm. We assumed that measuring the mass attenuation coefficient at off-axis can be mirrored on all sides as energy spectrum of all off-axis is approximately same for flattened and UF x ray beams.

2.B | Beam transmission and attenuation
The beam transmission and attenuation due to dose perturbation have been measured for both stainless steel (SS316) and titanium alloy (Grade 5) for all available energies of flattened and unflattened x ray beams. The measurement was carried out with CC13 ionization chamber placed at a depth of 10 cm in RW3 slab phantom with 100 cm SSD for 50 MUs. RW3 slab with block of high-Z materials (SS316 and Ti Grade 5) were inserted at 5 cm depth from the surface. Measurements were made with and without high-Z material for field sizes 3 × 3 cm 2 and 30 × 30 cm 2 at central axis for all available flattened and unflattened x ray beams.
The measurement was done at off-axis distances (3, 5, 8, 10, and 12 cm) along inline and cross-line direction by keeping constant field size of 30 × 30 cm 2 with and without high-Z material at respective off-axis distances. 21 The field size of 30 × 30 cm 2 was used because the maximum filed size is commonly used in clinical circumstances.

2.C | Profile measurements
The change in dose profile due to high-Z material for different energies at various depths for given field size was thoroughly studied to quantitate the beam hardening, dose enhancement due to lateral scatter contribution under the shielding for flattened and unflattened beams.
The profile measurements were acquired using a radiation field analyzer (Blue Phantom2 RFA -IBA Dosimetry, Germany) and CC13 field and reference ionization chamber. High Z material was placed at a depth of 5 cm form the water surface using 2 mm perspex tray bridge. Profiles were measured at depths of 10 and 20 cm T A B L E 1 Physical and chemical properties of stainless steel (SS316) and titanium alloy (Grade 5).

2.D | Interface effect
The interface effect at the junction of tissue and high-Z material was studied in an RW3 slab phantom, which has a mass density of 1.043 g/cm 3 that is approximately equivalent to that of water. Here, two components were studied; the backscattered dose perturbation factor [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] at the entrance side of high-Z inhomogeneity and the forward dose perturbation factor 37 on the exit side of the inhomogeneity. These two effects were studied for all flattened and unflattened x ray beam energies. Performing measurements at front interface using chamber proved difficult. Hence, we used gafchromic film dosimetry with flatbed scanner (EPSON Expression 10000XL).
The setup for measurement of BSDF was made using small piece of (5 × 5 cm 2 ) gafchromic film placed parallel to high Z surface at peak point, calculated by AXB algorithm (±1 mm) in treatment planning system and perpendicular to central axis. Each measurement was done multiple times (5) to reduce the inherent film uncertainty.
The high Z materials were (stainless steel and titanium alloy) placed  38 The measured values were compared with TPS data, calculated by Acuros AXB algorithm for all flattened and UF x ray beams.

3.A | Mass attenuation coefficient
The chamber based measurement of MAC with narrow beam geometry for flattened and unflattened x ray beam shows (  (15 cm) shows that flattened beam has more variation in mean energy than unflattened beam. Introduction of flattening filter in the beam not only reduces dose rate but also mean energy at off-axis.
But for unflattened beam, the difference in mean energy is considerably small from central axis to off-axis. Furthermore, the transmission factor toward off-axis (Table 3) in all direction gets marginally decreased due to lower phantom scatter contribution for all energies but the contribution of spectral variation (lower mean energy) at off-axis with deeper depth is negligible.

3.C | Profile measurement
The measured profile under stainless steel and titanium for different energies 6FF, 10FF, 15FF, 6UF, and 10UF at different depths 10 T A B L E 2 Measured mass attenuation coefficient for stainless steel (SS316) and Titanium alloy (Grade 5) at central and off-axis.

3.D | Interface effect
The interface effect like back scatter dose perturbation factor was studied with gafchromic films at peak position ( because of lateral scattering contribution due to increase in field size and was found to be 4.8, 4.8, 4.7, 4.7, and 4.6 cm for the energies 6FF, 10FF, 15FF, 6UF, and 10UF, respectively (Fig. 2).
The measured BSDF for flattened and unflattened x ray beam was in correlation with the calculated BSDF from Acuros XB algorithm. Figure 3 shows the FDPF for both flattened and unflattened beam measured through AXB algorithm and chamber (PPC40) with field size 3 × 3 cm 2 .

| CONCLUSION S
The MAC decreases with increase in energy for both flattened and unflattened x ray beam for stainless steel and titanium alloy. MAC is less for unflattened x ray beam compared to flattened x ray beam of same energy since the mean energy for UF x ray beam is lower than flattened beam due to beam softening caused by removal of the flattening filter away from beam path. The measured x ray beam transmission data states that the transmission factor varies with respect to T A B L E 3 Beam transmission factor of flattened and unflattened x ray beam for stainless steel (SS316) and titanium (Grade 5) at off-axis. T A B L E 5 Calculated and measured BSDF using Acuros XB and film for 3 × 3 cm 2 and 10 × 10 cm 2 for flattened and unflattened x ray beams.  The interface effect between RW3 slab and high Z materials of stainless steel and titanium interface was studied in detail using the factors namely BSDF and FDPF. The measured values and that calculated with Acuros AXB algorithm were compared and the result shows that the measured initial buildup of dose in front face of high Z medium characterized by BSDF was found to be in good agreement with Acuros AXB algorithm. Dose buildup due to backscatter electron of unflattened beam is lower than flattened x ray beams due to decrease in backscatter electron. Likewise, the FDPF was measured using parallel plate chamber and was compared with data from Acuros AXB algorithm which shows that the FDPF is low for UF x ray beam than flatted x ray beam due to less forward scatter electron since unflattened x ray beam has less mean energy than flattened x ray beam. The dosimetric properties of x ray photon beam interaction parameters were studied comprehensively in the presence of high Z material like stainless steel and titanium using both flattened and UF x ray beams to understand and incorporate the concept in clinical condition due to the variation in energy spectrum from FF to UF x ray beam in the treatment vicinity with high Z implants.

ACKNOWLEDGMENTS
We thank Mr. Sunder from Kalyani Radiotherapy Specialty India (P) Ltd for helping us to design RW3 phantom and high Z materials for this project. We also thank Mr. Giridharan Iyer from Varian Medical System India (P) Ltd in providing us the TrueBeam machine data for data incorporation. We sincerely thank our Managing Trustee Dr. B.
S. Srinath for his motivation in doing this work.

CONFLI CT OF INTEREST
No conflict of interest.