Gammaknife QA
Transcript of Gammaknife QA
By: Michael Curry
Daily QA Head Frame Attachment Image Acquisition Bubble Measurements Treatment Planning Treatment Begins
Spot checks that are Pass/Fail or Yes/No
Treatment
Couch In APS Couch Out
28. Helmet Changer (weekly)
Turn on APS Wait for green lights to come on
▪ Initiate QA test run
APS begins by moving to the Docking position (100, 130, 100) Check to see that the positioning is correct
Attach APS QA tool▪ Then APS will move to (100,100, 100) then (140, 42,
58.5)
▪ Check to see that the positioning is correct
▪ Then APS will check the z direction moving to 62.5
The long engravings are the 0 marks Each of the smaller engravings are ±0.1
and ±0.5mm respectively▪ Tolerance on the x and y coordinates is ±0.5mm
The spring loaded tip of the tool presses against the helmet at z=62 The reading should be 0 ±0.5 mm on the
ruler jig attached to the tool This test is done at tome left and
bottom right of the helmet
The frame attaches at four points around the patient's head with pins that penetrate about 2 millimeters.
Patient is given local anesthetics
Frame fitting takes approx. 20 mins
MRI
CT
Angiograph?
The Skull Scaling instrument “bubble helmet” is used to calculate the distance between the center of the Leksell® stereotactic space and the surface of the patient's skull for Leksell Gamma Knife® treatmentplanning with GammaPlan
Leksell GammaPlan
Contour lesions
Determine helmet size and dosimetry
Usual prescription is 20Gy to the 50% isodose line.
After all steps have been completed the treatment begins
Start with APS beams, then do those requiring trunnions
Physicists are involved in every helmet change, collision check, dock & verify, and changing from APS to trunnion.
Complete all daily and weekly checks Helmet sensor checks Trunnion centricity check Measurement of Absorbed Dose Rate Dose Rate Linearity LGP 30Gy calculation Timer Accuracy Couch Movement Time UPS system
Helmet Sensor Checks
Cap Sensor Test
▪ Green LED lit when cap in place
Helmet ID test
▪ Yellow LEDs lit in code for which helmet
Helmet Microswitch Test
▪ Place shim between microsensor
and adjustment ring. Red No LED
should light in “helmet position”
Helmet positionHelmet ID
Helmet Cap
Helmet ID Table Helmet Cap sensor
Adjustment Ring
Trunnion Centricity
Attach Trunnion
Attach the Helmet test tool
Push trunnion into bushing slots and tighten the trunnions
The reading should be 100±0.1mm and the bushings should be able to rotate.
Helmet test tool
Bushings
Data Acqusition Attach trunnion and 18 mm helmet are
Mount the 16cm diameter polystyrene phantom on the trunnion and insert the Exradin A16 I.C.
Take four 1 min measurements
Take one measurement for 4, 10, & 15 mins
Average the 1 min measurements (Av)
1min=R1, 4min=R4, 10min=R10,
15min=R15
Measurement of Absorbed Dose Rate (Dw)
Determine transit error: Tolerance <0.05min
Determine corrected Rdg/min:
Determine CTP:
Determine Dw: [cGy/min]
▪ Where:
4
4 1
4(Av)(min)
R
R R
min
CRdg Rdg
t
273.2 1013.2
295.2TP
TC
P
(Av) 21TPDw CRdg C TG
21 /plast w
elec gas wall ion repl en plastairTG C N L P P P
Calculation of Absorbed Dose Rate
Determine Decay:
Using the time since the last calibration and the output at that time we can calculate what the Absorbed Dose Rate should be on the day:
Then compare the Calculated and Measured absorbed dose rate.
▪ Tolerance is <±3%
1/2(.693 )t tte e
0
t
w wD D e
Dose Rate Linearity
Determined by: Tolerance <±2%
Dose Rate Constancy
Determined by: Tolerance <±2%
max
min
1 100linearity
CRdgR t
CRdg
1max1
1min
tan ( ) 1 100R
cons cy RR
LGP 30Gy calculation
Print the QA plan and see what time it says is needed to deliver 30Gy
Divide the 3000cGy by the time given to get the dose rate
Compare this to the previously Calculated dose rate
▪ Tolerance <±2%
Timer Accuracy Use an independent timer to determine the time
that the helmet is docked for a 15 minute treatment run.
▪ Tolerance: 0< t <6sec
Couch Movement Time Start a treatment and press the pause button
Use the independent timer to measure the time from interruption until the shielding door is completely closed
▪ Tolerance: 30< t <40sec
UPS system
Start a treatment run
Switch the input button off
The system should run on the backup battery for 20 minutes
If during that 20 minutes the output voltage falls below a certain level a system alarm will occur
If a system alarm occurs contact Elekta
Indicator light Input button
Site Kit Radiation Survey: Wipe test (semi-annually)
Source Housing (leakage)Radiation Levels (outside rm)
Training: General Radiation SafetyEmergency Procedures
Accuracy in Dose Delivery Precision in Dose Delivery Output Factors (helmet factors)
Treatment process Daily QA Monthly QA Annual QA