Axial Dose Distribution
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3D-CRT, IMRT, and VMAT differ fundamentally in how beam fluence is shaped and delivered. The dose heatmaps and DVH below illustrate real clinical consequences for target coverage and organ-at-risk (OAR) sparing.
Beams are shaped to match the PTV projected outline (beam's-eye view) using MLCs or custom blocks. Beam intensity within each field is uniform (or modified by a wedge for dose homogeneity). Planning is forward: the planner sets beam parameters and evaluates the resulting DVH manually, iterating until clinically acceptable.
The fundamental constraint: because each field is uniform, a single beam cannot simultaneously spare a structure anterior to the target while covering the target itself. OAR sparing is achieved by beam direction choice alone, not by fluence shaping.
For prostate 3D-CRT (4-field box, 70–78 Gy):
The 4-field box irradiates bladder wall and anterior rectum at full prescription dose — both sit within the geometric intersection of the 4 beams.
IMRT uses inverse planning: the planner defines dose objectives (target V100%, OAR V70Gy <15%, etc.) and an optimizer determines MLC leaf positions to meet them. Each beam is divided into many small segments (step-and-shoot) or delivered with continuously moving leaves (sliding window) to create a non-uniform fluence map.
The modulated fluence allows creating concave high-dose regions — impossible with uniform beams. A beam passing through the rectum can have its intensity reduced in the sub-apertures that cross the rectum, while maintaining full intensity through the target.
Step-and-shoot: beam off during leaf motion, on only when a planned segment is open.
Sliding window: leaves move continuously while beam is on; faster delivery.
Prostate IMRT vs 3D-CRT (randomized data, Peeters et al.; Zelefsky et al.):
VMAT delivers radiation as a continuous arc while simultaneously varying: gantry speed, MLC leaf positions, and dose rate. This is the critical difference from IMRT: instead of discrete fixed beams, VMAT optimizes over hundreds of control points around the arc, with each control point specifying a unique MLC aperture, gantry angle, and output rate.
The result is effectively infinite beam directions contributing to the dose distribution, but with the delivery completed in a single gantry rotation (typically 1–2 arcs). Gantry speed slows at angles where more dose is needed and accelerates through less important angles.
Introduced as RapidArc (Varian) and VMAT (Elekta); uses the same inverse optimization principle as IMRT but with the arc geometry constraint.
| Parameter | 3D-CRT | IMRT | VMAT |
|---|---|---|---|
| Beam delivery | Static fields, uniform fluence | Static fields, modulated fluence | Continuous arc, modulated fluence |
| Planning method | Forward (manual) | Inverse (optimizer) | Inverse (arc-constrained) |
| Concave distributions | No | Yes | Yes |
| Number of beams / arcs | 3–7 static | 5–9 static | 1–3 arcs (178–360° each) |
| Control points | 1 per beam | 10–100 segments/beam | ~178 control points/arc |
| Typical total MU (prostate) | 150–400 | 500–1500 | 300–700 |
| Delivery time | 4–10 min | 12–20 min | 2–5 min |
| Prostate rectum V70Gy | ~30–40% | <15% | <12% |
| Prostate bladder V65Gy | ~35–50% | ~20–25% | ~18–22% |
| H&N parotid mean dose | 35–45 Gy | 22–28 Gy | 20–26 Gy |
| Low-dose bath | Lowest | Higher (7–9 directions) | Moderate (arc — but 360°) |
| Intrafraction motion risk | Low | Higher (long tx) | Low (fast delivery) |
| QA complexity | Simple | Complex (per-field) | Complex (rotational) |
| Secondary cancer risk | Lowest | Higher | Intermediate |
| Current clinical role | Palliative; simple geometry | Complex sites when VMAT unavailable | Preferred for most modulated treatments |