Table 3.
Quality assurance of a tomotherapy Hi-ART machine
| QA | Item | Description | Performed by |
|---|---|---|---|
| Daily | Temperature | Gantry temperature control system reads 40°C after machine is turned on. | Radiation |
| QA | Warm up procedure | Beam runs at lower output for 10 min and warms up the components to stable temperature. | Therapist |
| Air Scan | With the couch out of the beam, the machine is run in the imaging mode, which corrects for the detector response. | ||
| Laser check | Green and red lasers fall on top of the marks on walls and floor created during commissioning. | ||
| Green and red lasers coincide at the virtual isocenter in the home position. | |||
| Output check | With a nine-chamber electrometer (Victoreen 7200) output is measured for a 5 × 20 cm2 open field on central axis and two off-axis points on each shoulder of the cone profile, to within ±3%. | ||
| Door Interlock | Machine cancels procedure when the entrance door is opened. | ||
| Weekly QA | Output check | With an A1SL ion chamber and electrometer the output is measured for a 5 × 20 cm2 open field at 1.5 cm depth in solid water in SAD setup (tolerance ±3%). | Physicist |
| Energy check | A 4-cm solid water slab is placed on the previous setup and the ratio of the two readings gives a measure of the beam energy at 5.5 cm depth (tolerance ±2%). | ||
| Laser check | With the cheese phantom, the transverse laser (X-Z plane) localization procedure is performed in which MVCT scans are done. During setup, the red lasers move by 2, 8 and 4 cm in the X, Y and −Z directions, respectively. From the scans and registration of images, the X, Y and Z shifts are within 0 ± 1, 0 ± 1 and 5 ± 1 mm, respectively. | ||
| Monthly | Output check | Same as in weekly QA | Physicist |
| QA | Energy check | Same as in weekly QA | |
| Laser check | Same as in weekly QA | ||
| Virtual isocenter check | With a ready pack film set up horizontally at the virtual isocenter and the green laser line marked on the film, the procedure moves the film 70 cm into the bore and exposes for an open field. The center of the exposure is within ±0.5 mm of the laser line. | ||
| Penumbra profile check | With a ready pack film set up horizontally at isocenter at 1.5 cm depth and SSD of 85 cm in solid water an exposure is made for all open leaves for each beam. The penumbra profiles are measured to within ±1 mm of FWHM at commissioning. | ||
| IMRT plan check | IMRT plans are measured for cylindrical targets in a cheese phantom for on axis and off axis locations to within ±3% for the 1, 2.5 and 5 cm beam. | ||
| Output drift check | With an ion chamber and Tomoelectrometer, an open beam at fixed gantry is sampled for every 100 ms for 10 min. The converted dose rate should be within ±2% of the average value. | ||
| Couch motion | A 40 × 1 cm2 open field is exposed on a film for the gantry at 0° and the couch moving at a constant speed for 20 cm. The optical density scan in the Y-direction should be constant within ±2% for constant speed and output. | ||
| Rotational variation | A procedure to monitor the beam output and energy is carried out by measuring the signal on the monitor chamber of the machine and the xenon detectors with the help of service engineers (tolerance ±2%). | ||
| Safety interlock | As described in the acceptance tests. | ||
| Annual QA | Alignment checks | Jaw shift (tolerance ± 0.3 mm), Y-axis misalignment (tolerance ± 0.5 mm), MLC center – Gantry isocenter alignment (tolerance ± 0.5 mm), Tongue and groove test (tolerance <2%) are checked. | Physicist |
| Beam Characteristics | Water scanning system is set up to measure (1) PDD, (2) Cone profiles and (3) Penumbra profiles at several depths for all the three beam widths and compared with commissioning data. | ||
| Displayed dose rate | Absolute dose output is measured for a set time and dose rate is calculated. This is set as the displayed dose rate at the operator station. | ||
| MVCT images | Tomoimage reconstruction is examined for coarse, normal and fine settings for no artifacts and good contrast resolution. MVCT image dose should be < 4 cGy. | ||
| Output check | With an A1SL ion chamber in water medium, the output is measured for a 5 × 40 cm2 beam at 1.5 cm depth in SAD set up (tolerance ± 2%). | ||
| Energy Check | The ion chamber is moved to a depth of 10 cm in water and the readings taken in SAD set up. The ratio of the readings in the previous step and this is a measure of the energy check (tolerance ± 2%). | ||
| Laser check | Same as in monthly QA | ||
| Virtual isocenter | Same as in monthly QA | ||
| IMRT plans | Same as in monthly QA | ||
| Output drift | Measure over 1000s (drift tolerance within 2%) | ||
| Safety interlocks | Same as described in Acceptance test | ||
| Couch motion | Same as in monthly QA |