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New Guideline for Radiotherapy Treatment with 'Small Fields'

IAEA regional training course on ‘small field’ dosimetry, Johannesburg, South Africa ( Photo: D. van der Merwe/IAEA)

A new code of practice, published last week, will help guide medical physicists to implement methodologies that can improve the accuracy of radiation doses for cancer patients when smaller fields and more targeted radiation is employed. Such so called ‘small field’ radiation is used to treat smaller tumours in the early stages of cancer, as well as larger ones whose treatment requires differentiated intensity of the radiotherapy to deliver varying doses to various parts of the tumour. Small field radiation is a rapidly growing treatment modality thanks, in part, to early detection of cancer and more sophisticated treatment machines.

The new code of practice will be applicable mainly to advanced radiotherapy techniques. “Radiotherapy has been changing very rapidly,” said May Abdel-Wahab, Director of the IAEA’s Division of Human Health. “For example, about 15 years ago, intensity modulated radiotherapy, a radiotherapeutic technique that allows modulation of the radiation beam intensity to enable better conformity to the contours of the target or tumour, was not commonly used around the world,” she said.

Another treatment technique that uses small fields is stereotactic body radiotherapy, which treats tumours with millimetre precision using a large dose given in just one to five sessions while simultaneously sparing healthy tissue. Accurate dosimetry measurements are crucial to make sure radiation treatment is safe and effective. For conventional radiotherapy, this has been achieved by the universally adopted existing codes of practice, which enable common procedures to be followed. However, the existing codes are not applicable to small fields because of differences in the underlying physics.

In response, the IAEA, in collaboration with the American Association of Physicists in Medicine (AAPM), published the first international code of practice for the dosimetry of small static fields. The publication will contribute to the understanding of small field dosimetry and consistency in the measured dose worldwide, Abdel-Wahab said.

Melissa Martin, the President of the AAPM, said that “a key step in the radiotherapy process is the requirement for consistent reference dosimetry traceable to metrological primary standards and common procedures to be used within a country. This joint document is vital to achieve this important goal.”

The new code of practice is a comprehensive guideline that details the theory of small field dosimetry and gives data and a methodology for accurate dosimetry.

“In the future, new radiotherapy technologies and novel machines will continue to be developed and used for cancer radiotherapy,” Abdel-Wahab said. “Radiotherapy continues to be a rapidly changing field and it is important to provide appropriate guidance for the safe and effective use of these technologies to Member States.”

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