IMRT/VMAT/STEREO COMMISSIONING

Question 1

• Explain how you would go about commissioning IMRT:

Answer 1

See TG-119: IMRT commissioning with multi-institution comparison. Use phantom studies to verify that treatments can be planned, prepared for treatment, and delivered with sufficient accuracy. Gamma criteria of 3%/3 mm are used. It is common to only analyze pixels with doses > 10% of maximum dose. Alternatively, rectangular ROI may be set to jaw settings. Typically require 95% pass rate (2 sigma confidence interval). Commissioning studies should mimic the types of target and structure geometries and target doses and dose constraints that are likely to be encountered in the clinic.

Question 2

• Explain process for commissioning and clinical implementation of SRS/SBRT on conventional C-arm linac. What equipment/measurements are required?

Answer 2

o Beam measurements with small fields are challenging due to charged particle non-equilibrium, non-negligible detector perturbation effects, enhanced influence of finite source size. Challenges in defining field size.
 Typically stereo commissioning data must go to field sizes smaller than for conventional RT. Extrapolation of commissioning data potentially leads to errors in dose calculation.
o Some TG-142 QA test tolerances are more stringent (e.g., coincidence of mechanical and radiation isocentres, measured using Winston-Lutz test)
o The commissioning of MLC commonly includes mechanical stability checks [e.g., Winston-Lutz test with MLC defined field, spoke shot], leaf position verification [accuracy/reproducibility using picket fence test, log file analysis], beam data acquisition, leaf transmission, leaf leakage, verification of beam penumbra, and the dynamic leaf gap test [I.e., dynamic picket fence test]
o IGRT system: image quality, isocentricity
o End to end test with anthropomorphic phantom with embedded target to test full workflow including e.g., target localization ability

Question 3

• You have been assigned the project of implementing VMAT for head and neck treatment in a facility that does not yet use this technique. How would go about it? What resources (human, equipment, etc.) would you need?

Answer 3

o Would need to do a series of end-to-end tests to make sure entire process from simulation to delivery is running properly.
o Testing synchronization of gantry position, leaf position and dose rate.
o MLC leaf positioning accuracy, reproducibility.
o Dosimetry tests (e.g., flatness, symmetry and output as a function of gantry speed & dose rate)
o Since it is no longer step and shoot, must expand Q&A routine to account for the fact that the beam is on while the gantry rotates, and while the MLC (and possibly also the collimator, depending on implementation) is moving.
o Verify mechanical constraints on the machine (VMAT optimizer requires this information).
o Interruption/resumption tests
o Leaf leakage characterization (interleaf, through leaves, through leaf ends)
o Absolute dose measurements with a calibrated ion chamber to make sure TPS calculates dose properly to a point.
o Compare plan objectives obtainable with VMAT to plan objectives achievable with old method (e.g., IMRT) to ensure that acceptable plan quality is achievable; comparison with VMAT treatments from the literature.