A new formalism for reference dosimetry of small and nonstandard fields

TLDR

The increasing use of small and composite fields in radiotherapy, enabled by advanced multileaf collimators and new treatment units, has led to larger dosimetric errors because conventional reference conditions cannot be established and absorbed‑dose‑to‑water measurements are not standardized. This paper proposes a new formalism to extend conventional Codes of Practice for clinical reference dosimetry of small and composite fields based on absorbed dose to water. An international IAEA–AAPM working group has developed the formalism, which introduces two intermediate calibration fields—a static machine‑specific reference field and a plan‑class specific reference field—to standardize dosimetry for modalities lacking conventional reference conditions. The working group invites comments from the medical physics community before finalizing a Code of Practice or other recommendation.

Abstract

The use of small fields in radiotherapy techniques has increased substantially, in particular in stereotactic treatments and large uniform or nonuniform fields that are composed of small fields such as for intensity modulated radiation therapy (IMRT). This has been facilitated by the increased availability of standard and add‐on multileaf collimators and a variety of new treatment units. For these fields, dosimetric errors have become considerably larger than in conventional beams mostly due to two reasons; (i) the reference conditions recommended by conventional Codes of Practice (CoPs) cannot be established in some machines and (ii) the measurement of absorbed dose to water in composite fields is not standardized. In order to develop standardized recommendations for dosimetry procedures and detectors, an international working group on reference dosimetry of small and nonstandard fields has been established by the International Atomic Energy Agency (IAEA) in cooperation with the American Association of Physicists in Medicine (AAPM) Therapy Physics Committee. This paper outlines a new formalism for the dosimetry of small and composite fields with the intention to extend recommendations given in conventional CoPs for clinical reference dosimetry based on absorbed dose to water. This formalism introduces the concept of two new intermediate calibration fields: (i) a static machine‐specific reference field for those modalities that cannot establish conventional reference conditions and (ii) a plan‐class specific reference field closer to the patient‐specific clinical fields thereby facilitating standardization of composite field dosimetry. Prior to progressing with developing a CoP or other form of recommendation, the members of this IAEA working group welcome comments from the international medical physics community on the formalism presented here.