Target volumes: The concept of GTV , CTV , PTV , and ITV described in ICRU 50 and 62 for SRS also applies to SBRT planning (Medin et al. 2010; ICRU 1993). PTV margins depend on body site, treatment device, localization technique, and imaging frequency. Typical margins range from 2 to 5 mm for SBRT.

Dose conformity: the high-dose volume conforms tightly around the target.

Dose heterogeneity : hot spots located within the target are often considered not only acceptable, but also desirable. The prescription dose is typically 50–90 % of the maximum dose depending on the treatment delivery and treatment planning systems.

Dose gradient : the dose fall-off away from the target is steep. The volume of normal tissue receiving high doses of radiation is kept at a minimum. This is in comparison with other treatment techniques like 3D conformal.

Beam energy: 6 MV photons offer the best compromise between beam penetration and penumbra characteristics. Many techniques use unflattened beams .

Beam shaping: Radiation is collimated to a small field using heavy metal cones (circular field 4–60 mm diameters), multileaf collimator (MLCs), or microMLCs (2.5 mm leaves width). MLCs and micro MLCs are used to deliver treatments developed with conformal beams, intensity modulated fields, dynamic conformal arcs , or a combination of these (ICRU 1999).

Treatments are delivered via coplanar and non-coplanar beam arrangements.

Circular fields provide a sharper penumbra than microMLCs.

Beam geometry: multiple non-overlapping beams concentrically pointing to the target; 5–12 coplanar or non-coplanar beams; 1 or 2 coplanar or non-coplanar arcs; a continuously rotating fan beam; hundreds of non-coplanar pencil beams pointing to different parts of the target (non-isocentric beam arrangement) or to the same point (isocentric beam arrangement).

Forward planning : the user manually adjusts beam arrangement, field shapes, and weights until the desirable dose distribution is achieved.

Inverse planning : the user specifies plan objectives for target and normal structures and a dose optimization algorithm calculates field shapes and weights based on the minimization of a mathematical cost function.

3-Dimensional conformal radiation therapy (3D-CRT ): typically forward planned. It might be advantageous for moving targets, as the target is always in the open radiation field.

Intensity-modulated radiation therapy (IMRT ): typically inverse planned (although the field-in-field technique is forward planned).

Arc therapy (RapidArc , VMAT ).

Pencil beam algorithms using radiological path length corrections to account for tissue heterogeneities. Not accurate in conditions of electronic disequilibrium . In these cases, heterogeneity corrections explicitly accounting for the transport of secondary electrons must be employed. While the most accurate technique for dose calculation is Monte Carlo , convolution-superposition methods are sufficiently accurate in most clinical situations.

Calculation grid : should be less than 2 × 2 × 2 mm³.