Standards for Small Fields in External Beam Therapy

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Objective for A.15.0

  • Establish and disseminate standards for Small Fields in External Beam Therapy.

Background: Radiotherapy (RT) technology is rapidly evolving to narrow the treatment fields and spare the neighboring critical structures. In addition, very small treatment fields are becoming more widely utilized due to the advances in RT delivery devices. Some of these include banks of miniaturized lead leaves called micro multileaf collimators (MLCs). Another example is an adjustable iris for the CyberKnife delivery system that collimates fields down to 5mm diameter. There are expectations from most radiation oncology centers that treatment beams will be available with smaller fields and that the medical physicist will commission them as accurately as the large fields.

The most common method of external-beam RT involves using a linear accelerator. The linac typically generates electrons with a range of energies up to 20 MeV, which are then used to produce x-rays. Stereotactic radiotherapy or radiosurgery (SRT / SRS) has been used for many years to delivery fields as small as 4mm with the Co-60 Gamma Knife (GK) system. However when conventional linacs are required to perform the special task of small field delivery, the delivery mechanisms are more complex than the relatively straightforward treatments of the GK using fixed collimators. It should be noted that even though GK treatments have been in use for decades, the dosimetry of the smallest fields has been uncertain due to the challenges in measuring the dose in such small fields.

There is a need to accurately measure doses from small fields (<2cm). Dosimeters have finite volumes and dimensions, which lead to volume averaging effects. The larger the volume, the higher the effect. Even ion chambers considered to be micro chambers still occupy a space large enough to peturb the ideal measurement of the dose to a point on the central axis of the field. Thus, smaller dosimeters such as diodes, diamond detectors, scintillators, gels, and MOSFETs have been investigated for their applications in small-field dosimetry, but none have proven to be perfect.

Action Items:

1 – Establish and disseminate recommendations for small-field dosimetry with specific emphasis on the proper use of detectors in small and nonstandard treatment fields.

Resource Requirements:

1 –??? Partnerships between NIST, ADCL’s and the medical community are essential in this area.