Radiation therapy CT simulator: a large-bore scanner with a flat couch top and external marking lasers, chosen by bore, scan field, tube power and laser accuracy

01

Gantry opening of 82, 85 or 90 cm and the scan field that is actually reconstructed

The IAEA cites breast, lung and vulva patients scanned with their limbs held away from the torso, and head and neck patients in immobilisation devices, as cases where a 70 cm bore forces a change of position, giving a less reproducible set-up or a mismatch between imaging and treatment positions. [2] The IAEA gives 70 cm as the bore of conventional CT scanners (typ.), allowing objects up to 50 cm across (typ.), and describes large-bore scanners above 70 cm as designed for radiotherapy needs. [2] The 510(k) summaries below show that a wider gantry does not always come with an equally wide reconstructed field.

Canon's predicate in the same filing, a diagnostic Aquilion Prime SP, has a 780 mm opening, a 500 mm maximum field and ±30° tilt, which puts the 900 mm, no-tilt large-bore model in context. [5] Philips files its Big Bore in an Oncology configuration and a Radiology (Base) configuration, with intended use that includes images for oncology treatment preparation and radiation therapy planning. [3]

The IAEA warns that on some scanners CT numbers towards the periphery of large-field images are not reliable, which matters to dose calculation algorithms that convert CT numbers to density; it asks that the effect on dose calculation be evaluated at commissioning. [2] Editorial calculation: an 850 mm bore is 150 mm wider than a 70 cm bore, 75 mm more clearance on each side of the isocentre; the 85 cm Siemens models reconstruct 60 cm, leaving 25 cm of the opening outside the standard field.

Ask for the gantry opening and the maximum scan field with and without extended-field reconstruction, and for CT-number accuracy measured at the edge of the largest field you will use for planning.

02

Tube power and heat capacity for long planning scans at 3 mm slices

Planning scans cover longer volumes than diagnostic scans, preferably with 3 mm slices, and should be acquired quickly to limit motion; the IAEA names the heat storage capacity of single-slice CT tubes as the limit, lists the compromises otherwise made (fewer images, thicker slices, lower mAs or a higher pitch) and says greater heat storage capacity reduces them. [2]

Siemens lists maximum image acquisition lengths of 1600 mm and 2000 mm, depending on the patient table, for its go. platform scanners. [4] Editorial calculation: at the 3 mm slice thickness the IAEA prefers, a 2000 mm acquisition is about 667 contiguous images; Philips lists helical slice thicknesses from 0.67 to 5 mm and a 0.42 s rotation for its Big Bore. [3]

Give the supplier your longest planning scan (length, slice thickness, kV and mAs, 4D or not) and ask whether it runs in one acquisition without tube-cooling pauses.

03

Movable sagittal laser, indexed flat couch top and the ±2 mm daily checks

The IAEA treats a laser marking system as necessary for a CT used in radiotherapy, prefers at least the overhead sagittal laser to be movable, but puts alignment accuracy and stability ahead of movability, and asks for a laser line at least 50 cm long at the tomographic plane. [2] Fixed and movable wall lasers, it says, give the same quality of treatment. [2] Siemens includes control of RTP laser systems among the functions of its tablet workflow software. [4]

The in-gantry alignment lights of a diagnostic scanner are held only to ±5 mm, which the IAEA calls outside acceptable limits for radiotherapy; if they are used for marking they need to be tightened to ±2 mm, and on some scanners that is not sustainable because the lights sit on a vibrating fibreglass cover. [2] Couch checks are run with 70 kg and then 140 kg on the top to represent a patient. [2]

For the couch top itself, the IAEA asks that the CT and the linac tops share the same width, flex and sag under load and accept the same indexing of immobilisation devices, so the patient is set up in the same place on both. [2]

Ask which lasers are supplied (gantry, wall, ceiling; fixed or movable), their stated line width and positioning accuracy, and the couch top width, sag under 70 and 140 kg, and indexing pitch, compared with your linac couch.

04

Fluoroscopic simulator or CT simulator: two FDA device classifications

US regulation still has a separate class II device type, the radiation therapy simulation system, defined as a fluoroscopic or radiographic x-ray system used to localise the volume to be treated and confirm the position and size of the treatment field; it may include display equipment, patient supports and planning programs. [1] The large-bore scanners in this page's tables are instead cleared as computed tomography x-ray systems under 21 CFR 892.1750, product code JAK. [3] [4]

Virtual simulation software takes over the conventional simulator's role on screen: the IAEA expects it to show gantry, couch, collimator and jaw motion, source–surface distance and beam divergence, to build digitally reconstructed radiographs, and to design portals with blocks and multileaf collimators. [2]

05

Data for a radiotherapy CT simulator

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Patients: [sites treated; widest set-up, e.g. breast board, arms up, head and neck mask]
Bore: [gantry opening; scan field, standard and extended]
Tube: [generator kW; heat capacity; longest planning scan and slice thickness]
Couch top: [flat top width; indexing; sag at 70 and 140 kg; linac couch to match]
Lasers: [gantry and wall lasers; movable sagittal laser; accuracy; control from the console]
Software: [virtual simulation; 4D or respiratory gating; DICOM export to the TPS]
Acceptance: [laser and couch checks; CT number to density at the largest field]

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