Intensity Modulated Radiation Therapy Delivery Types

Forward Planning: The user defines geometry (gantry, collimator, couch), collimation (jaws, MLC shapes), and fluence (wedges, open fields, MU). IMRT can be forward planned by manually defining fluence or subfields.
Inverse Planning: The user defines dosimetric criteria and desired weighting. An optimization algorithm defines the collimation and beam fluence.
Beamlets: Beam fluence is divided into "beamlets" typically measuring $0.2-1.0\,\text{cm}$ along the leaf motion direction and the leaf width in the cross-leaf direction.
Mathematical Model: Dose in voxel $i$ is calculated as $D_i = \sum_{j=1}^J a_{ij} w_j$ , where $w_j$ is the intensity of the $j^{th}$ beamlet and $a_{ij}$ is the dose deposited in the $i^{th}$ voxel per unit fluence.
Optimization Algorithms:
- Gradient descent: Fast but prone to getting stuck in local minima.
- Simulated Annealing: A stochastic method that adds randomness to avoid local minima; it is slower but more robust.

Mechanism: Uses physical material (e.g., tin granules or tungsten powder) to modulate primary fluence.
Materials: Max thickness is approximately $5\,\text{cm}$ . Percent transmission ranges from $100\% - 38\%$ for $6X$ tin and $100\% - 18\%$ for $6X$ tungsten powder.
Advantages: Simple implementation, static delivery, and no interplay between intensity modulation and organ motion.
Disadvantages: Requires manual entry to the room per field, lacks automation, and provides limited modulation compared to MLC-based methods.

Step & Shoot (Static MLC / SMLC-IMRT): The leaves travel to discrete positions (step) and the radiation beam turns on (shoot). Radiation delivery alternates with MLC movement.
Dynamic MLC (Sliding Window / DMLC-IMRT): Leaves sweep across the field while the beam is on. The leaf sequence is translated into leaf positions as a function of MUs.
Direct Machine Parameter Optimization (DMPO/DAO): Directly optimizes leaf positions and machine parameters rather than optimizing fluence first and calculating the leaf sequence afterward. This ensures "what you see is what you get" at optimization.
Leaf Sequencing Limitations: Must account for leaf transmission (lower bound on intensity), leaf speed, and the "tongue & groove" effect.

Beam Setup: Typically utilizes $7-12$ equi-spaced beams with the isocenter placed near the center of the PTV.
Dosimetric Impact: IMRT allows for dose escalation and significant sparing of Organs at Risk (OARs) such as the spinal cord, larynx, and parotid glands. Larynx mean dose can be reduced from $53\,\text{Gy}$ (3D) to $26\,\text{Gy}$ (IMRT).
Efficiency and Hardware: IMRT Monitor Units (MU) are typically $3-5$ times higher than 3D-CRT. Lower energy beams are often used to reduce neutron production.
Advantages: Highly conformal dose distributions and reduced complications in surrounding tissues.
Disadvantages: Labor-intensive planning, extended delivery time, higher whole-body dose due to increased MU, and potentially less homogenous PTV dose distribution.