Acute skin toxicity has been reported for early-stage NSCLC SBRT treatments [3]. The authors reported that “Distance from the tumor to the skin on the patient’s back <5 cm (p = 0.006), treatment with three beams (p = 0.0007), and a maximum back skin dose >50 % of prescribed dose on the planning scan (p = 0.02) were all significantly associated with those patients that developed Grade 2 or higher skin reaction.” It is also important to carefully assess dose in skin folds, especially posterior skin folds which may not be as immediately obvious as anterior skin folds. For beams passing through the treatment couch and any setup devices, it is important to verify these to be accurately modeled by the treatment planning systems. Several couch models have been shown to change the dose delivered by oblique posterior beams by up to 6 %. Carbon fiber couches used in conjunction with electromagnetic tracking systems have been measured to be at the upper limit, or even exceeding, this level of dosimetric impact. The upcoming AAPM Task Group 176 “Dosimetric Effect of Immobilization Devices” will summarize a clinical review of the dosimetric effect of setup devices, including recommendations for clinical dosimetry.

While the main focus of plan evaluation is naturally on the high-dose regions, the distribution of peripheral dose has significant impact both on short-term as well as long-term complication rates.

Many delivery devices, such as Gamma Knife, CyberKnife and Tomotherapy, are limited to one beam energy. When planning highly modulated beams from many directions, and especially when using non-coplanar or non-isocentric planning techniques, the planner must pay close attention to dose delivered in the periphery of the targeted treatment area. These “hot spots” can be more pronounced for obese patients, where the path-length from skin to target exceeds ~15 cm. In planning systems which use a calculation grid to define the body volume in which the dose is calculated and displayed for planning purposes, it is important for the planner to open up the calculation grid and evaluate the plan for peripheral hot spots. Effective ways to minimize the potential for peripheral hot spots include the use of a higher number of beams, limiting the number of MUs per beam direction, and using soft tissue constraints or phantom structures to control the peripheral dose.

Another consideration must be the peripheral dose distribution into peripheral OARs. With increasing cancer survival rates, considering low-dose regions to radiation sensitive organs may affect secondary cancer rates in the decades to follow. One of many examples of increased secondary malignancy caused by low peripheral doses to an OAR was published by Kleinerman et al. [22]. There are four sources for this dose: (1) Low-dose regions of the treatment plan itself, (2) Imaging dose, (3) Leakage, and (4) internal scatter. Leakage and internal scatter are machine-dependent. Imaging dose management is part of simulation and treatment delivery design. During the planning process, low-dose regions is the only peripheral dose parameter which can be influences by the planner under physician guidance.

Dose prescriptions schemes for brain tumors are based on several decades of experience with the Gamma Knife. Dose prescriptions are based on published complication rates based on lesion diameter [31]. The largest lesion diameter considered for SRS is typically 3 cm.

The development of frameless SRS has enabled physicians to develop fractionated treatment courses to alleviate the risk of necrosis associated with high doses in the brain. A special consideration to make when planning SRS to the brain is the fact that many cases are metastatic and therefore have a high probability of returning for further treatment, either to the same lesion or to new lesions. It is therefore advantageous to plan each case with consideration of dose to critical structures such as the lenses of the eyes, the optic chiasm and the brain stem regardless of their proximity, as well as to “normal” brain tissue.

The utilization of SRS for spine has increased with the implementation of frameless, image-guided SRS techniques [32]. The predominant fractionation scheme is still a single fraction of 10–18 Gy [33–36] although radiobiological considerations of cord tolerance for tumors in close proximity to the spinal cord or invading into the epidural space have led to fractionated treatments of 10 Gy × 3 or 80 Gy × 3. The tumor contour is typically standardized to include the complete vertebral body and the peduncles. The most significant OAR is of course the spinal cord. Contouring, and in consequence DVH constraints, vary between contouring the spinal canal (thecal sac) or the visible cord. Several authors have published retrospective studies on spinal cord tolerance [33, 37–39]; Choi et al. [40] published a study on cord tolerance for SRS in recurrent spinal malignancies.

The Canadian Association of Radiation Oncology (CARO) Stereotactic Body Radiotherapy task force has published guidance recommendations [41] for spine SRS which are listed in Table 6.1.

The most widely used dose-fractionation scheme of 60 Gy in 3 fractions is based on the RTOG-0236 and RTOG-0618 protocols. These protocols used dose calculation algorithms based on homogeneous tissue models and have been shown to overestimate the dose by approximately 10 % [42]. As a result of these studies, some physicians have advocated using 54 Gy × 3 if inhomogeneity-correcting dose calculation algorithms are used. Other publications have studied outcomes as a function of tumor size, concluding a volume-adaptive dosing for lung SBRT could improve local control [43]. Table 6.2 summarizes selected society and clinical trial recommendations for lung SBRT dose-fractionation.

Pneumonitis is the predominant reported toxicity for lung treatments. Because lung cancer patients are often elderly and have limited lung function, a V20 dose limit of 20 Gy is often used as a hard OAR constraint. Research efforts are underway to distinguish high-functioning from low-functioning lung regions with the goal to steer the OAR dose towards the low-functioning lung regions [44].

An added consideration is the dose delivered to the chest wall. Woody et al. [45] developed a predictive model for different dose-fractionation schemes based on a retrospective data analysis of 102 SBRT patients using a modified EUD model.

SBRT for Pancreas is warranted for resections, locally advanced cases, local recurrences and Oligometastatic Pancreas Cancers. The doses are designed by applying a tolerance-based approach, meaning that the proximity of the lesion within the Pancreases to critical structures, i.e., the stomach and duodenum, governs the total dose (Fig. 6.2) [46].

The liver, kidneys, spinal cord and bowel are among the other dose constraints to consider when planning SBRT to the pancreas.

SBRT for liver cancer is one of many possible therapeutic options to treat liver malignancies. Therefore, it is not utilized as often as e.g. SBRT for lung lesions. Early dose escalation studies [47–49] confirmed the safety of liver SBRT as non-surgical treatment option. Table 6.3 lists a selection of current society recommendations and clinical trials related to liver SBRT.

Prostate SBRT is the most recent development in SBRT targets. The biological characteristics of early-stage prostate cancer require long-term follow-up studies to provide data on local control, late effects, and biochemically disease-free intervals. Because prostate SBRT trails at this point have only relatively short clinical follow-up, it is highly recommended limit the clinical use of this technique on a clinical trial basis. Until results are published, caution is advised in applying dose constraints for prostate SBRT.

There are currently two trials on hypo-fractionated prostate treatments. The major difference between them is that one aims for dose distributions with heterogeneities modeling prostate brachytherapy [50], while the other chooses a homogeneous dose approach [51]. The latter published their early prospective Phase II study results [52] which includes the treatment planning constraints found in Table 6.4.

4D CT in planning is most commonly used to determine a gating window, and create an ITV (Internal Target Volume) based on the width of the gating window chosen for a particular patient. If compression is used to manage respiratory motion, the 4D CT can be used to generate a MIP image which determines the residual motion to be covered by the ITV.

If the treatment delivery system is capable of tumor and/or fiducial tracking instead of compression, breath-hold or gating, there are several additional issues to assess in the planning phase: deformation, rotation, and the relative position of the tumor to the OARs. Deformation and rotation margins can be determined by fusing the tumor center of mass in different respiratory phases and determining the ITV. The relative position of tumor to critical organs is much more difficult to assess with current technology. Deformable image registration algorithms have very limited suitability for this task, because relative voxel motion is not based on anatomically correct modeling of tissue motion.