SRS and SBRT

17.1

Introduction

Stereotactic radiosurgery (SRS) was originally defined as fulfilling all of the following conditions:

• 1.

  Single-fraction treatment

• 2.

  High dose per fraction (>5 Gy)

• 3.

  Target with diameter of <3.5 cm in the brain

• 4.

  Delivery accuracy of <1 mm as defined by the Winston-Lutz test

• 5.

  No planning target volume (PTV) or internal target volume (ITV) margins are used; clinical target volume (CTV) margins may be used

SRS was later expanded to also include single-fraction treatments of spinal lesions and then also to include fractionated treatments. The current definition includes:

• 1.

  1 to 5 fraction treatment

• 2.

  High dose per fraction (>5 Gy)

• 3.

  Target with diameter of <3.5 cm in the central nervous system (CNS) (brain or spine)

• 4.

  Delivery accuracy of <1 mm as defined by an end-to-end test

Stereotactic body radiation therapy (SBRT), also sometimes called SABR (stereotactic ablative radiation therapy), is defined by the following conditions:

• 1.

  1 to 5 fraction treatment (up to 8 in Canada)

• 2.

  High dose per fraction (>5 Gy)

• 3.

  Target size up to 4 cm diameter in the lung or 5 cm to 7 cm for target locations in the thoracic and abdomino-pelvic cavity

• 4.

  Technical delivery accuracy of <1.5 mm to 2 mm as defined by an end-to-end test

• 5.

  ITV and PTV margins are used to compensate interfraction and intrafraction motion and deformation. Most targets except prostate require respiratory motion management.

17.2

Technical Requirements

The high doses and steep dose gradients of SRS/SBRT delivered in very few fractions result in a much smaller margin of error than for conventionally fractionated radiation therapy. A machine being used for the delivery of SRS/SBRT must therefore at minimum fulfill the following stringent technical requirements outlined in Part I of this book:

• 1.

  Chapter 4 : The machine must meet the mechanical tolerances outlined in American Association of Physicists in Medicine Task Group (AAPM TG)-142

• 2.

  Chapters 1 / 2 : Small field beam commissioning

• 3.

  Chapter 7 : Image guidance capabilities (or frame for SRS)

• 4.

  Chapter 16 : End-to-end (E2E) test results <1 mm and delivery quality assurance (DQA) results <3%/2 mm

• 5.

  Chapter 19 : Respiratory motion management techniques

The E2E test is an adaptation of the Winston-Lutz test for the era of image-guided, frameless SRS/SBRT. Ideally, each of the steps would be performed by the treatment team member(s) who will perform the same function for a patient treatment. Performing an E2E test in this way as part of the commissioning process can help clarify the workflow and information flow and serve as a tool to develop the initial set of policies and procedures:

• 1.

  An appropriate anthropomorphic phantom for the treatment site, equipped with fiducials for localization similar to what will be used in a patient, is loaded with in-vivo dosimetry tools such as thermoluminescent dosimeter (TLD), optically stimulated luminescent detector (OSLD), metal-oxide semiconductor field-effect transistors (MOSFETs), gel, or film (physicist).

• 2.

  The phantom is immobilized and scanned, using the same immobilization devices and scan protocols that would be used for the patient (radiation therapist).

• 3.

  The images are imported into the treatment planning system, and the hidden target is contoured (physician).

• 4.

  A treatment plan is developed fulfilling the prescribed dose constraints (dosimetrist).

• 5.

  The plan is signed and reviewed (physician).

• 6.

  The plan documentation is processed and the plan exported to the treatment unit (dosimetrist).

• 7.

  A second check, secondary monitor unit (MU) calculation, and patient-specific QA measurements if applicable are performed (physicist).

• 8.

  The treatment is delivered (radiation therapist).

• 9.

  The dose measurement result is analyzed (physicist).

17.3

Policies and Procedures

Before starting an SRS/SBRT program, a set of policies and procedures (P&Ps) must be developed in a multidisciplinary setting of all care providers involved. To determine which P&Ps are needed, a patient flowchart can serve as a useful tool. This flowchart will also serve as a quality control tool to make sure all members of the patient treatment team understand their role in the care process, the order of care steps, and the information flow.

Table 17.1 outlines a sample set of P&Ps to meet the American College of Radiology (ACR) practice accreditation guidelines and the Canadian Association of Radiation Oncologists (CARO) scope of practice guidelines for lung, liver, and spine SBRT recommendations. The ASTRO white paper on quality and safety considerations in SBRT emphasizes that SRS/SBRT is not one technique/modality and that expertise in one anatomic target area does not constitute expertise for other sites. It is therefore necessary to develop procedures that are specific to the technology in use at the institution, as well as specific to disease site(s).

The P&Ps should be tested using a mock patient treatment scenario, preferably including the image acquisition, treatment planning, and treatment delivery of a patient care plan to a phantom. After the initial patients have been treated, the P&Ps should be revised. After the initial revision, the P&Ps should be reviewed and, if applicable, updated on an annual basis.

In lieu of binders at multiple locations, which are difficult to keep current, an electronic system should be used to centrally store the P&P manual. The P&P should be accessible to all members of the Radiation Oncology department, be reachable from the majority of clinical workstations, and offer version tracking and sign-off capabilities to easily identify the most up-to-date iteration of the P&P.