A new formalism for reference dosimetry of small and nonstandard fields

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.

Characteristics of a novel treatment system for linear accelerator–based stereotactic radiosurgery

“…Therefore, it is crucial to choose the correct detector, considering the size, energy dependence, and perturbation, for example. A new formalism has been developed for the dosimetry of small field (39) . For the Edge commissioning, the machine‐specific reference field is defined at $3\times 3 {\text{cm}}^2$, since the conventional $10\times 10 {\text{cm}}^2$ cannot be established for all detectors considering the energy dependence of the diodes and volume averaging effect of the ion chambers.…”

“…For the Edge commissioning, the machine‐specific reference field is defined at $3\times 3 {\text{cm}}^2$, since the conventional $10\times 10 {\text{cm}}^2$ cannot be established for all detectors considering the energy dependence of the diodes and volume averaging effect of the ion chambers. The field factor ${\Omega }_{Q_{cline,}{Q}_{msr}}^{f_{clin},f_{msr}}$, under the notion proposed by Alfonso et al, (39) which converts the absorbed dose to water for the machine‐specific reference field ($3\times 3 {\text{cm}}^2$) to the absorbed dose to water for the small clinical field, should be carefully evaluated to account for the difference of the detector response and beam quality at two different field sizes. A Monte Carlo calculated actor ${\Omega }_{Q_{cline,}{Q}_{msr}}^{f_{clin},f_{msr}}$ was recommended to correct the field factor.…”

Small field detector correction factors: effects of the flattening filter for Elekta and Varian linear accelerators

“…Where necessary, small field correction factors $false(k_{Q_{clin}{Q}_{msr}}^{f_{clin},f_{msr}}false)$, as defined by Alfonso et al, (18) have been provided. These correction factors account for detector over‐ or under response compared to a water‐equivalent detector.…”

Robotic radiosurgery system patient‐specific QA for extracranial treatments using the planar ion chamber array and the cylindrical diode array

“…The output factor is the ratio of the dose reading of the center detector for an individual cone to the dose reading of the 60 mm cone 31 , 32 . The measured output factors were compared with the commissioning data measured with a pinpoint ion chamber (PTW, Freiburg, Germany).…”