1 IMRT Delivery System QA Thomas LoSasso, PhD Memorial Sloan Kettering Cancer Center IMRT Dose Delivery • Acceptance testing • Commissioning • Quality assurance • Verification QA Static IMRT Leaf affects borders only affects dose within target position 1-2 mm OK 0.2 mm OK Why: specific tests for IMRT? Accuracy of leaf positioning (gaps) Gap error Dose error 0.0 5.0 10.0 15.0 20.0 0 1 2 3 4 5 Nominal gap (cm) % Dose error Range of gap width 2.0 1.0 0.5 0.2 Gap error (mm) Acceptance testing • MLC alignment • MLC leaf / gap calibration • Timing errors • Beam characteristics at small MU A60 A31 B30 B1 B60 B31 A30 A1 MLC Alignment A60 A31 B30 B1 A1 A30 B31 B60 Film 1 Film 2 Field B 90° Field B 270° Field A 90° Field A 90°
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IMRT Delivery System QA IMRT Dose DeliveryIMRT Dose Delivery • Acceptance testing • Commissioning • Quality assurance • Verification Q A Static IMRT Leaf affects borders only
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6a Field Size Defined by MLC at 100 cm SAD by Light Field: Date: __________ Initials: __________ Tolerance: 0.5 mm
Digital setting (cm) A carriage B carriage A leaves B leaves 20.0 16.0 12.0 8.0 4.0 0.5 -4.0 -8.0 -12.0 -16.0
Note: If leaf positions exceed tolerance, reinitialize and recheck. If this does
not correct the problem, verify that the correction parameters in files MLCTABLE.TXT and MLCXCAL.TXT have not changed.
4 cm
Leaf / carriage positioning vs off-axis distance
7
← - 0.5 mm
→ + 0.5 mm
← - 0.2 mm
→ + 0.2 mm
errors introducedLeaf positioning
1 mm bands 17b DMLC Output (Gap width) Check
15 MV x-rays, File: DMLC_QA2.dva, Dose = 100 MU, Dose rate = 240 MU/min SAD: 100cm, Depth = 3.1cm, Backup = 10cm, C.S. = 10x10cm2 Tolerance: (The ratios should be within 2% (< 0.2 mm gap) of the annual calibration.)
Measure the output for the scanned field before and after reinitializing the MLC. Measure the output for a 10x10cm2 reference field size defined by the jaws (MLC retracted). Calculate the ratio of the scanned field to the reference field outputs.
Reference10x10
DMLC - before reinitialization
DMLC - after reinitialization
Reference 10x10
Average Annual Ratio
17c Beam Holdoff Functionality
Date: Initials
Same setup as 17b, Dose = 5 MU Record the output and the Tx time. Check if the holdoff is being applied. Reading Functioning
15 MV x-rays, File: DMLC_QA2.dva, Dose = 100 MU, Dose rate = 240 MU/min SAD: 100cm, Spokas in air w/buildup, C.S. = 10x10cm2 Tolerance: (The ratios should be within 3% (< 0.2 mm gap) of each other.) Position a cylindrical chamber in air at isocenter.
For each gantry angle, measure the open field and the scanned field outputs. At gantry angles of 90° and 270°, also measure output for collimator angles of 90°. Calculate the ratio of the scanned field to the reference field outputs.