Comparing flattened and unflattened beams in radiotherapy

Comparing Dosimetric Characteristics of New Flattened Versus Unflattened Beams requires more than checking whether two plans deliver the same prescribed dose. Flattened beams, traditionally shaped with a flattening filter, and flattening-filter-free (FFF) beams produce different profiles, dose rates, out-of-field radiation levels, and commissioning demands. Those differences can affect treatment efficiency and the confidence of a clinical physics team. Learn more about Accomodations.

For medical physicists, the useful comparison begins with measurement conditions. Field size, beam energy, source-to-surface distance, detector choice, phantom material, depth, and dose-per-pulse all influence the result. A statement that an FFF beam is “more efficient” has little value unless efficiency is defined and linked to a treatment technique, such as stereotactic body radiotherapy, volumetric-modulated arc therapy, or conventional three-dimensional conformal treatment.

The archived Canadian Organization of Medical Physicists meeting in Banff provides a useful professional context for this subject. Its scientific programme reflects the way conference delegates approach emerging accelerator technology: by connecting beam data, Monte Carlo modelling, quality assurance, and clinical implementation rather than treating any one measurement as decisive.

The topic remains relevant in Australia, where metropolitan cancer centres in Sydney, Melbourne, Brisbane, Perth, and Adelaide may operate mixed linac fleets. A new machine must fit existing workflows, local licensing requirements, staffing patterns, and the realities of patients travelling from regional areas. The best beam is therefore the one that delivers a defensible clinical benefit within the whole service.

Why beam flattening changes the dose picture

A flattening filter is designed to make the central portion of a photon beam more uniform at a selected depth and field size. Removing it produces a forward-peaked profile, with higher dose rates and a softer low-dose tail outside the field in many configurations. The unflattened beam can shorten beam-on time, but it does not behave like a flattened beam with the filter simply taken away.

The profile shape is one of the first visible differences. FFF beams commonly show a pronounced central-axis peak and greater off-axis fall-off, while flattened beams offer a broader plateau. Wedges, jaws, multileaf collimators, and intensity-modulated delivery can compensate for profile characteristics, but the compensation must be demonstrated through measured data and treatment planning validation.

Energy labels also require care. Removing the filter changes the beam spectrum, contaminant electron contribution, head scatter, and dose-per-pulse. Comparing “6 MV” flattened and “6 MV FFF” beams as though they were dosimetrically identical can obscure changes in percentage depth dose, tissue-phantom ratios, lateral profiles, and surface dose.

Measurements that separate useful differences

A disciplined beam-data set should include central-axis and off-axis measurements across clinically relevant field sizes and depths. The aim is to distinguish changes caused by the filter from changes caused by detector volume averaging, positioning uncertainty, phantom scatter, or an unsuitable measurement range.

Useful comparison points include:

• Percentage depth dose or tissue-phantom ratio at reference and treatment depths
• In-plane and cross-plane profiles, including penumbra and field-edge gradients
• Output factors for small, intermediate, and large fields
• Surface dose, buildup-region behaviour, and electron contamination
• Head scatter, leakage, and dose outside the treatment field
• Dose-rate stability, monitor-unit linearity, and delivery-time performance

Detector selection becomes especially important for small fields and steep gradients. A large-volume ionisation chamber can smooth the apparent profile and underestimate the true penumbra, while diodes or microchambers may introduce energy, angular, or polarity effects. Film, radiochromic film, two-dimensional arrays, and appropriately corrected solid-state detectors can provide complementary evidence.

The reference conditions should be recorded with enough detail for another physicist to reproduce them. Australian departments often need to compare data across sites, vendors, and service contracts, so a consistent naming convention and traceable calibration pathway are practical safeguards. Results should be interpreted with the same care used for any external audit or annual linac review.

Commissioning and treatment-planning consequences

FFF beams can be attractive for stereotactic treatments because their high dose rate may reduce treatment time and the period during which intrafraction motion can occur. That advantage is most meaningful when the plan remains robust under delivery constraints. A shorter beam-on interval does not automatically mean a shorter appointment if imaging, immobilisation, couch correction, and patient changeover dominate the workflow.

Treatment-planning algorithms must reproduce the measured characteristics of the beam. Pencil-beam methods may struggle with sharp gradients, heterogeneities, and small fields, whereas collapsed-cone and Monte Carlo approaches can offer stronger modelling in challenging geometries. The Banff archive’s Monte Carlo abstracts illustrate why transport modelling remains valuable when measured dose alone cannot explain the behaviour.

Commissioning should test both routine and exceptional use cases. Plans should be created for small stereotactic targets, large palliative fields, irregular apertures, arcs, and situations involving lung, bone, or air cavities. Gamma analysis or similar pass-rate statistics can support review, but the clinical meaning of a discrepancy must be considered alongside dose gradients, target location, and organ-at-risk proximity.

Practical choices for Australian services

An Australian clinic assessing a new FFF-capable linac must consider the local market as well as the physics. Vendor support, spare-parts availability, software licensing, service response times, and compatibility with an existing oncology information system can influence the total value of a machine. A high dose rate is less useful if a department cannot obtain timely engineering support or train enough staff to maintain the configuration.

Radiation use is regulated through a combination of national guidance and state or territory legislation. Facilities should align commissioning, shielding assessment, radiation surveys, and ongoing quality assurance with relevant ARPANSA recommendations and the requirements of the jurisdiction in which the service operates. Documentation must be clear enough for internal governance, accreditation, and inspection, with responsibilities assigned to qualified medical physicists and radiation safety personnel.

Planning should also reflect everyday clinical habits. Early morning lists, long patient travel from regional New South Wales or northern Queensland, and tight imaging schedules can make reliable treatment duration particularly valuable. At the same time, a busy public hospital may prioritise throughput and resilience, while a private metropolitan centre may place greater emphasis on premium stereotactic capability and predictable appointment times.

Before installation, teams can use a focused evidence checklist:

• Define the clinical techniques that genuinely require or benefit from FFF delivery
• Agree on reference conditions, detector systems, and data formats
• Budget for commissioning time, staff education, and independent verification
• Confirm local radiation licensing, shielding, and acceptance-test responsibilities
• Test integration with planning, record-and-verify, imaging, and motion-management systems
• Establish escalation pathways for abnormal output, profile drift, or software changes

From beam data to a defensible clinical decision

The central comparison should be framed around patient outcomes and service performance. Flattened beams may remain preferable for broad, uniform fields or established workflows where profile uniformity simplifies planning and verification. FFF beams may offer advantages in high-dose-rate stereotactic delivery, selected arc treatments, and cases where reducing beam-on time has a meaningful motion-management benefit.

The decision should use a balanced set of endpoints: target coverage, conformity, homogeneity where relevant, organ-at-risk dose, monitor units, delivery time, image guidance requirements, leakage, and quality-assurance burden. A plan that is marginally faster but substantially more sensitive to setup error may not be the better choice. Likewise, a beam with higher monitor units may still be clinically attractive if it improves robustness or reduces low-dose exposure in a specific treatment pathway.

The conference setting also highlights the value of shared professional review. Comparing results with published data, vendor specifications, local commissioning records, and peer institutions can expose hidden assumptions. Australian physicists can add further value by documenting how beam selection affects rural referral pathways, public-sector capacity, workforce training, and compliance with state-based radiation controls.

A well-designed comparison therefore ends with an implementation decision, not a single winning curve. Record the measured evidence, the uncertainties, the treatment sites included, and the conditions under which the recommendation applies. When that record is revisited during annual review or a future accelerator replacement, it becomes a practical clinical resource rather than a one-time commissioning file.

Use the archived meeting resources alongside current vendor data, ARPANSA guidance, and your department’s own measurements to build a comparison that is clinically relevant, locally compliant, and reproducible. A careful beam-characterisation programme can turn the choice between flattened and unflattened delivery into a transparent improvement in radiotherapy practice.