Breath-hold CT scans can also potentially be used during respiratory-gated and real-time tumor tracking irradiation; however, it should be noted that a 4D CT scan is preferred over a breath-hold CT scan taken at the same respiratory position, because the respiratory muscles used during breath-holding and free breathing may differ, and any tumor lag occurring during free breathing will be absent during breath-hold. It is also known that the positions of implanted markers do not always represent the tumor position because the tumor and the markers move at different rates during respiration [38]. The ITV should consider such intrafraction variations and potential changes in internal-external correlation factors [39, 40], both within the gating window and the entire cycle of respiration.

The literature shows clearly shows that the single largest systemic source of error in radiation therapy is target delineation. Such errors can be minimized only by using site-specific delineation protocols and consensus delineation atlases. Inconsistencies in target delineation are considered to be significant sources of uncertainty in treatment planning [41]. In defining the gross tumor volume (GTV), clinical target volume (CTV), ITV, and planning target volume (PTV) when performing SBRT, the data in the International Commission on Radiation Units and Measurements (ICRU) Reports 50 [42] and 62 [43] must be taken into account.

The goal of SBRT treatment is to ablate tissues within the PTV; destruction of such tissues is not considered to cause complications. Dose inhomogeneity inside a PTV is usually considered to be acceptable and need not be addressed as a priority during planning. Thus, a maximum point dose of up to around 160 % of the prescription dose is common in SBRT plans (Figs. 9.1, 9.2, and 9.3). Recently, treatment planning has been accomplished using standard beams, static conformal arcs, dynamic conformal arcs, IMRT, VMAT, or hybrid plans. Three-to-six non-coplanar arcs and five-to-nine (or more) non-coplanar static fields are commonly used. Such an approach optimally requires that radiation should converge on the target in as concentric a manner as possible, from many directions. Adjustment of the MLC margin is a key in terms of the dose distribution [51] (Fig. 9.1). If the MLC margin is close to the beam penumbra [Fig. 9.1 (Left)], a homogeneous PTV dose, the maximum of which is ~110 % of the prescription dose, can be achieved (Figs. 9.2 and 9.3); however, dose fall-off outside the PTV is slow. If the MLC margin is fit to or much less than the beam penumbra [Fig. 9.1 (Right)], dose fall-off outside the PTV is rapid, but the PTV dose can be inhomogeneous in that a maximum dose of ~125 % or more of the prescription dose can be attained (Figs. 9.2 and 9.3).

For small beams, such as those commonly used in SBRT, the higher the beam energy, the larger the beam penumbra, due to lateral transport of electrons in the medium. In a low-density medium, such as lung tissue, this effect becomes more significant. Use of a lower photon beam, such as 6-MV, available on most modern treatment platforms, affords a reasonable compromise between beam penetration and the required penumbral characteristics for SBRT lung applications.

It is preferable that CT images used for dose calculation reflect respiratory status during beam delivery. As shown in Table 9.2, dose calculation should be performed using the CT dataset that is most appropriate for each patient.

The dose calculation algorithm (including a heterogeneity correction), and the grid size for calculation, used in any treatment planning system, affect the accuracy of the calculated dose distribution. Inaccurate dose calculation causes large discrepancies between planned and actually delivered doses; therefore, it is recommended that high-precision dose calculation algorithms with fine grid sizes be employed. The technical details of dose calculation have been discussed in this chapter.

Normal tissue dose limits for SBRT differ considerably from those of conventional radiotherapy because the dose/fractionation schemes are extreme. Thus, normal tissue dose limits for SBRT should not be directly extrapolated from conventional radiotherapy data. Particular attention should be paid to fraction size, total dose, time elapsing between fraction deliveries, and overall treatment time; these are important radiobiological factors that need to be maintained within clinically established parameters (often described in the SBRT literature). This issue assumes particular importance when planning new hypofractionated schedules and trials for which no reliable mechanism has yet been established to estimate the radiobiological effects. Therefore, in a clinical trial situation, not only fraction size, but also treatment frequency and overall treatment time, should be maintained for all patients throughout the entire trial, to obtain reliable outcome data. Critical organ tolerance doses based on SBRT experiences appearing in the evolving peer-reviewed literature must be respected [52, 53].

SBRT treatment plans often use a large numbers of beams, unconventional dose fractionations, and varying delivery frequencies. It is critical to accurately communicate the details of the treatment plan and the execution thereof to the treatment team. The quality of planned SBRT dose distributions can be evaluated using parameters characterizing target coverage, dose homogeneity, dose delivered outside of the defined target, and the volume of normal tissue exposed to lower doses. Simple methods of describing these parameters may use combinations of DVHs for different organs and tables showing dose allocations to different subvolumes of such organs. The following metrics are relevant: “respiratory motion range”, “dose calculation algorithm”, “prescription dose”, “prescription ICRU reference point or dose/volume”, “number of treatment fractions”, “total treatment delivery period”, “target coverage”, “heterogeneity index”, “conformity index”, and “dose to organs at risk”.