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Annual Scientific Meeting of the Canadian Organization of Medical Physicists — July 9–12, 2014, The Banff Centre, Banff, Alberta, Canada

Choosing Beam Energy Wisely for Stereotactic Radiosurgery

Stereotactic radiosurgery (SRS) demands a level of precision that makes beam energy selection a clinical and engineering decision, rather than a default setting on a treatment machine. The ideal photon beam must deliver a highly conformal dose to a small intracranial target while limiting exposure to healthy brain, optic structures, the brainstem and other sensitive organs at risk.

Energy affects penetration, lateral penumbra, monitor-unit requirements, surface dose, head scatter and the performance of the multileaf collimator (MLC). Those effects become especially important when targets are small, irregular or located close to critical anatomy. A treatment team therefore needs to assess the entire delivery chain, from imaging and contouring through planning, verification and patient-specific quality assurance.

The scientific discussions preserved through the 2014 Annual Scientific Meeting of the Canadian Organization of Medical Physicists offer a useful professional context for this topic. Although the meeting took place in Banff, Alberta, the same reasoning applies to Australian services, whether a high-volume metropolitan centre in Sydney or Melbourne, a public hospital in Brisbane, or a regional network supporting patients across large distances.

Clinical Questions Before Energy Selection

The first decision is not simply whether to use 6 MV, 10 MV or a flattening-filter-free beam. The team should define the clinical objective: single-fraction radiosurgery, hypofractionated stereotactic radiotherapy, or a multi-target plan delivered with fixed fields, dynamic conformal arcs or VMAT. Target depth, volume, shape and proximity to organs at risk should be documented before comparing beam options.

For most intracranial SRS cases, 6 MV photons provide a practical balance between penetration and small-field behaviour. At modest depths, a lower-energy beam can achieve sharp dose fall-off and manageable output factors. Higher energies may offer useful penetration for deeper or larger targets, but their benefits must be weighed against changes in scatter, surface dose, MLC transmission and the possible production of unwanted neutron radiation at sufficiently high energies.

Physical Effects In Small Fields

Small stereotactic fields do not behave like the broad reference fields used for conventional radiotherapy calibration. Lateral electronic disequilibrium, detector-volume averaging and partial occlusion of the source can all affect measured output. A beam that appears attractive in a planning system may perform differently when delivered through a small cone or a tightly shaped MLC aperture.

Flattening-filter-free (FFF) beams can increase dose rate and shorten treatment time, which is valuable when a patient must remain immobilised with submillimetre accuracy. Their altered profile and increased dose per pulse also require suitable detector selection, validated algorithms and careful commissioning. The choice between 6 MV FFF and a flattened beam should be based on measured dosimetric performance, not on delivery speed alone.

Evidence From Planning And Verification

Comparative planning is one of the strongest methods for selecting energy. Create clinically realistic plans at the available energies and compare conformity index, gradient index, dose to organs at risk, treatment time, total monitor units and the robustness of the result after image-guidance corrections. For a deep lesion near the optic chiasm, the best plan may differ from the best plan for a superficial convexity target.

Measurement must include the conditions in which the beam will actually be used. Commissioning should cover output factors, depth-dose curves, profiles, penumbra, MLC leakage, end-to-end accuracy and the effect of couch and immobilisation accessories. Independent checks using radiochromic film, small-volume ionisation chambers or appropriately characterised solid-state detectors can expose issues hidden by a treatment planning system.

A useful conference-era habit is to connect technical work with professional governance. The archived conference schedule reflects the broad mix of scientific sessions, examinations, business meetings and practical events that support safe clinical practice. Beam selection should be reviewed through the same multidisciplinary lens, involving radiation oncologists, dosimetrists, radiographers, engineers and medical physicists.

Adapting The Approach To Australian Practice

Australian centres work across a varied service landscape. A tertiary facility such as Peter MacCallum Cancer Centre or Royal North Shore Hospital may have several linacs, dedicated SRS platforms and a large specialist team, while a regional service may depend on one machine and refer complex cases interstate. Energy selection must therefore account for local resilience, backup capacity, staff expertise and the time required to repeat measurements after a major software or hardware change.

The Australian market also makes equipment availability and vendor support practical considerations. A plan based on a 10 MV FFF option is of limited value if that energy is unavailable on the commissioned machine, poorly supported locally or difficult to verify with the service’s existing detectors. Public and private providers may face different purchasing cycles, service contracts and patient throughput pressures, while Medicare funding and referral patterns influence how often complex SRS is offered.

Local language and workflow matter as well. A physicist might describe a plan as “good to go” only after image guidance, couch corrections and independent checks are complete, not simply because the dose distribution looks tidy. In a busy Melbourne or Perth department, reducing an extended treatment by several minutes can help the list run on time, but that operational gain must not displace robust patient-specific QA. For patients travelling from regional Queensland or Western Australia, reliable scheduling can also reduce the burden of repeat visits.

Commissioning, Standards And Ongoing Review

Energy selection should be documented in a commissioning report that explains the clinical rationale, measurement methods, tolerances and limitations. Reference dosimetry may follow the locally adopted protocol, while small-field work needs a recognised framework such as the IAEA TRS-483 code of practice. The equipment, detector models, software versions and analysis methods should be recorded so that results remain reproducible.

Australian services can align their governance with ACPSEM guidance, relevant RANZCR expectations and facility-based radiation safety requirements. Periodic review is important after MLC replacement, beam steering changes, new imaging systems, planning-system upgrades or the introduction of a new immobilisation device. Trend analysis of output, profiles and end-to-end tests can identify gradual drift before it affects a patient.

Professional participation strengthens that process. The COMP archive’s material on committee volunteer guidance illustrates how technical communities rely on members who contribute beyond their immediate clinical lists. Australian physicists similarly benefit from sharing commissioning experience through ACPSEM groups, local networks, educational meetings and peer review.

A Practical Selection Framework

A defensible choice combines physics, clinical need and service capability. The following checks can keep the decision focused:

  • Define target depth, size, shape and proximity to critical structures before choosing an energy.
  • Compare flattened and FFF options using measured small-field data rather than nominal beam labels.
  • Evaluate conformity, dose gradient, organ-at-risk exposure, monitor units and treatment time together.
  • Confirm that the detector, calculation algorithm and QA method are suitable for the selected field sizes.
  • Include image guidance, immobilisation, couch attenuation and end-to-end accuracy in the assessment.
  • Record machine availability, maintenance support, staffing and the needs of patients travelling long distances.
  • Revisit the decision after software, MLC, imaging or beam-delivery changes.

The final selection should be clinically explainable: the team should be able to state why a particular energy provides the best balance for the target and the service. In many intracranial cases, 6 MV or 6 MV FFF will remain a strong starting point, but the correct answer depends on measured performance and the treatment geometry.

Use this framework during the next SRS protocol review, compare the available energies with representative patient plans, and document the evidence in the department’s commissioning and quality-assurance records. A carefully justified beam choice turns a routine machine setting into a traceable part of safe, high-precision care.