Reducing VMAT Patient Specific Machine The Christie

Transcription

1 Reducing VMAT Patient Specific Machine The Christie Phil Whitehurst Christie Medical Physics & Engineering Wilmslow Road Manchester M20 4BX

2 Background Outline Christie IMRT/VMAT Techniques The Problem Why do we need to reduce machine verification. What is VMAT verification actually for? What verification do we actually need per patient? Examples from the Christie Outcomes

3 Background Techniques Christie VMAT IMRT Examples (Main Sites) IMRT Breast IMRT Lung IMRT Prostate IMRT (Art. Hips) Sarcoma IMRT Whole CNS IMRT VMAT H&N VMAT Prostate VMAT Gynae VMAT Lung VMAT SABR VMAT Bladder (Std * Adapt) VMAT Rectum VMAT Brain VMAT

4 Background - The Problem Christie IMRT numbers

5 Background - The Problem Christie IMRT numbers Predicted 2016 IMRT = 3900/yr (Includes Breast IMRT) Inv. IMRT = 2300/yr End of 2016/17 Predicted 90% of the radical activity will be IMRT

6 Background Set the Scene Christie VMAT The Christie Configuration Machines (Only 6 & 10MV) Elekta Versa HDs Elekta Precise with Agility Head Elekta Precise with MLCi and MLCii (old machines) VMAT no longer planned on MLCi Varian 2100 HD MLC Delivery/R&V Mosaiq 2.62

7 VMAT Background Set the Scene Christie VMAT All the techniques are standardised (heavily Scripted/Automated) (very large staff group ) Objective/constraints Arc length Number and time of the Arc (this has a significant effect on the modulation). Arc spacing and grid resolution controlled

8 Background Set the Scene Christie VMAT VMAT Majority of the VMAT plans are optimised at 4 deg spacing & 3mm ACC grid resolution. 4Deg/3mm vs 2 deg/2mm Negligible affect on verification (Chamber/Film/Delta4/Array).Except : SABR (/FFF delivery) Optimise at 4Deg/2mm ACC grid Interpolate 2 Deg Lung SABR Final Calc CCC (this did influence Max 0.5cc/1cc reporting) Brain VMAT 2Deg spacing / 3mm ACC

9 What is VMAT verification actually for?

10 To check the TPS model IMRT / VMAT uses transmitted dose and beam penumbrae within beams, plus smaller fields TPS needs to be more accurate to model these than for conventional RT IMRT / VMAT can be more demanding for inhomogeneity corrections and near-surface calculations VMAT needs dose rates / MLC leaf speeds included in TPS VMAT planning needs sufficient gantry angle sampling in TPS TPS needs to divide deliveries sensibly between arcs

11 To check machine delivery accuracy VMAT accuracy can depend on: Accurate MLC calibration Control of dose rate and gantry speed as well as MLC position Interpretation of TPS plan by linac how does it handle high leaf/gantry speeds? Accurate and rapid information transfer between control software and delivery system

12 As a second MU check Every plan needs an independent MU check Simple hand calculations or spreadsheet methods don t work for VMAT Requires measurement or more complex software calculation

13 To check plan transfer In 2005 in New York, a H&N patient was treated for 3# with open fields instead of sliding window MLC s (and died 2007). TPS software hung while transferring the prescription MLC positions were not sent Patient received around 14Gy per #

14 As an End to End test Sometimes we need to test the system all the way through from CT imaging to planning to delivery to verification tests every part of system

15 Audit / Trials QA Independent outside check of your system Boosts confidence Increases safety Examples: IPEM Rotational Audit VMAT trials QA Between centre audits

16 What verification do we actually need per patient?

17 Per patient? To check the TPS model Commissioning & initial patient cohort To check machine delivery accuracy Commissioning & routine check As a second MU/independent TPS check To check plan transfer As an End to End test Commissioning & occasional check Audit / Trials QA Occasional

18 Proposed framework for stopping per-patient verifications 1 Documented analysis of verification results per site /class solution showing your system is stable Risk assessment demonstrating all risks have been considered and mitigated Every patient needs still needs a second MU check VMAT compatible secondary dose check software Every patient still needs a transfer check - software check needed for VMAT

19 Proposed framework for stopping per-patient verifications 2 Methods for detecting unusual / abnormal plans. E.g. normal MU ranges, complexity indices. Regular checks of the whole process to cover all linacs Consider tightening up frequency of other QC checks Due to UK recommendations in vivo dosimetry (not diodes) or documented justification for not performing in vivo dosimetry Everything documented and justified

20 Examples So.. We ve only stopped patient specific Machine Verification for H&N VMAT and Prostate VMAT

21 Examples verification results analysis Prostate VMAT(2014) Analysed >700 patients. Only failures >550MU, so MU range (single arc)

22 Patient analysis

23 Examples verification results analysis Prostate VMAT(2014) Analysed >700 patients. Only failures >550MU, so MU range (single arc) Complexity indices investigated, but no correlation found. Literature reviewed (tried published matrices) Extending the work to our own matrices.

24 Examples verification results analysis H&N VMAT Pre-Agility we sometimes saw failures (e.g. Naso s) Re-analysed 693 patients treated on 5 Agility linacs between Jan 2013 and July Handful of failures fine when re-analysed or re-measured. All passed at 4%/4mm (>95%). High point dose reference within 2% Normal MU ranges established (<800 MU dual arc)

25 Examples So what about the independent MU check?

26 Mobius

27 Mobius

28 Examples How do we check the plan consistency and the electronic plan transfer to Mosaiq? AutoLock & transfer check

29 AutoLock Software Automate the things that can be checked by a computer. Creates a properties file that is used to check standard parameters Standardisation Allows the user/checker to concentrate on other checks Reduced plan rejections improved efficiency

30 Transfer check software Compares TPS values (Autolock properties file) with prescription file exported from R&V system Pre-delivery check Checks all the machine settings (including every MLC and CP MU) Skip Pass Warning Fail Critical Check was not made Parameters match Parameters need to be reviewed e.g. names do not quite match Parameters do not match and need to be authorised Parameters do not match, no authorisation will be allowed

31 Transfer check software

32 Examples regular system checks Routine verifications per linac Concluded that the current Machine QC frequency and test acceptable Still verify other clinical sites frequently (all energies) Monthly test of Prostate & H&N

33 Prostate >550MU (60Gy/20# Prescription) Prostate with Nodes (very low numbers) H&N >800 MU (Dual Arc) Any Exclusions Nasopharynx (increased complexity) Trial Patients (e.g. ARTFORCE integral PET boost)

34 Justification Critical to have it all in place & accessible Reports Risk assessments Justifications Clear information on which plans must still be physically verified

35 Outcomes At the Christie saving us approx. 25 VMAT patient verifications per week significant reduction! Allowing new VMAT techniques to come online Time spent on more productive and useful work NHS - helps funding pressures (CiP).

36 What Next. Verifications for Gynae and Rectum to be reviewed aim to stop verification by Apr 17 Free up verification slots for the new sites Evolve AutoLock to check more parameters Improve the robustness of Mobius (i.e. a backup server purchased failover). What about transit dosimetry options? Investigate Elekta solution.

37 Conclusions It is possible to stop per patient verifications for selected sites Need to have all the framework steps in place Document everything!

38 Any Questions? Thanks to Linnéa Freear Alistair Pooler Geoff Budgell Rest of Christie team.