Verification of Relative Output Factor (ROF) Measurement for Radiosurgery Small Photon Beams

Transcription

1 Verification of Relative Output Factor (ROF) Measurement for Radiosurgery Small Photon Beams Reduan A a, Mazurawati M b, Nur Iziana M a, Nik Ruzman NI b, Ahmad Z a and Ahmad Lutfi Y b a School of Health Sciences, Health Campus USM, Universiti Sains Malaysia, Kubang Kerian, Kelantan, Malaysia b Department of Nuclear Medicine, Radiotherapy and Oncology, School of Medical Sciences, Health Campus, Universiti Sains Malaysia,16150 Kubang Kerian, Kelantan, Malaysia *Corresponding author: reduan@usm.my ABSTRACT: Particular attention may be paid to reassess for treatment planning systems (TPSs) that deal with specialized techniques such as stereotactic radiosurgery. It is important for clinical medical physicist to be able to quantify errors involved in the dosimetric parameter data before it can be downloaded into the TPS. The purpose of this study was to verify the measurement of relative output factor (ROF), which is one of the dosimetric parameter data required for Stereotactic Radiosurgery (SRS) treatment planning. ROF of 5 mm to 45 mm diameter Radionic circular cone collimators were measured using three types of ionization chambers, which were A12 Extradin Farmer-type Chamber (0.65 cc), A14 Extradin Microchamber (0.016 cc) and PTW Pinpoint Ionization Chamber (0.016 cc). The ROF measurements were verified using a Monte Carlo (EGSnrc) validated model and other previous studies. In conclusion, pinpoint ionization chamber gave the most reliable ROF result for field size from 5 mm to 45 mm compared to other ionization chambers used in this study. The ROF measurement using pinpoint ionization chamber was in good agreement with the Monte Carlo calculation (± 4.9% percentage deviation). Therefore for measurement of ROF for SRS circular field size, pinpoint ionization chamber was suggested due to its accuracy. This ROF validation test can be carried out during the commissioning and installation of the new TPS and also for annually quality assurance (QA). 20

2 Keywords: Relative Output Factor (ROF), Stereotactic Radiosurgery (SRS), Monte Carlo, Small field dosimetry Introduction For installing and commissioning a new stereotactic radiosurgery (SRS) treatment planning computer, a comprehensive set of beam data of the linear accelerator (LINAC) machine must be measured and then downloaded into the treatment planning software (Abdullah et al., 2015). Three basic beam parameters to be measured for dose calculation are tissue phantom ratios (TMR) or percentage depth dose (PDD), relative output factors (ROF) and single beam profiles. There are two crucial components in SRS in order to achieve the aims of a treatment planning. The first component is to deliver a precise and uniform dose to the target lesion with multiple non-coplanar beams. The second factor is the accuracy in patient positioning, which can be achieved by introducing the stereotactic apparatus. With the combination of these two factors, SRS can provide uniform dose and sharp dose falloff for treating small lesion and at the same time minimizing the damage to critical organs surrounding the lesion (Khan et al., 2011). Although dose calculation can be carried out using dosimetric parameters as beam data, the method of collecting beam data presents challenges to the physicists. In particular, narrow stereotactic radiosurgery beams with sharp dose gradient require detectors and chambers with high spatial resolution. For this reason, the smallest detector available should be used when performing small field dosimetry. For central axis measurement such as depth dose, tissue phantom ratio and ROF, the detector dimensions should be significantly smaller than the field sizes. ROF is the ratio of output for a stereotactic radiosurgery cone diameter size to the reference condition (field size 10 x 10 cm 2 at the reference depth inside the water phantom and at 100 cm of source to surface distance (SSD)). Particular attention may be paid to reassess for treatment planning systems (TPSs) that deal with specialized techniques such as stereotactic radiosurgery. It is important for clinical medical physicist to be able to quantify errors involved in the input data (measured beam data) before it can be downloaded into the TPS. All input data should be correct and accurate before it can be used clinically. In this study, ROF verification is carried out to make sure the 21

3 input data is accurate by comparing the measured data of ROF and ROF calculated using MC. This ROF validation test can be carried out during the commissioning and installation of new TPS and also for annually quality assurance (QA). Materials and Methods Measurement of ROF In this study, the main focus was to measure the relative output factor (ROF) for Radionic circular cones used for SRS treatment from 5 mm to 45 mm. The measurement of ROF was conducted inside a mini water phantom with the dimension of 35 cm x 35 cm x 50 cm. Firstly, the gantry and collimator position was positioned at zero degree and SSD was at 100 cm. Then, the pinpoint ionization chamber was placed 5 cm from the water surface (Figure 1). The 6 MV photon used was energized by Primus (Siemens Medical, USA) linear accelerator. The irradiations were repeated three times and the average values were calculated. The circular cone of 45 mm diameter was then replaced with other circular cones available in the department and the procedure was repeated with smaller circular cone size. The ROF for each circular cone was calculated by finding the ratio of charge obtained at 50 mm depth to the charge (C w,ic,a ), obtained at similar depth for reference field size (100 x 100 mm 2 ) (C w,ic,ref ). ROFs were calculated using the formula from Equation 1: ROF (A) = C w,ic,a / C w,ic,ref (Equation 1) The procedures were repeated by using different types of ionization chamber: A14 microchamber and A12 Farmer type ionization chamber. 22

4 Figure 1: The experimental setup for relative output factor measurement Modelling and Simulation Recently, Monte Carlo (MC) calculation method has reported as most accurate method of predicting dose distribution in radiotherapy (Abdul Aziz et al., 2011). In this present work, MC simulation and calculation is a useful tool in evaluating the ROF measurement results. Calculation of ROF for Radionic circular cones used for SRS treatment from 5 mm to 45 mm was performed using BEAMnrc and DOSXYZnrc (National Research Council Canada, NRCC) source code (EGSnrc package) on personal desktop. BEAMnrc is a general purpose MC simulation system for modelling radiotherapy source while DOSXYZnrc is used to calculate dose in three dimensional (3D) medium (Abdul Aziz et al., 2011). The simulations were performed in three stages. BEAMnrc was used to simulate the Siemens Primus linac head to obtain phase space data for the first 20 cm after the target, right after the projection mirror. This phase space file was used in the subsequent stereotactic cones and 100 x 100 mm 2 field sizes. In this stage, 50 million particle histories were used. In the second stage, all thirteen Radionic stereotactic cones diameter starting from 5 mm to 45 mm were simulated. The cone was simulated as stainless steel materials with a density of 8.03 g/cm 3. The geometry of each cone was measured manually since the technical drawing of the component was unavailable. The Radionic assembly holder and the cones were simulated using BEAMnrc s flattening filter (FLATFILT) component module. The 50 x 50 mm 2 (phase 23

5 space file was used for the second stage) and space phase files were generated for all cones at SSD = 80 cm. The values for energy cutoff for photon (PCUT) and electron (ECUT) transport were set to 0.01 and 0.7 MeV, respectively. The phase space files generated during the second stage of simulations were used in the final stage to calculate the three dimensional (3D) dose distributions in water phantom using DOSXYZ software. Results and Discussion The measurement of ROF with 6 cm x 6 cm MLC size in comparison with Sharma et al. (2007) is presented in Figure 2. ROFs were measured for both studies using the same type of pinpoint ionization chamber. In general, both studies showed good agreement of ROF values for field size from 12.5 mm to 45 mm. The highest difference was 2.1% for field size 22.5 mm. X jaws or multi-leaves collimator (MLC) and Y jaws were shaped according to the radiation field size. It was important to verify by inspection that the area outside the central part of the conical collimators was completely shut by the jaws. The X and Y jaws were also ensured to not overlap with the circular collimator opening. This situation could happen when the MLC is stuck and not calibrated properly. The advantage of using smaller jaws size is to avoid radiation leakage at the Linac s head. Contributions to the output factor or scatter factor may arise from two groups of photon beams. Primary photons arise from the Linac s head, while scattered photons arise from collimation devices including Y jaws, MLC, collimator housing and circular cone. ROF are strongly dependent on the field size and the energy of the photon beam. If more photons pass through the collimator opening and reach the point of interest inside the water medium, then the lateral scatter equilibrium effect is said to exist, which arises from the collimate field size. As the diameter of field size increases, the primary photons will remain the same, but the contribution from the scattered photon from the cone size differs. In small field dosimetry, less contribution of scattering radiation from the medium (water), called the phantom scatter, is observed. The effect of phantom scatter on increased field size was also explained in Khan (1994). The explanation is general and agreed with what happens in the small field size dosimetry. The effect of field size on the scatter factor due to phantom scatter alone is significant as long as 24

6 ROF the distance between the point of measurement and the edge is shorter than the range of the lateral scatter electron produced. When the certain distance is reached, there is no further increased in the scatter factor caused by the phantom scatter. When the field size is reduced below the level required for lateral scatter equilibrium, the scatter factor decreases rapidly Cone Diameter (mm) Measurement Sharma et al Figure 2: ROF vs. Radionic cone diameter measured using pinpoint ionization chamber. Figure 3 illustrates our ROF results compared to that of the study done by Shepard (2009) which measured ROF using BrainLab assembly holding the cones and circular cones. The graph shows a small variation difference between both studies for field size from 17.5 mm to 30 mm. Stereotactic radiosurgery beams exhibit a sharp decrease in the output with decreasing field size (Shepard, 2009). It is obviously observed in Figure 3 that ROF of small fields showed strong field size dependence with rapidly decreasing ROF as the field diameter decreased (Sharma et al., 2007). This is due to the rapid decrease in the primary photon for field size smaller than lateral electron range, where the lateral electron equilibrium no longer exists. Figure 3 also reveals that ROF of BrainLab (BrainLab, 2012) and Radionic circular cones were close to each other (within 2%) for every cone diameters. The slight variation in the ROF of the SRS circular cones from these two manufacturers may be due to the difference in the physical dimension and material of the cone (Sharma et al., 2007). Radionic s collimator housing geometry was longer compared to the collimator housing of BrainLab, which caused the collimation of field to be closer to the isocenter. The distance from the isocenter to the 25

7 ROF lower end of the mounted collimator (with gantry in 0 degree angle) was 230 mm. It allows for more primary radiation and scattering radiation coming from the circular cone collimator to reach the detector and reduce the scattering radiation produced by the interaction with air because the air gap between the surface of the circular cones and the isocenter was reduced. Khelashvili et al. (2012) reported that for small fields, the x-ray target is partially occluded by the field boundaries, due to the divergence of the small fields. In effect, the extrapolations of the openings toward the target converge slightly above the target position. Therefore, the projection of the field opening at the position of the target is smaller than the focal spot size and a partial occlusion of x-ray source occurs. If part of the target is occluded, the geometrical penumbras from opposite edges of the fields are overlapped, resulting in less ROF value in comparison to the bigger field size, in which the entire focal spot on the target is viewed by the detector. Hence, this occlusion effect is strongly dependent on the design of the collimator housing with the circular cones and also the divergence of the small field to the target Cone Diameter (mm) Measurement Shepard 2009 Figure 3: ROF vs. cone diameter of different SRS cones manufacturers Figure 4 shows the measurement of ROF with different sizes of multi-leaves collimator (MLC) opening. The graph shows the variation of ROF with circular cone field sizes, from 5 mm to 45 mm, measured using pinpoint ionization chamber with different MLC sizes. From Figure 4, it is shown that the ROF values for 5 cm x 5 cm MLC size was consistently low compared to ROF values of 6 cm x 6 cm MLC size except for the smallest cone, 5 mm. The 26

8 ROF ROF values of 6 cm x 6 cm MLC size were 1.06% higher than ROF of 5 cm x 5 cm MLC size. Smaller MLC size caused larger amount primary beams to pass through. It also reduced the production of scattered photon by limiting the collision between the primary beam to MLC and Y jaws inside the Linac s head before passing through the circular cone and reaching the target area cm x 6 cm 5 cm x 5 cm Cone Diameter (mm) Figure 4: ROF for circular cone diameters with diffrent MLC size Further comparison of ROF for circular cone field sizes measured using three different types of ionization chambers was plotted in Figure 5. Generally, ROF values increased with field sizes. From Figure 5, it is observed that ROF measured using pinpoint ionization chambers showed small deviation of difference compared to ROF calculated using the MC method. ROF values measured using the pinpoint chamber were consistently lower than those measured using A14 microchamber for all Radionic radiosurgery cones sizes, except for the 5 mm cone. However, ROF measured using A12 Farmer type ionization chamber showed large differences compared to those measured using both pinpoint and A14 microchamber for all field sizes. The differences range from 35% to 80%, except for the largest circular cone field size, 45 mm (3.9% difference). This was because in smaller field, less scattering photons were produced due to fewer lateral scattering from the primary radiation. As a result, fewer ion pair produced and small amount of charge was collected by the ionization chamber. For 27

9 ROF ionization chambers that have larger active volume such as A12 Farmer, the charge collected by the ionization chamber was very small and thus leading to the volume averaging effect Cone Diameter (mm) MC PP A12 A14 Figure 5: ROF measured using different types of ionization chambers Table 1: Results of ROF measured using different types of ionization chambers and ROF calculated with MC A14 A12 PP MC Cone (mm) ROF ROF ROF ROF

10 Table 2: The percentage deviation values of ROF measured using A14, A12 and pinpoint ionization chambers compare with MC-calculated A14/ MC A12/ MC PP/ MC Cone (mm) % % % A comparison of ROF for circular fields measured using different types of ionization chamber detectors and MC-calculation is summarized in Table 1 and Table 2. The results of ROF measured using A12 was too small for 5 mm field size compared to both A14 and pinpoint measurement. ROF measured using A14 microchamber compared with MCcalculation, with less than 7 % deviation for all measurements except for 5mm cone size. ROF measured using pinpoint ionization chamber showed a good agreement with MCcalculation, with less than 5 % deviation for all measurements. Conclusion ROF measurement must be done carefully than the usual care for small field sizes dosimetry. Smaller field can cause greater error. This study indicates that the absence of lateral electronic equilibrium due to small size circular cone affect the result of ROF measured. Besides that, other factors such as the volume averaging effect due to the detector itself, energy dependence and misalignment of positioning the detector rightly at the central axis of the beam during ROF measurement can contribute to ROF errors. Even though the detector was considered as small, the size was probably large enough for small beam dosimetry. 29

11 From our study, pinpoint ionization chamber gave the most reliable result for ROF for field size from 5 mm to 45 mm compared to other ionization chambers which were A14 microchamber and A12 Farmer type chamber. The results were also supported by the agreement with MC-calculation and previous measurements conducted by Shepard (2009) and Sharma et al. (2007). Therefore for measurement of ROF for SRS circular field size, pinpoint ionization chamber was suggested due to its accuracy. Acknowledgements The authors would like to thank all oncologists, physicists and therapy radiographers in the Department of Nuclear medicine, radiotherapy and oncology of the Hospital Universiti Sains Malaysia and also all supporting staff in Medical Radiation Programme of School of Health Sciences for their cooperation and help. In addition, we would like to express our gratitude to Dr. Ahmad Lutfi Yusof for the MC simulations. References Abdul Aziz, M.Z., Salikin, M.S., Yusoff, A.L. and Abdullah, R. (2011). Monte Carlo simulation of electron beam in 3D water phantom. World Academy of Science and Technology, 60: Abdullah, R., Nik Idris, N.R., Zakaria, A., Yusoff, A.L., Mohamed, M., and Mohsin, N.I.(2015). Measurement of dosimetric parameters and dose verification in stereotactic radiosurgery (SRS). Jurnal Sains Kesihatan Malaysia, 13(1): BrainLab Physic. (2012). Technical Reference Guide Rev USA: BrainLab. Khan, F.M. (1994). The Physic s of Radiation Therapy. Second Edition. USA: William and Wilkins Khan, F.M., Gibbons, J., Mihailidis, D., and Alkhatib, H. ( 2011). Khan 's Lectures: Handbook of the Physics of Radiation Therapy. USA: Lippincot Williams and Wilkins. 30

12 Khelashvili, G., Chu, J., Diaz, A. and Turian, J. (2012). Dosimetric characteristics of small diameter BrainLab cones used for stereotactic radiosurgery. Journal of Applied Clinical Medical Physics, 13(1): Sharma, S.D., Kumar, S., Dagaonkar, S.S., Bisht, G., Dayanand, S., Devi, R., Deshpande, S.S., Chaudry, S., Bhatt, B.C. and Karman, S. (2007). Dosimetric comparision of linear accelerator-based stereotactic radiosurgery system. Journal of Medical Physics, 32(1): Shepard, D. ( 2009). Quality assurance in stereotactic radiosurgery and fractionated stereotactic radiotherapy. AAPM 51st Annual Meeting. California, July. 31