Plan deliverability metrics study and outcome

Plan evaluation metrics

1. Analysis of Plan Deliverability Metrics and their impact on Patient Specific Quality Assurance Aayushi Paul (Resident Medical Physicist) November, 2023

2. Outline Background Patient Specific Quality Assurance Complexity Metrics Deliverability Metrics Objectives Treatment Planning Portal Dosimetry Gamma Passing Rate Analysis Defined Deliverability Metrics

3. Background

4. Patient Specific Quality Assurance IMRT QA verification is an important process employed to check the accuracy of IMRT plan dose calculations. Measurement based Independent Monitor Unit/ dose calculation Portal Dosimetry

5. Patient Specific Quality Assurance contd. A plan evaluated against the gamma index will be considered clinically acceptable if the percentage of all points with minimum gamma index of less than 1 is greater than a threshold value, usually 95% or 97%.

6. Complexity Metrics Based on identified sources of errors in VMAT delivery (MLC leaf position, gantry rotation, beam stability) and TPS calculation (MLC modelling, off-axis and irregular field modelling, output factors for small field sizes).

7. Deliverability Metrics Focus on the machine capability of delivering the treatment as planned due to the variation of mechanical (gantry, MLC) and dosimetric (dose rate, MU) machine parameters. The advantage of these metrics is that they are easily interpreted due to their physical meaning.

8. Objectives Evaluation of the potential of addition of subsequent arcs to a VMAT treatment plan optimized for single photon energy, vertically mounted linear accelerator to reduce complexity, enhance deliverability, and account for dosimetric variations without sacrificing overall plan quality for hypo-fractionated (i.e. a high dose per fraction) treatment delivery. Correlation of machine deliverability metrics with γ passing rate (GPR) has been conducted to predict a decrease in dosimetric uncertainties for treatment plans with greater number of arcs.

9. Treatment Planning 11 patients who had previously undergone Short Course Radiation Therapy in rectal cancer. 25Gy in 5 fractions with additional 30Gy in 5 fractions in case of nodal involvement. Two treatment plans being generated for each patient involving 2 and 4 full rotational arcs while keeping identical optimization objectives. Plans optimized on Eclipse TPS v16.1. Varian UNIQUE LINAC (6MV FF beam)

10. Treatment Planning contd.

11. Portal Dosimetry

12. Gamma Passing Rate (GPR) Analysis PATIENT 2_ARC (3%,3mm) 4_ARC (3%,3mm) 2_ARC (2%,2mm) 4_ARC (2%, 2mm) 1 96.9 97.1 88.3 90.6 2 94.3 92.7 83.5 82.2 3 97.1 98.3 87.2 89.3 4 97 97.6 89.5 90.4 5 96.3 97.1 90 94.2 6 98.2 98 99.9 98.1 7 94.9 97 82.5 87.3 8 94.6 97.8 83.7 86.4 9 87.7 93.4 72.9 85.1 10 99.2 99.6 93.9 95.5 11 99.3 99.6 92.3 95.6 GPR (97%) MEAN ± ST DEV p - VALUE 2_ARC 4_ARC 3%,3mm 95.95 ± 3.07 97.11 ± 2.10 0.04 2%, 2mm 87.61 ± 6.72 90.43 ± 4.76 0.02

13. GPR Analysis contd.

14. Defined Deliverability Metrics (MU -1 ). ( (degree/s) -1 ) ((MU/min) -1 )   New metrics have been introduced for this particular study to observe the improvement in deliverability and dosimetric variations for 4 arc plans over 2 arcs ones .

15. Defined Deliverability Metrics contd. The gantry speed and the dose rate follows an inverse relationship. Product of the two quantities for both the plans for each patient are almost constant . The 2 arc plans were optimized at a higher dose rate and lower gantry speed while the 4 arc plans were optimized for lower dose rates but a higher gantry speed , leading to a slightly higher value for the product (MDR*MGS) for the 2 arc plans.

16. Defined Deliverability Metrics contd. A nearly flat curve for the 2 arc plans show a very high dose rate (close to the maximum dose rate) being maintained throughout the delivery.

17. Defined Deliverability Metrics contd. A nearly flat curve for the 4 arc plans show a high gantry speed (close to the maximum gantry) being maintained throughout the delivery.

18. Defined Deliverability Metrics contd. A combination of a higher gantry speed and a moderate dose rate corresponds to a higher GPR.

19. Defined Deliverability Metrics contd.

20. Defined Deliverability Metrics contd. Low MU/CP (MU/CP < 3) is potentially demanding in terms of MLC motion and modulation

21. References Chiavassa S, Bessieres I, Edouard M, Mathot M, Moignier A. Complexity metrics for IMRT and VMAT plans: a review of current literature and applications. Br J Radiol 2019; 92 : 20190270 Tolerance limits and methodologies for IMRT measurement-based verification QA: Recommendations of AAPM Task Group No. 218 Park, J.M., Kim, Ji ., Park, SY.  et al.  Reliability of the gamma index analysis as a verification method of volumetric modulated arc therapy plans.  Radiat Oncol   13 , 175 (2018). https://doi.org/10.1186/s13014-018-1123-x