Stereotactic Body Irradiation: Extracranial Tumors

Safety Considerations and Technical Aspects of Stereotactic Body Radiotherapy

Relative to cranial SRS, the major additional practical hurdle that must typically be overcome to target an extracranial tumor with SBRT accurately and precisely involves accounting for breathing-related motion, present to some extent for nearly all cases apart from largely immobile spine and paraspinous tumors. Periodic breathing motion displacements have been both quantified and, if necessary, controlled with equipment and procedures used for simulation/planning and consistently applied toward treatment. Positioning errors (e.g., misalignment) either during a treatment or between treatments would also otherwise require expansion of targets. Linear accelerator (LINAC) manufacturers now offer SBRT-ready treatment delivery systems that integrate a high-performance LINAC with one or more forms of image-guidance technologies, thus ensuring proper target relocalization and beam alignment. Collectively, these SBRT systems and procedures allow the use of considerably smaller fields compared with conventional radiotherapy without missing the target.

The American Society for Radiation Oncology (ASTRO) has issued guidelines on SBRT. The American Association of Physicists in Medicine (AAPM) Task Group 101 report expands on the ASTRO guidelines. ASTRO has also produced a white paper on quality and safety in SRS and SBRT. In addition to describing the personnel and training requirements, the reports highlight key features of the quality assurance (QA) measures necessary to ensure the safety of the procedure.

The complete process of care for a patient receiving SBRT consists of multiple steps: patient immobilization, motion assessments (and motion management if necessary), planning computed tomography (CT) image acquisition, analysis and processing of any four-dimensional image sets, fusion of the planning images with relevant diagnostic image sets, target delineation, dosimetric planning, patient-specific QA testing, patient setup on the treatment couch, acquisition of guidance images to allow target relocalization, deployment of any motion-managing techniques or devices, proper initialization and commencement of the patient's unique beam and/or arc sequence, real-time monitoring of the integrity of the treatment delivery process, and the patient's stability and tolerance. At each step along the way are opportunities for systematic errors, miscalibrations, miscalculations, and any number of operator mistakes.

Regarding the possible purely technical sources of errors, such as impaired performance of gantry motion or incorrect registration of pretreatment cone beam CT scans with the planning images in the image-guidance software, careful initial commissioning and subsequent regular maintenance checks of individual components of the system are mandatory. Additionally, it is strongly recommended that there be end-to-end testing that incorporates all of the linked component parts to ensure that the additive impact of numerous small errors do not accumulate to a clinically relevant systematic targeting error whereby the planning target volume (PTV) is not properly irradiated for any combination of reasons. The possibility of human error can never be completely eliminated. However, as noted in the chapter on stereotactic irradiation of central nervous system (CNS) tumors ( Chapter 27 ), steps to reduce the chance of errors include the use of checklists and the development of a general culture of safety in which communication flows freely and nonjudgmentally among colleagues, with the common goal of quality patient care.

SBRT requires proper patient repositioning, target localization, and management of breathing-related motion. Commercially available immobilization devices include several types of body frames with external fiducial markers, but these or any frameless system must always be used with accompanying image guidance that involves ultrasound, kV radiographs, CT scan, or magnetic resonance imaging (MRI) to verify the location of internal targets relative to the beams to be used ( Fig. 28.1 ). Because SBRT treatment sessions take more time than conventional external beam treatments, patient comfort is an important issue to lessen the chance that patients might shift their position between the time of image guidance and treatment delivery.

Breathing motion management can be accomplished in one of several ways. Motion-dampening techniques involve light to moderate abdominal compression coupled with thoughtful patient coaching intended to transfer the predominant breathing forces from primarily abdominal (diaphragmatic contraction) to primarily chest wall (external intercostals, scalene, and sternocleidomastoid contraction). In this way, breathing forces that otherwise put a tumor into motion with larger displacements are reduced while still facilitating an adequate tidal volume. Also within this category are coached breath hold maneuvers to “freeze” the tumor in space by drawing in and holding a constant tidal volume (e.g., deep inspiration) and holding that volume while the radiation beam is engaged. Gating systems may be used for SBRT in the same manner as for conventionally fractionated radiotherapy: the movement of external surface markers is correlated with phases of the breathing cycle, and beam output is triggered only when the markers are located within a preselected segment of the breathing cycle, implying tumor location within the expected range of motion. Tracking or “chasing” systems move the radiation beam to follow the movement of the tumor. Both breath-hold and gating methods have a duty cycle whereby the beam is turned on and off for periods of time, thus lengthening the treatment time. To some degree, these motion management techniques can be used in conjunction (e.g., abdominal compression and gating) to reduce the overall target displacements or duty cycle.

Most clinical implementation of SBRT thus far has involved high-energy photons as the source of therapeutic radiation. However, charged particles could also be used. There is no absolute standard or consensus class solution for the combination of beam or arc angles best suited for any given clinical situation, and each case can present idiosyncratic challenges. In general, though, to achieve a tightly focused high-dose distribution within the PTV and rapid dose fall-off outside the PTV, a combination of multiple (often 7–10) noncoplanar beams or multiple arcs is often required ( Fig. 28.2 ). Intensity modulation across the individual beams or arc segments can be incorporated within SBRT, but it is important to bear in mind that, as for cranial SRS, it is typically advantageous to allow for a dose hot spot to accumulate inside the PTV to steepen the dose fall-off outside the target, exploiting the natural gradient of the lateral beam penumbra to some extent in this regard. One notable exception to this practice would be in the case of prostate SBRT, in which it is important to avoid a hot spot in the region of the urethra.

Clinical Radiobiology and Normal-Tissue Dose Constraints

The emerging knowledge about high-dose per-fraction irradiation is reviewed in more detail in the chapter on stereotactic irradiation of CNS tumors ( Chapter 27 ). Briefly, the mathematical models established to describe the relationship between radiation dose and biological effect in tumor and normal tissue that are generally reliable for conventionally fractionated radiotherapy are less certain in the setting of high-dose per-fraction irradiation. Furthermore, the complex milieu of the tumor microenvironment includes capillaries, stroma, and components of the immune system, each with its own independent response to ionizing radiation and interconnected impact on tumor control and toxicity. Given these caveats concerning traditional radiobiological models, for the purpose of predicting the effect of SBRT on tumors and normal tissues it is safest to rely whenever possible on empirical observations that relate dose or dose-volume parameters to directly observed rates of tumor control or normal-tissue toxicity.