RapidArc plan verification using ArcCHECK™ Dr Craig Edwards University Hospital of North Staffordshire The Cancer Centre, Stoke-on-Trent, Staffordshire, ST4 6QG. England

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

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

IMRT IMRT = Intensity Modulated RadioTherapy Modulate incident photon beam Highly conformal technique Mould dose around structures Spare radio-sensitive structures Increase delivered dose

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: 0 - 3 cm/s Dose rate: 0 - 600 MU/min

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 ?

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!!

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

Performance Tests Response reproducibility Response linearity with dose Dose rate dependence Consistency of chamber response with orientation Stability of calibration with time

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)

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 129.71 cGy Comparison with Eclipse - 0.15 % Standard deviation: 0.13 cGy CV: 0.10 %

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

Response linearity with dose Results Comparison with Eclipse: max diff 0.3% (@10 MU) above 100 MU, diff -0.1% When normalised against 100 MU response; Standard deviation: 0.38 cGy CV: 0.29 %

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)

Dose Rate Dependence Results Standard deviation: 0.70 cGy CV: 0.54 % Comparison with Eclipse; Maximum difference -1.4% (@100 MU.min-1) Difference at 600MU.min-1 -0.1% Standard deviation: 0.70 cGy CV: 0.54 %

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 129.52 cGy Standard deviation: 0.68 cGy CV: 0.50 %

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 129.72 cGy Standard deviation: 0.23 cGy CV: 0.18 %

Clinical tests Verification of IMRT treatment Verification of RapidArc treatment

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%

Verification of IMRT Results Gamma evaluation pass rate 98.5 % Point dose evaluation measured -0.2 %

Verification of RapidArc Method Same as IMRT except there is only 1 field!!

Verification of RapidArc Results Gamma evaluation shows 99.4 % pixel pass rate Point dose evaluation measured -0.7 %

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