Jean-Pierre Bissonnette, Peter A. Balter, Lei Dong, Katja M. Langen, D. Michael Lovelock, Moyed Miften, Douglas J. Moseley, Jean Pouliot, Jan-Jakob Sonke, Sua Yoo
5 min
Image-guided radiation therapy (IGRT) has become essential for managing geometric uncertainties in patient setup and internal organ motion. As clinics adopt various CT-based imaging technologies—such as kilovoltage cone-beam CT (kV-CBCT), megavoltage cone-beam CT (MV-CBCT), fan-beam MVCT, and CT-on-rails—the need for standardized quality assurance (QA) has grown. The AAPM Task Group 179 (TG-179) provides a consensus-based, vendor-independent framework to ensure that these systems maintain the high geometric fidelity required for modern radiation therapy, including stereotactic body radiation therapy (SBRT).
The report emphasizes that the primary goal of IGRT is to detect and correct positional errors. Consequently, geometric accuracy—specifically the alignment between the imaging system and the treatment beam isocenter—is the most critical component of the QA program. The task group recommends daily geometric checks to identify gross errors resulting from system collisions, software upgrades, or maintenance. While more precise monthly and annual tests are required to calibrate and verify system performance, daily checks provide a rapid, efficient way to ensure the system is safe for patient treatment.
Beyond geometry, the report addresses image quality and imaging dose. Unlike diagnostic CT, IGRT systems often operate under different physical constraints, such as large cone angles and scatter environments, which can degrade image contrast and uniformity. The report suggests that while image quality tests (e.g., spatial resolution, low-contrast detectability) are necessary, their frequency can be reduced to a semiannual schedule once a system's stability is established. Furthermore, the report highlights the importance of balancing the clinical benefit of IGRT with the cumulative imaging dose, particularly for patients with long life expectancies.
Successful IGRT implementation requires a multidisciplinary approach involving physicists, therapists, and physicians. The report outlines the resource requirements for commissioning and maintaining these programs, noting that clinics should start with end-to-end tests using phantoms to build confidence before transitioning to clinical patient studies. By documenting site-specific protocols and maintaining rigorous QA, clinics can leverage IGRT to improve tumor control probability and reduce toxicity through tighter treatment margins.
PURPOSE: Commercial CT-based image-guided radiotherapy (IGRT) systems allow widespread management of geometric variations in patient setup and internal organ motion. This document provides consensus recommendations for quality assurance protocols that ensure patient safety and patient treatment fidelity for such systems. METHODS: The AAPM TG-179 reviews clinical implementation and quality assurance aspects for commercially available CT-based IGRT, each with their unique capabilities and underlying physics. The systems described are kilovolt and megavolt cone-beam CT, fan-beam MVCT, and CT-on-rails. A summary of the literature describing current clinical usage is also provided. RESULTS: This report proposes a generic quality assurance program for CT-based IGRT systems in an effort to provide a vendor-independent program for clinical users. Published data from long-term, repeated quality control tests form the basis of the proposed test frequencies and tolerances. CONCLUSION: A program for quality control of CT-based image-guidance systems has been produced, with focus on geometry, image quality, image dose, system operation, and safety. Agreement and clarification with respect to reports from the AAPM TG-101, TG-104, TG-142, and TG-148 has been addressed.
Alex: [curious, leaning in] Does the report handle image quality with the same kind of unified approach, or is that more fragmented?
Sam: [measured, precise] It's a hybrid. The report borrows general principles from fan-beam diagnostic CT, but cone-beam geometry — kilovoltage or megavoltage — introduces its own artifacts. Larger cone angles mean more scatter reaching the detector, which produces cupping and capping artifacts you don't typically see on a standard diagnostic scanner.
Alex: [analytical, processing] Right, because the scatter-to-primary ratio is so much higher in cone-beam systems. So how do you standardize QA for something inherently less stable than a diagnostic scanner?
Sam: [steady, grounded] They shift the target from absolute image quality to relative stability. You establish a baseline during commissioning, then monitor for deviation from that baseline over time. For something like low-contrast or spatial resolution, you're not trying to match a diagnostic scanner's performance — you're checking that your specific system hasn't drifted from where it started.
Alex: [probing] But doesn't that drift measurement depend on reproducing the exact same phantom and setup each time? Change the scatter environment and the comparison falls apart.
Sam: [nodding in voice, clear] Exactly — and the report is explicit about that. Change the phantom size or field size and you've invalidated the comparison. The goal isn't to chase theoretical perfection in every scan; it's to catch sudden, gross failures — a detector element going bad, for instance — before they affect treatment. [[RP_SECTION:dose-optimization-strategies|Dose Optimization Strategies]]
Alex: [reflective, slower] So it's failure detection, not performance characterization. What about dose? Daily imaging on top of daily treatment sounds like it adds up.
Sam: [measured, building the case] That's the real trade-off the report wrestles with. Imaging dose accumulates over a full course of treatment, so the recommendation is to tailor dose to the clinical task rather than defaulting to a single protocol. If you only need to confirm bony anatomy for positioning, you don't need the same mAs as a soft-tissue verification scan.
Alex: [deliberate, checking understanding] So the physicist is optimizing a dose-to-information ratio for each anatomical site?
Sam: [quiet confidence, precise] Precisely, and it's meant to be a documented risk-benefit decision, not an ad hoc one. The report pushes a tiered approach — start with high-contrast bony matching, and only move to more complex soft-tissue protocols once the team has shown the system is stable and the safety margins are established.
Alex: [reflective] So the overall picture is a report that unifies process across vendors, but still leaves the physics — and the judgment calls around dose and drift — squarely in the physicist's hands.
Sam: [measured] That's a fair summary. It's a consensus document for clinical process design, not a substitute for understanding the physics of your own machine.
Alex: If you want the specific test protocols and figures we skipped, you can generate a deep dive of this paper. The report has the rest either way.
Sam: Thanks for listening.