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S. Marcié Unité de Physique Centre Antoine-Lacassagne (CAL) Nice. Rth de haute technicité 1- La RCMI. Historique. Pourquoi cette technique ? Pourquoi maintenant ? Multilames (depuis 10 ans), Pilotage des accélérateurs, Puissance des systèmes de calcul. x2. x2. x2. x2. Méthodes.
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S. Marcié Unité de Physique Centre Antoine-Lacassagne (CAL) Nice Rth de haute technicité1- La RCMI Physique/CAL 2012-2013
Physique/CAL 2012-2013 Historique Pourquoi cette technique ? Pourquoi maintenant ? Multilames (depuis 10 ans), Pilotage des accélérateurs, Puissance des systèmes de calcul
x2 x2 x2 x2
Physique/CAL 2012-2013 Méthodes Statique dite « step and shoot » Dynamique dite « sliding windows » ou dynamique Arcthérapie modulée
Physique/CAL 2012-2013 Matériels Accélérateur avec collimateur multilames, Système de calcul avec logiciel inverse, Moyens de contrôle de qualité
Physique/CAL 2012-2013 Le collimateur multilames du Primus
Physique/CAL 2012-2013 Several MLC
Physique/CAL 2012-2013 Several MLC MLC jaw Y jaw Y jaw X Y backup MLC MLC jaw X ELEKTA replacement Of the upper jaw VARIAN additionnal collimator SIEMENS replacement of the inferior jaw - Arc movement- C.Mubata, SFPH Tours 99)
Contrôles de qualité Les collimateurs multilames Physique/CAL 2012-2013
Physique/CAL 2012-2013 MLC and IMRT: "overtravel" Profiles (Step-Shoot) 4 segments and 3 levels axis Intensity levels 4 Leaves displacement right left 3 2 1
Physique/CAL 2012-2013 MLC and IMRT: "overtravel" Complex profiles => "overtravel" about 1/2 width of the max field Dynamic mode
Physique/CAL 2012-2013 MLC and IMRT: leaves width Study of the homogeneity and the % dose in OAR in IMRT (Shepard et al., Med.phys 26 1212-1221) e => better homogeneity in the target, dose reduction in OAR
Physique/CAL 2012-2013 MLC and IMRT: leakage / transmission In Step and Shoot, segments number: In SS, MU number x 3 / 3DCRT => leakage and transmission x 3 => Transmission /leakage MLC < 2% (IEC 601-2-1, 1998) UM If transmission MLC of 1%: Ttotale = 9 seg x10 MU x1% = 0.9 UM = 1,8% total dose 50 8 9 40 6 7 30 4 5 20 2 3 10 1
Physique/CAL 2012-2013 MLC and IMRT: leakage / transmission In SW, during irradiation, each field point has more time under the MLC : about 4 to 6% of the total dose totale is due to the transmission => better modelisation by the TPS
Physique/CAL 2012-2013 MLC and IMRT: geometry (1) The segmentation algorithm suppose the independance of adjacent leaves => the MLC must atminimum be focused in the direction of adjacent leaves Idealy, the MLC is also focused in the direction of opposite leaves MLC double focused
Physique/CAL 2012-2013 MLC and IMRT: geometry (2) non focused leaves and rounds (cste penombra): - Variation of the transmission along the leaf (=> offset of 1 mm) - Correspondance light/irradied field ? => corrections (files/offset) Offset de 1mm LoSasso et al., Med. Phys. (25) 1998
Physique/CAL 2012-2013 Effet « tongue and groove » ou tenon-mortaise
Physique/CAL 2012-2013 MLC and IMRT: geometry (3) Shape "tongue-and-groove" of adjacent leaves ( to limite leakages) gives an under-dosage at the junction => the segmentation can avoid this effect 1st segment 2nd segment tongue groove Under-dosage 50%
Physique/CAL 2012-2013 MLC and IMRT: accuracy of leaves Static mode: Superposition of segments => lines of junction on the edges of segments Accuracy of the positioning => affects the dose distribution on the field edges Variation of "Output Factor" OF for small fields => influence on the dose/ small segment
Physique/CAL 2012-2013 MLC : accuracy of leaves (SS) => Step-and-shoot: MLC accuracy < 2 mm (field > 1cm) B. Rhein, DKFZ, Heidelberg
Physique/CAL 2012-2013 Multileaf collimator
Physique/CAL 2012-2013 MLC and IMRT: accuracy of leaves Dynamic mode: Positioning errors of leaves => errors on the dose over the field Prescribed position Real position Real dose different From prescribed dose gap
Physique/CAL 2012-2013 MLC and IMRT: leaves position Tolerance on the leaf position gap => dynamic: accuracy MLC < 0.2 mm LoSasso et al., Med. Phys. (25) 1998
Physique/CAL 2012-2013 "classics" tests in 3DCRT: Perpendicularity of leaves Rotation axis of MLC, of gantry Penumbra Gravity effect ... MLC: quality control
Physique/CAL 2012-2013 Ratio dose under the MLC closed, dose for a 10x10 cm² field at the reference depth (film, chamber) Leakage and transmission 2.0 On Primus 10 cm 1.5 1.0 0.5 - tolerance: 2% - Once a year
Physique/CAL 2012-2013 900 800 Dose for 1200 UM 700 600 500 Profile axis 400 300 Ideal profile 200 100 Measured profile 0 -50 -40 -30 -20 -10 0 10 20 30 40 50 in plane (mm) Influence on dose profile of misalignment of some leaf
Physique/CAL 2012-2013 Example of DKFZ, Siemens - tolerance +/- 1mm in SS - once a month Leaves calibration (SS) On Primus Dose 100% 50% Distance (mm) 3 x 40 cm²
Physique/CAL 2012-2013 Real/prescribed position (SS) Superposition of 2 x1/2 fields (junction line) - tolerance +/- 1mm in SS - twice a week On Primus Prescribed position
Physique/CAL 2012-2013 5 fields of 0,4 x 40 cm² at different axis distances - The leaves must be aligned - The 5 lines must have the same width - Twice a week Accuracy of leaves position (SS) On Primus 0.4 x 40 cm²
Physique/CAL 2012-2013 "Continous stripe test": To verify the leaves position, the speed, the acceleration /deceleration - - if v=cste => uniform dose - twice a week Quality control of the MLC (SW) Volontaries errors - 0.5 mm - 0.2 mm Lame gauche UM 1 mm Lame droite + 0.2 mm + 0.5 mm position LoSasso et al., Med. Phys. (25) 1998
Physique/CAL 2012-2013 Speed stability The adjacent leaves move at different speeds but cste => bands of different but uniform intensities - uniform dose profiles if speed is stable Quality control of MLC (SW) MU Left leaf N°3 Right leaf N°1 N°2 N°2 N°1 N°3 position LoSasso et al., Med. Phys. (23) 1996
Physique/CAL 2012-2013 Sliding window: To verify the gravity effect + position + speed + acceleration For 4 angles of the gantry, a field of 0,5 x 25 cm² move uniformly - the dose and the dose profiles must be uniform for the 4 positions Quality control of MLC (SW) Acceleration effect Stop of leaves can be previous (effect of the acceleration and deceleration) LoSasso et al., Med. Phys. (25) 1998
Autres points Physique/CAL 2012-2013
Physique/CAL 2012-2013 Small number of MU - Small number of MU : flatness, symetry, dose = f(MU) On Primus
Dosimetry of Small Monitor Unit Segments in IMRT Delivery • Deviation from Mean Dose for 30 Fraction Segment for 1 MU. • Deviation from Mean Dose for 20 Fraction Segment for 2 MU. • Deviation approaches < 0.5% for higher (>5) MU Physique/CAL 2012-2013 Courtesy by Fox Chase Cancer Center, Philadelphia
Verification of homogeneity and symmetry of LOW MU Results for 1-10 MU • Beam Flatness (PRIMUS): • 1.48-1.78% (in-plane) • 1.45-1.7% (cross-plane) • Symmetry: • 0.24-1.26% (in-plane) • 0.53-1.08% (cross-plane) Physique/CAL 2012-2013 Courtesy by Fox Chase Cancer Center, Philadelphia