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