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

Measuring surface dose in megavoltage beams for modern radiotherapy

The build-up region of a megavoltage photon beam is the most studied yet practically elusive part of radiotherapy. The dose at the skin, and within the first few millimetres of tissue, depends on contamination by low-energy electrons from the treatment head, lateral scatter and accessories such as wedges and immobilisation masks. Getting that number right informs bolus placement, field size limits and the safety of paediatric craniospinal irradiation.

In Australian radiotherapy departments, surface dose measurements shape daily workflow more than many physicists first realise. The Peter MacCallum Cancer Centre in Melbourne weighs skin-sparing benefits against target coverage for chest wall disease, while the Crown Princess Mary Cancer Centre in Westmead needs a defensible estimate at the bolus interface. Regional centres in Townsville, Cairns and Launceston, often operating with a single physicist, rely on protocols executable without lengthy setup.

Regulatory expectations reinforce the importance of rigorous technique. The Australian Radiation Protection and Nuclear Safety Agency publishes the foundational code for medical exposure, and the NSW Environment Protection Authority translates those national expectations into licence conditions for linear accelerators. A documented surface dose protocol is a routine requirement during commissioning, after major upgrades and following new techniques.

This overview summarises the principal methods available to a working medical physicist, with attention to practical implementation across Australian cancer centres. The aim is to give a junior physicist a defensible starting point and to refresh the working knowledge of an experienced practitioner who may last have performed a build-up measurement several years ago.

Physical foundations of the build-up region

Beneath the skin, the dose distribution in a megavoltage beam is governed by a transient charged-particle equilibrium that has not yet been established. Photons entering the patient liberate high-energy electrons, which travel a finite distance before depositing their energy. Until the depth of maximum dose, dmax, is reached, the dose rises with depth, producing the characteristic build-up curve. The dose at depth zero reflects contaminant electrons generated in the flattening filter, jaws, monitor chamber and the air path between source and patient.

These contaminant electrons have a broad energy spectrum, but the practical implication is straightforward: a 6 MV beam giving 50 to 60 percent of dmax at the surface for a 10 by 10 centimetre field will show higher surface dose for larger fields, oblique incidence and physical wedges. Erythema and moist desquamation can occur with tangential breast fields, total body irradiation setups and total skin electron techniques in some Sydney and Brisbane mycosis programmes.

For modern flattening-filter-free beams, the contamination profile is altered but not eliminated, and the surface dose remains a measurement to be made rather than assumed from manufacturer data. Reporting conditions therefore need to specify field size, SSD, wedge and any immobilisation devices in contact with the patient.

Extrapolation chamber dosimetry

The extrapolation chamber is the closest thing the field has to a primary standard for surface dose measurement. A variable-volume parallel-plate ionisation chamber allows the user to plot ionisation against electrode separation and extrapolate the curve to zero gap, where the chamber approximates a true surface. The technique, described in IAEA Technical Reports Series 398 and AAPM Task Group Report 8, remains the reference against which other dosimeters are compared.

In practice, the extrapolation chamber is slow, requires mechanical care and a stable environment, and is rarely used for routine patient measurements. Most Australian academic centres, including those at the University of Sydney and the University of Adelaide, keep an extrapolation chamber for commissioning and cross-checks but rely on faster detectors for clinical QA. A well-executed measurement can settle a dispute between two diode systems and provide the absolute reference for a chamber-specific correction factor.

Thermoluminescent and optically stimulated dosimeters

Thermoluminescent dosimeters, particularly TLD-100 chips and microcubes, have been the workhorse of skin dose audits for over forty years. Their small size, near tissue-equivalence and ability to be taped to a patient or phantom suit them to measurement at multiple points along a field. The Australian national dosimetry audit, run with ARPANSA's secondary standards laboratory, has used mailed TLD capsules for intercomparison across providers for many years.

Optically stimulated luminescence dosimeters, such as the nanoDot, have largely superseded TLDs for many in-clinic applications because they can be read in a tabletop system within minutes and reused after annealing. The OSL response is energy-dependent at low energies, however, and the user must apply a correction when the dosimeter is used at the surface where the spectrum is soft. The trade-off between convenience and accuracy is one that every Australian department negotiates in its own QA documentation.

Solid-state detectors for clinical workflow

Silicon diodes offer real-time readout, small active volumes and excellent reproducibility, which has made them popular for routine surface dose checks. The over-response of unshielded diodes to low-energy photons is well documented, and a wide-shield or a specifically commissioned detector can mitigate that issue. In busy photon and electron programmes, a single diode may serve as the daily QA tool for IMRT, VMAT and electron match lines, with periodic checks against film and TLD.

MOSFET detectors extend the same logic to single-dose measurements with immediate electronic readout. They have been used in Australian centres for in-vivo dosimetry during total marrow irradiation and for verification of paediatric treatments where any extra measurement time must be justified. The finite lifetime of the MOSFET, its angular dependence and the need for a stable bias supply all influence the operational protocol, but the detector fills a useful niche for measurements that cannot wait for film processing or TLD readout.

Radiochromic film and emerging techniques

Gafchromic EBT3 film remains a popular two-dimensional dosimeter for surface and near-surface measurements, particularly when the geometric detail of an irregular field or bolus interface needs to be preserved. With careful calibration in the relevant energy range, the film can resolve dose gradients beyond the spatial resolution of any single-point detector. Many Australian centres use EBT3 as an arbitration tool when a clinical decision rests on a single measurement, such as whether to remove bolus from the final fraction.

Plastic scintillators, EPID-based dosimetry and Monte Carlo validated planning system calculations are extending the practical range of surface dose work. Readers interested in a broader survey of where the discipline is heading may find the advances in radiation therapy dosimetry page a useful companion. None of these newer tools replaces the underlying physical measurement; they sit alongside it, each contributing to a layered QA programme that protects the patient at the surface, where the first centimetres of tissue meet the beam.

Practical recommendations for surface dose programmes

  • Commission at least two independent dosimetry systems, such as an extrapolation chamber and EBT3 film, so that no single technique is the sole arbiter of a clinical decision.
  • Document the energy dependence of every solid-state detector used at the surface, and re-verify the correction factor after any change to the linac target or flattening filter.
  • Specify reference field sizes, SSDs, wedge angles and immobilisation hardware in every surface dose report so that measurements can be reproduced at a later audit.
  • Use TLD or OSL mail audits, including those coordinated through ARPANSA-linked networks, as an external check on internal protocols.
  • For tangential breast and total body irradiation fields, compare measured surface dose against TPS predictions at plan approval, and flag plans that exceed locally agreed action levels.
  • Keep an annotated log of all surface dose measurements, including the reason for the test, the detector used and any clinical action taken; this is invaluable for accreditation reviews and incident investigation.
  • Review the surface dose protocol at least every two years, or whenever a new treatment technique, immobilisation device or beam modifier is introduced into clinical use.

Physicists attending the meeting in Banff can carry these recommendations into hallway conversations and formal QA committee discussions, where the practical challenge of measuring what cannot easily be seen is shared across the profession. The interdisciplinary clinical collaboration between dosimetrists, radiation oncologists and medical physicists is what allows new surface dose techniques to be implemented safely, and Australian centres have a long tradition of cross-institutional intercomparison that should be sustained as the discipline evolves.