Understanding CT planning, V-Sim and TPS

Before the development of CT planning the simulator was used to plan or 'simulate' radiotherapy treatment.

A simulator is a machine that is mechanically the same as a linac but instead of producing high energy x-rays for treatment it procuces lower 'diagnostic' energy x-rays for imaging. The patient is positioned on the simulator table, the same as in the linac, and the beam angles and positions are planned or 'simulated'. The images of the beams are acquired to be approved by the clinician and compared with images taken during treatment on the linac.

A 2D treatment plan was designed on a physical outline of the patient taken at the simulator appointment (often by bending lead wire around the patient and then drawing round the wire on a piece of paper). In 2D planning it is assumed the patient is the same shape along their length and that the patient is 'made of water', i.e. that the x-ray absorption everywhere in the patient is the same as in water (clearly not the case in lung and bone etc.).

Many departments still use a simulator and 2D planning for some or all of their patients. Patients undergoing treatment at a GenesisCare centre will have a CT planning scan and are CT planned.

CT planning scan

The CT Planning Scan is carried out on regular diagnostic CT scanners. "Wide-bore" scanners have bore sizes from 80cm to 90cm, to more easily scan patients in the radiotherapy treatment position (with appropriate immobilisation equipment).

A flat table-top with indexing (for immobilistaion devices) is required to replicate the linac table top.

Diagnostic scanner tables may need to be upgraded to ensure their movements are geometrically accurate enough for radiotherapy planning.

External laser systems are usually installed as the internal lasers on CT scanners are not easy to adjust and are not helpful in setting up the patient further down the bed. GenesisCare install fixed side-wall lasers and a moving overhead sagittal laser.

Scan protocols are similar to diagnostic but altered to maximise geometric accuracy and linearity of Hounsfield Units.

HUs (Hounsfield Units) are the units of xray absorption in a CT scan:

  • A CT scan is made up of slices of data.
  • Each slice of data is a matrix of voxels.
  • Each voxel has a HU value proportional to it's xray absorption, which is proportional to its electron density, which is fairly proportional to its physical density:
    • Air has HU = -1000.
    • Water has HU = 0.
    • Bone has HU = 1200 to 1800ish.
  • A CT scan is thus a 3D map of xray absorption in the patient which is used by treatment planning software to determine the absorption of xrays anywhere in the patient taking into account the 3D shape of the patient and the different absorption of different tissues within the patient. The treatment planning software uses the CT scan to accurately calculate the dose absorbed anywhere in the patient and produce a 3D treatment plan and dose distribution.

Virtual simulation (V-Sim) and treatment planning systems (TPS)

Systems in use globally, including:

  • virtual simulation software:
    • AcQSim (Philips).
    • Focal (Elekta CMS).
    • Pro Soma (Medcom).
    • Somavision (Varian).
  • treatment planning software:
    • Pinnacle (Philips).
    • Monaco (Elekta CMS).
    • Xio (Elekta CMS).
    • MasterPlan (Oncentra).
    • Eclipse (Varian).

Virtual simulation software has been developed to replace the physical simulator. Using Philips AcQSim V-Sim software GenesisCare treatment planners can import the patient CT planning scan and then:

  • Position beams around the virtual patient with different gantry & collimator angles and table rotations (warning if the position is not possible to deliver on the linac).
  • Set appropriate field sizes and MLC shapes.
  • Produce DRRs (digitally reconstructed radiographs) which are images that look like radiographs taken from the treatment field geometry (like a simulator image).
  • Visualize the patient surface, showing the light field of the treatment field on the patient surface and showing patient marks that have been placed on during the planning CT scan (ie on tattoos, or surgery scars).
  • May allow 'contouring' / 'voluming' of patient anatomical structures within the patient CT dataset (eg draw the lungs etc.) which can be used by the TPS to calculate dose statistics for the separate anatomical structures.

V-Sim software can do most things the Treatment Planning software can do except that it can't compute the dose distribution.

Treatment planning software usually includes some V-Sim functionality and also enables the calculation of the dose distribution within the patient. Using Philips Pinnacle TPS GenesisCare treatment planners can import the patient CT Planning Scan and then:

  • Position beams around the virtual patient with different gantry & collimator angles and table rotations (warning if the position is not possible to deliver on the linac).
  • Set appropriate field sizes and MLC shapes.
  • Produce DRRs (digitally reconstructed radiographs) which are images that look like radiographs taken from the treatment field geometry (like a simulator image).
  • Visualize the patient surface, showing the light field of the treatment field on the patient surface and showing patient marks that have been placed on during the planning CT scan (ie on tattoos, or surgery scars).
  • May allow 'contouring' / 'voluming' of patient anatomical structures within the patient CT dataset (eg draw the lungs etc.) which can be used by the TPS to calculate dose statistics for the separate anatomical structures.
  • Calculate dose in the patient and display the dose distribution on the planning CT scan.
  • Calculte DVH (Dose Volume Histogram) information.
  • A plot of dose versus the amount of tissue receiving that dose, which may be for the whole patient or for individual anatomical structures (if they have been volumed / contoured), and is a tool for judging how good a treatment plan is.
  • Philips DMPO (Dirct Machine Parameter Optimisation) IMRT inverse planning software enables GenesisCare planners to carry out fast & accurate computation of IMRT dose distributions.
  • Philips Syntegra image fusion software allows other scans (MRI, PET etc.) to be fused with the CT planning scan.

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