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RapidArc plan verification using ArcCHECK™
Dr Craig Edwards University Hospital of North Staffordshire The Cancer Centre, Stoke-on-Trent, Staffordshire, ST4 6QG. England
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The University Hospital of North Staffordshire (UHNS)
Radiotherapy Physics; Moved to the new Cancer Centre in June 2009 Cancer Centre features; Eclipse treatment planning system 4 Varian iX linear accelerators HDR Brachytherapy Unit An Acuity simulator GE lightspeed CT scanner
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IMRT and RapidArc at UHNS
Dynamic IMRT 2005 Commissioned on 600C & 2100C Varian linacs ~ 15 patients per year (2:1 H&N:prostate) Dynamic IMRT 2010 4 matched 2100iX Varian linacs ~ 175 patients per year (8:1 prostate:H&N) have treated uterus, cordoma, breast RapidArc 2011 Attended Varian training course MSc Project Commissioned patient QA system First clinical patient summer 2011
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IMRT IMRT = Intensity Modulated RadioTherapy
Modulate incident photon beam Highly conformal technique Mould dose around structures Spare radio-sensitive structures Increase delivered dose
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Arc Therapy - RapidArc An advanced form of IMRT
Uses 177 control points Gantry arm rotates 358° Three forms of modulation Gantry speed: 0.5 – 4.8 °/s Leaf speed: cm/s Dose rate: MU/min
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Patient QA measurements
IMRT patient Every field’s dose fluence measured and compared Gamma analysis used to evaluate (3mm/3%) Point dose measurement compared with expected (+/- 5%) Arc Therapy Replicate conventional IMRT ?
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System Choice Several available Selection – ArcCheck ArcCheck Delta4
MatriXX / cube Octavius Selection – ArcCheck Light Single cable set-up Absolute dose Potentially 3D DVH mapping looked right - cylindrical!!
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Rotation QA system ArcCheck Cylindrical with diameter of 26.6 cm
Water equivalent 1386 diode detectors Diodes at a depth of 2.9 cm Diodes 1 cm apart Arranged helically Central cavity insert
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Performance Tests Response reproducibility
Response linearity with dose Dose rate dependence Consistency of chamber response with orientation Stability of calibration with time
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Calibration Varian 2100iX linear accelerator
Operating energies of 6 and 10 MV Dose rate 600 MU.min-1 Field size 10 x 10 cm2 ArcCheck centre positioned at isocentre (SSD 86.7cm) 200 MU delivered Dose at detectors 6 MV – 2.47 Gy (1.32 Gy at isocentre) 10 MV – 2.60 Gy (1.54 Gy at isocentre)
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Response reproducibility
Method Irradiated central diode 100 MU, 10 x 10 cm2 field 5 irradiations Record response & compare with planning system Determine coefficient of variation (CV) Results Mean reading cGy Comparison with Eclipse % Standard deviation: cGy CV: %
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Response linearity with dose
Method Irradiated central diode Varied delivered dose from 10 to 500 MU 10x10 cm2 field Record response & compare with planning system Determine CV
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Response linearity with dose
Results Comparison with Eclipse: max diff 0.3% MU) above 100 MU, diff -0.1% When normalised against 100 MU response; Standard deviation: cGy CV: %
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Dose Rate Dependence Method Irradiated central diode
100 MU, 10 x 10 cm2 field 100 to 600 MU.min-1 Record response & compare with planning system Determine coefficient of variation (CV)
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Dose Rate Dependence Results Standard deviation: 0.70 cGy CV: 0.54 %
Comparison with Eclipse; Maximum difference % MU.min-1) Difference at 600MU.min % Standard deviation: cGy CV: %
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Dependence upon irradiated angle
Method Irradiated central diode 100 MU, 10 x 10 cm2 field ArcCheck rotated to each angle Record response Determine coefficient of variation (CV) Results Mean reading cGy Standard deviation: cGy CV: %
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Stability of calibration
Method Irradiated central diode 100 MU, 10 x 10 cm2 field Response recorded every 20 mins for 2 hours Determine coefficient of variation (CV) Results Mean reading cGy Standard deviation: cGy CV: %
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Clinical tests Verification of IMRT treatment
Verification of RapidArc treatment
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Verification of IMRT Method ArcCheck CT scanned
Image used as template for verification plan Export the treatment plan to ArcCheck image Export re-calculated fluences MapCheck Expected dose fluence compared to measured for every field and the sum of all fields Fields evaluated using gamma calculation with setting of 3mm/3%
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Verification of IMRT Results Gamma evaluation pass rate 98.5 %
Point dose evaluation measured -0.2 %
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Verification of RapidArc
Method Same as IMRT except there is only 1 field!!
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Verification of RapidArc
Results Gamma evaluation shows 99.4 % pixel pass rate Point dose evaluation measured -0.7 %
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Conclusions The ArcCheck dosimetry system;
is simple and quick to set-up has a quick and efficient software solution to calibrate diodes in absolute dose detectors demonstrated response characteristic which would make it a suitable dosimetry tool is an excellent solution for the verification of both IMRT and RapidArc treatment
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