4.6 Daily Linac Quality Assurance per AAPM TG-142 Specifications

4.6 Daily Linac Quality Assurance per AAPM TG-142 Specifications

Quality assurance (QA) in radiation therapy is a mandatory operational framework designed to guarantee that linear accelerators deliver precise radiation doses safely, accurately, and consistently. In 2009, the American Association of Physicists in Medicine (AAPM) published Task Group 142 (TG-142), titled Quality Assurance of Medical Accelerators. TG-142 updated and expanded previous recommendations (TG-40), incorporating specific QA guidelines for modern technologies such as asymmetric jaws, Multi-Leaf Collimators (MLC), portal imaging, Cone-Beam CT (CBCT), and stereotactic radiosurgery (SRS/SBRT). Radiation therapists bear primary responsibility for performing and documenting daily linac QA prior to clinical patient treatments.

Rationale and Regulatory Framework of AAPM TG-142

AAPM TG-142 classifies linear accelerator QA tests into three temporal frequencies: Daily, Monthly, and Annual. The daily QA battery is designed to quickly detect significant machine malfunctions, dosimetric output drifts, laser misalignment, imaging errors, or safety interlock failures that could cause patient injury or treatment misadministration.

Daily QA must be completed every morning prior to initiating clinical treatments. If any test parameter falls outside the defined TG-142 tolerance limit, radiation therapists must follow established institutional action levels, withhold clinical treatment delivery, and immediately notify a qualified medical physicist.

Daily Dosimetric Quality Assurance Protocols

Dosimetric QA verifies that the linear accelerator output (dose delivered per Monitor Unit) remains stable and constant compared to baseline values established during physics calibration.

Output Constancy Testing

Therapists perform daily output checks using dedicated commercial daily QA devices (e.g., Sun Nuclear Daily QA3, PTW QUICKCHECK) containing arrayed ionization chambers or solid-state diode detectors placed at a fixed SSD (typically 100 cm). Output constancy is checked for all clinical photon energies (e.g., 6 MV, 10 MV, 6 MV FFF) and selected electron energies.

• Photon Beam Output Constancy Tolerance: ±3.0% relative to baseline.
• Electron Beam Output Constancy Tolerance: ±3.0% relative to baseline.

If the measured daily output drift exceeds ±3.0%, the therapist must refrain from treating patients. Output errors between 3% and 5% require physics investigation and recalibration before clinical use. Output variations exceeding 5% indicate severe machine failure (such as pulse modulator instability or beam steering failures).

Daily Mechanical and Alignment Verification Tests

Mechanical checks ensure that optical indicators, lasers, and distance measuring systems accurately reflect the position of the radiation isocenter.

Laser Alignment

Room positioning lasers must intersect precisely at the mechanical isocenter. Therapists align the daily QA phantom to the side and ceiling room lasers and measure spatial displacement.

• Non-SRS/SBRT Machine Tolerance: ±2.0 mm
• SRS/SBRT Dedicated Machine Tolerance: ±1.0 mm

Optical Distance Indicator (ODI)

The ODI projects the SSD reading onto the phantom skin surface. It is verified against a rigid mechanical distance pointer at 100 cm SSD.

• ODI Tolerance: ±2.0 mm at nominal SSD.

Collimator & Field Size Indicators

Visual light field sizes (e.g., 10x10 cm²) projected onto the phantom surface are verified against digital console indicators.

• Collimator Indicator Tolerance: ±2.0 mm (or ±1.0 mm for SRS/SBRT).

Daily Safety System and Door Interlock Audits

Safety tests verify that all safety systems, switches, and interlocks operate correctly to protect patients and clinical staff. All safety items are evaluated on a Pass / Fail basis:

1. Vault Entrance Door Interlock: Opening the vault door while beam-on is initiated must immediately terminate radiation delivery.
2. Audio-Visual Systems: Both treatment room HD video cameras and full-duplex audio intercoms must function clearly.
3. Beam-On Warning Lights: Room entrance lights and console indicators must illuminate when high voltage / RF power is energized.
4. Stereotactic Auxiliary Interlocks: Interlocks connected to specialized SRS cones or motorized add-on accessories must function.
5. Collision Anti-Touch Sensors: Physical pressure applied to gantry faceplate or detector touch guards must halt motorized motion.

Daily Imaging QA for Planar and CBCT Systems

Because modern linacs utilize IGRT for daily patient positioning, the imaging system's spatial agreement with the treatment beam isocenter must be verified daily.

Image Positioning & Repositioning Accuracy

The daily QA phantom contains radiopaque targets placed at known coordinates. Therapists acquire planar kV/MV images and volumetric CBCT scans, perform automated image registration, and apply phantom shift corrections.

• Non-SRS/SBRT Machine Imaging Tolerance: Positioning and repositioning accuracy within ≤2.0 mm.
• SRS/SBRT Dedicated Machine Imaging Tolerance: Positioning and repositioning accuracy within ≤1.0 mm.

Additionally, therapists inspect planar and CBCT image quality for obvious artifacts, detector ring artifacts, or electronic noise.

Complete AAPM TG-142 Daily Linac QA Test Matrix

The following table summarizes the full daily QA test battery required under AAPM TG-142:

Strict execution and electronic documentation of daily QA ensures that linear accelerators remain compliant, safe, and precise for every patient treated.