Sudan Academy of Sciences, SAS Atomic Energy Council Evaluation of asymmetry collimator for a new generation telecobalt machine By Eshraga Elfaki Suliman A thesis Submitted to the Sudan Academy of Sciences in Partial Fulfillment of the Requirements For MSc in Medical Physics Supervisor: Mr. Mostafa M.Elhasan Jan 2008 Sudan Academy of Sciences Atomic Energy Council Examination Committee Name Title Signature Dr. Farouk Idris Habbani External Examiner Cr\dS\ ^3 ' Mr. Ammar Mohammed Academic Affairs Elamin Representative MvJ^-^4 Mr. Mostafa M.Elhasan Supervisor ^^ Jan 2008 Acknowledgements Sincere thanks To Mr.Mostafa M.Elhasanfor seeding the concept of this study. To all staff at the Department of Medical Physics at the Radiation and Isotopes Center of Khartoum (RICK) who have been generous in their help and support. DEDICATION To my Mother Father Sisters and Brothers who have shared their love and support. ABSTRACT A new model of the telecobalt unit, Theratron Equinox-100, (MDS Nordion, Canada) equipped with a single 60 degree motorized wedge (MW) and upper (X) and lower (Y) asymmetric jaws have been evaluated. Symmetrical jaws were commissioned in Pinnacle3 (Philips), the 3D treatment planning system (TPS). The profiles and central axis depth dose (CADD) were measured with Wellhofer blue water phantom for various field sizes using 0.13 cc thimble ionization chamber (Scanditronix Wellhofer, Uppsala, Sweden) and the data were commissioned in Pinnacle3.These profiles and CADD for symmetry jaws were compared with asymmetry jaws for various field sizes. Also beam profiles for 5x5, 10x10 and 20x20 cm2 for symmetry and asymmetry field sizes at 5 and 10 cm depths measured with 2D-Array (two dimensional detector array with 729 vented ionization chambers with a size of 5x5 mm2, PTW, Germany), are compared. A homogeneous phantom generated in Pinnacle .The dose calculated in this phantom at 10 cm depth for various field sizes of symmetry and asymmetry jaws using collapse cone convolution (cc convolution) model with a grid size of 4 mm , and compared with measured dose in a water phantom at 10 cm depth with a 0.6 cc thimble ion chamber FC- 65-G and DOSEl electrometer for field sizes of 5x5, 10x10 and 20x20 cm2 using IAEA dosimetry protocol TRS-398.The variation of measured and calculated doses at 10 cm depth were within 1%. 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A^JI a,iV<ml .ii\\\A\ JUJVI ciilj sjJiUlaJ! *J*3U! JjLJJ 2L.Uull dlL 729 t>° AjjS-a 5ijij-a-a jc Sjbc- jA j (2D-ARRAY) i-i*il j^SI (j* j^.1 p jj ahVimj aj Lijji j '10x10 '5x5 JJO'VI djli ^Ol*JI JjSail <UJLJI jjj-all IJJJS aj i_Lil£ll Ij^j t jjb Aijc. .SjJaLlLall JJ&J ajiaUloll A_12LIA*1I JjlaJ] 4 U..n\lj ajui 10 J 5 l>aaj' ^ 2ajal 20x20 3 Li..ll AjcUJiVI *cjaJI L-JL^ aj (Pinnacle TPS) d*fi\ J&$ U\U\'\\\ ^UJJ fhVi„ib OjiaUlall JJC.J BjiaLiLajl 4-iOLul (Jjivll ajoi^Q (J-a*jl -^ O^LAJJ aj (_jJI (jjajLvLajl Jauijll ,_i pjjl (j-a (jJVjJl AljC. al.YNUulj ^Lall TOjil! ^ L^-uAjS aJ ,^1 lilli *-« j»J^I *^* Ajjlia aJ (j^tj .^Lall ^ 4^laJI ^oUJiVl ^^j^Jl ^Lul (TRS-398) JAJJJJJI t^j (FC-65-G) i—kjjLaJ! A^mljj 1 g ;1 > D^> aJ ^pl! tdljj P-Lall (ji <uA£a]l \J^L±AA\ AC JZS\ jjj La LJ^ikVI (jl ^?-J 1 1 .0/0l J| JjLalj V ^-10 L^ ^ cPjJ^V L_iLa^ ^ Val^i^V (Pinnacle3 TPS) K'^1 s^uJJ ^ ^^^1' ^ (J^>^j c^ r^ LISTS OF TABLES Table No. Table Name Page No. Table 4.1 Collimator jaws setting to obtain two geometries of asymmetric fields. 50 For measurements, ion chamber was positioned at off-axis distance in the in-plane Oparallel to the gantry arm, directed towards the gantry) or cross-plane (from left to right). Table 5.1 Beam specifications (a) Symmetry %, (b) Flatness % and (c) 60 Penumbra (cm) values in all directions: Comparison between symmetry and asymmetry fields. Table 5.2 Percentage difference in (a) Symmetry % (b) Flatness % values 61 between symmetry and asymmetry fields Table 5.3 Absorbed doses to water for symmetric and asymmetric beams 65 measured at 10 cm depth in the blue water phantom using FC-65-G ion chamber and DOSEl electrometer following TRS-398 protocol from IAEA. Table 5.4 Treatment time calculated in the Pinnacle3 TPS using cc convolution 65 for symmetric and asymmetric beams with different field sizes at 10 cm depth. LISTS OF FIGURES Figure NO. Figure Name Page No. Figure 1.1 Definition of asymmetrical field. It defined by the distance separating 14 each jaw from the intersection (0) of the collimator axis. Figure 1.2 Definition of the beam axis for (a) symmetric, (b) and (c) asymmetric 14 beams. Figure 1.3 Treatment technique for breast cancer using independent collimators. 15 Figure 2.1 Typical dose distribution on the central axis of a megavoltage photon 18 beam striking a patient. Ds is the surface dose at the beam entrance side, Dex is the surface dose at the beam exit side. Dmax is the dose maximum often normalized to 100, resulting in a depth dose curve referred to as the percentage depth dose (PDD) distribution. The region between z = 0 and z = zmax is referred to as the dose buildup region. Figure 2.2 Geometry for PDD measurement and definition. Point Q is an arbitrary 22 point on the beam central axis at depth z, point P is the point at zmax on the beam central axis. The field size A is defined on the surface of the phantom. Figure 3.1 Vectorial framework for the convolution process. Array from the source 38 to the interaction site r" intersects the medium surface at ro. Dose at r is calculating due to terma at all interaction sites r" via the kernel value corresponding to the vector r - r\ Figure 3.2 (a) Convolution by the deposition position point of view, where the dose 40 in successive deposition voxels is calculated due to all interaction voxels, (b) The interaction point of view, where dose throughout the medium is calculated due to successive interaction voxels. Figure 4.1 The Blue Water Phantom tank on the electrical Lifting Table used for 43 CADD and beam profile measurements. Figure NO. Figure Name Page No. Figure 4.2 Compact Chamber CC13. Air ionization chamber, vented through 45 waterproof silicon sleeve and consists of inner and outer electrode made of Shonka C552 with nominal volume of 0.13 cm3. Figure 4.3 FC-65-G, Farmer chamber with active volume of 0.65 cm3. It is a 46 waterproof, vented chamber consists of outer electrode made of Graphite. Figure 4.4 PTW Freiburg 2D-ARRAY Seven 29. A two-dimensional detector array 47 with 729 vented ionization chambers uniformly arranged in a 27 x 27 matrix with an active area of 27 x 27 cm2 with size of 5mm x 5mm. Figure 4.5 Alignment cap mounted on the CC13 ion chamber used to fine leveling 48 of the Blue Water Phantom. Figure 4.6 Collimator jaws setting for symmetric fields where beam axis is 49 identical to collimator axis (Xi=X2 and Yi=Y2). Figure 4.7 Collimator jaws setting for asymmetry fields: (a) At off-axis distance in 51 the cross-plane direction. Xi and Yi were moved to the collimator axis to obtain quadratic asymmetric field. (Xi=Yi=0 and X2=Y2). (b) At off-axis distance in the in-plane direction where Yi moved to the collimator axis to obtain half asymmetric beam. (Yj=0 and X]=X2) Figure 5.1 Central axis depth dose curves for field sizes 5x5, 10x10 and 20x20 cm2 54 with symmetric fields (black lines) and asymmetric fields (red and green lines) measured in the Blue Water Phantom. Figure 5.2 In-plane beam profiles at 0.75, 5, 10 and 20 cm depths for symmetric 56 and asymmetric fields for (a) 5x5 (b) 10x10 and (c) 20x20 cm2 field size measured in the Blue Water Phantom. Figure 5.3 Cross-plane beam profiles at 0.75, 5, 10 and 20 cm depths for 57 symmetric and asymmetric fields for (a) 5x5 (b) 10x10 and (c) 20x20 cm2 field size measured in the blue water phantom. Figure NO. Figure Name Page No. Figure 5.4 Diagonal beam profiles at 0.75, 5, 10 and 20 cm depths for symmetric 58 and asymmetric fields for (a) 5x5 (b) 10x10 and (c) 20x20 cm2 field size measured in the Blue Water Phantom.