IMRT QA: Point Dose Measurements or 2D Array?

Transcription

1 QA for IMRT IMRT QA: Point Dose Measurements or D Array? General Linac QA for IMRT MLC checks Dose linearity at low MU Patient-specific measurements Performed using various methods Purpose is to verify dose delivery Andrew Hwang, PhD Necessity of IMRT QA Potential error sources Planning R&V Delivery accuracy of dose delivery should be documented by irradiating a phantom containing a calibrated dosimetry to verify that the dose delivered is the dose planned. ACR-ASTRO practice guideline 1

2 Objective Describe our experience with two different IMRT QA methods Compare the two methods Discuss ideas about interpreting results Phantom Measurements Two plastic phantoms Cylindrical phantom Slab phantom Phantom size chosen to approximate treatment site Dose measurements Central ion chamber Peripheral MOSFETs Procedure Copy treatment plan to CT scan of phantom Adjust isocenter position as needed Determine planned dose to ion chamber / MOSFET s Deliver plan Compare measurement w/ expected dose Results N=77 Mean deviation -0.1±.0% (ion chamber) 1.3 ±.3% (MOSFET) Consistently able to achieve our goal of 5% tolerance.

3 Phantom measurement pros and cons Advantages End-to-end check Easy to interpret results Disadvantages Checks limited portion of field Harder to set-up May be difficult to obtain good results in high gradient regions D Arrays Popular tool for performing IMRT QA Available from multiple vendors Sun Nuclear MapCheck 157 diodes, 7 mm spacing 3x6 cm active area PTW seven9 79 ion chambers 7x7 cm area IBA Matrixx 100 ion chambers cm area MapCheck Array Questions Pass / Fail criteria? Best way to use array? Rotating with gantry vs. stationary Field by field vs. composite What information can be determined from data? array Mounting frame 3

4 Gamma Analysis Method used for comparing two distributions Combines dose difference and distance to agreement (DTA) γ Γ ( r ) = min{ Γ( r, r )} [ r] m ( r, r ) m c = r m c ( r, r ) δ ( r, r ) + m c m c d M D M c Pass/Fail criteria 3%/3mm is the most common criteria chosen for gamma analysis (Nelms & Simon, 007) AAPM TG-119 (IMRT commissioning) recommends a 90% pass rate for 3%/3mm for per field analysis Stricter criteria may be more sensitive to dosimetric / MLC errors DTA criteria Dose criteria Value less than 1 = satisfactory result See Low, Harms, Mutic, and Purdy, Med Phys, vol. 5, Results Pass rates dependent on resolution of planar dose calculation Switching fluence (and dose grid) resolution from 4 mm to mm improved average passing rate Average gamma (3%/3mm) pass rates 97.1±.7% Composite pass rates did not correlate with field by field measurements. Average pass rate (3%/3mm) was 96.8±3.9% Gamma pass rates did not vary by beam angle Composite vs. Individual Fields Individual Field Pass Rate (%) Mean R = Minimum R = Composite Pass Rate (%) Pass rate for composite plotted vs. individual field pass rates Composite 3mm/3% pass rate: 96.8±3.8% 4

5 1 5 9 Effect of Gantry Angle Data from one linac plotted here ANOVA shows differences not significant Control Plots Plot of process vs. time Have control limits ( specifications) Based on short-term variation x± 3σ short n Proper design needed for maximum utility Help to identify special cause variation but does not identify cause Control Plots KD Potential error sources Plotted data by machine Subgroup by patient Red lines show control limits Pass Rate 3%/3mm (%) Patient # 9 8 KD Planning R&V Delivery Measurement & Analysis Standard Deviation (%) Patient # 5

6 Interpreting Results Unclear relationship between gamma index pass rates and 3D dose distribution MLC errors may be detected by D array measurements (Yan et al, 009). Systematic MLC errors (1- mm) result in significant dosimetric changes (Mu et al, 008) Gamma analysis of individual field measurements is insensitive to inaccuracies of overall plan (Kruse, 010) Gamma pass rates do not predict clinically relevant errors (Nelms et al, 011) Array Pros and Cons Advantages Check larger portion of the field Potentially able to detect atic MLC position errors Simple to use, less user dependent Quicker to set-up Disadvantages Difficult to interpret results Time Comparison Phantom Array Plan Preparation 5 minutes 5 minutes Set-up / take down time 0-5 minutes minutes Delivery Time minutes minutes Analysis / Documentation 5 minutes 10 minutes Total (1 plan) minutes minutes Conclusions Compared and contrasted two methods to perform patient-specific IMRT QA Most (~75%) IMRT QA currently done with D array Results demonstrated that our IMRT delivery is accurate Methods have different strengths and weaknesses and provide different information Tests are screening and not diagnostic Total (5 plans) minutes minutes 6

7 Acknowledgements Jeff Bellerose, Angelica Perez-Andujar, Devan Krishnamurthy, Josephine Chen, Ping Xia. References G Mu, E Ludlum, and P Xia, Impact of MLC leaf position errors on simple and complex IMRT plans for head and neck cancer, Phys Med Biol, vol. 53, pp , 008. JJ Kruse, On the insensitivity of single field planar dosimetry to IMRT inaccuracies, Med Phys, vol 37, pp , 010. BE Nelms, JA Simon, A Survey of planar IMRT QA analysis, J Appl Clin Med Phys, vol. 8, pp. 1-15, 007. BE Nelms, H Zhen, and W Tome, Per-beam IMRT QA passing rates do not predict clinically relevant patient dose errors, Med Phys, vol 38, pp , 011. G Yan, C Liu, TA Simon, L Peng, C Fox, JG Li, On the sensitivity of patient-specific IMRT QA to MLC positioning errors, J Appl Clin Med Phys, vol. 10, pp , 009. GA Ezzell, JW Burmeister, et al, IMRT commissioning: multiple institution planning and dosimetry comparisons: a report from AAPM task group 119, Med Phys, vol 36, pp ,