SRS QA Procedure: An Overview

David Huang

Stereotactic Radiosurgery (SRS) is a one-time application of a high radiation dose to a stereotactically defined target volume. It differs markedly from both conventional neurosurgery and conventional external-beam radiotherapy. It is a unique hybrid of surgery and radiotherapy. Unlike many procedures in neurosurgery or radiotherapy, a multidisciplinary approach that incorporates the unique knowledge of three separate medical disciplines, neurosurgery, radiation oncology, and medical physics, is required to produce an optimal result.

A high degree of dose conformity is a hallmark of SRS, which is generally achieved by using appropriate circular beams to fit the lesion, optimizing arc angles and weights and using multiple isocenters or dynamically shaping the field during arc rotations with mini (or micro) MLCs. Accuracy of beam delivery is another hallmark of SRS. It is strictly controlled by a specially designed stereotactic apparatus, which is used through all steps of the process: imaging, target localization, head immobilization, and treatment set-up. Because of the critical nature of brain tissue, elaborate quality assurance program is an essential for SRS procedure.

SRS is a special procedure, which requires careful commissioning followed by a rigorous quality assurance program to maintain its original accuracy specifications. Quality assurance involves both the clinical and physical aspects of SRS. The physical part may be divided into two categories: treatment QA and routine QA. The former includes checking or double-checking or triple-checking of the procedures and treatment parameters pertaining to individual patient. The latter is designed to periodically inspect the hardware and software performance to ensure compliance with original specifications.

Patient treatment involves many steps and procedures. It is highly desirable that detailed checklists are prepared to document these steps in a proper sequence. The objective of the checklist should be to ensure procedural accuracy at each step of the way and to minimize the chance of a treatment error.

Major components of treatment QA consist of checking,

(a) stereotactic frame accuracy including phantom base, CT/MRI/angiographic localizer, pedestal or couch mount, docking process for Gamma Knife;

(b) imaging data transfer, treatment plan parameters, target position, monitor unit/time calculation;

(c) frame alignment with gantry and couch eccentricity, congruence of target point with radiation isocenter, collimator setting, cone/helmet size, couch position, inserted block position, patient immobilization and safety locks; and

(d) treatment console programming of beam energy, MU/time, helmet size, arc angle, etc.

A routine QA program is designed to check the hardware/software performance of SRS equipment on a schedule frequency basis. For the Linac, the relevant QA protocol is the AAPM Report # 40. For the Gamma Knife, a routine QA program should be compliant with the NRC regulation. For the Cyberknife, a routine QA procedure should be compliant with related AAPM Reports. For the SRS apparatus, the routine QA schedule is recommended by the AAPM Report # 54.

In my talk, I will address the contents of a checklist with checking procedures and details of a routine QA program with examples for X-Knife, Gamma-Knife, and Cyber-Knife. I will also briefly describe the clinical procedures for SRS in my talk. Hence, the audience could have better idea and great understanding about the SRS procedures and its associated QA procedures as well.