Treatment Planning and Simulation
Treatment Planning and Simulation tests the chain from CT scan to an approved, deliverable plan: immobilization and indexing, image acquisition, volume definition, dose calculation, and plan evaluation against DVH criteria.
Simulation and image acquisition
- Reproducibility starts with immobilization indexed to the couch (wing board, vac-lok, thermoplastic mask) and reference marks placed at the intended isocenter.
- CT simulation: thin slices (2-3 mm routine, 1 mm for intracranial SRS), scan extent about 5 cm beyond the PTV, arms positioned out of the beam path, and a CT-to-density table validated with a phantom.
- 4DCT or breath-hold scanning for thoracic and upper-abdominal targets; the motion envelope from all phases defines the internal target volume.
Volumes and margins (ICRU 62/83)
- GTV is visible tumor; CTV adds microscopic spread; ITV adds internal motion; PTV adds setup and machine uncertainty.
- Margin recipes such as van Herk (2.5 sigma + 0.7 sigma) convert measured systematic and random errors into a PTV margin. A default 5 mm is not a recipe.
Dose calculation
- Type A pencil beam fails in heterogeneous tissue: it overestimates dose beyond lung and underestimates inside lung because it does not model lateral scatter.
- Type B (superposition/convolution, e.g. AAA) and Type C (Acuros, Monte Carlo) handle density heterogeneity better.
- Use a calculation grid of 3 mm or less for small fields, and confirm whether the algorithm reports dose to medium or dose to water.
Worked example
A 10 x 20 cm rectangular field needs an equivalent square for depth-dose lookup: 4A/P = 4(200)/(60) = 13.3 cm. Use the PDD for the 13.3 cm square, not for the 20 cm side length. When evaluating a 60 Gy prescription, report D95 (must reach at least 57 Gy) and the near-maximum dose (D2 or D0.03 cc) for serial structures; a plan that looks uniform on a color wash can still violate these numbers.
Traps
- Placing respiratory motion in the CTV instead of the ITV.
- Using the physical field size for a heavily MLC-blocked field.
- Accepting a DVH that looks acceptable while ignoring deliverability (leaf travel, jaw settings, gantry-couch collision).
- Forgetting that a plan without a signed approval is not treatable.