Small and non standard fields
The use of small fields in radiotherapy techniques has increased substantially, in particular in stereotactic treatments and large uniform or non-uniform fields that are composed of small fields such as for IMRT. This has been facilitated by the increased availability of standard and add-on multileaf collimators and a variety of new treatment units, such as GammaKnife (Leksell GammaKnife®, Elekta Instrument AB, Stockholm, Sweden), CyberKnife® Robotic Radiosurgery System and TomoTherapy® Hi-Art® (Accuray Inc., Sunnyvale, CA).
A number of dosimetry incidents have been reported due to the misunderstanding of small field dosimetry. For example in 2007, an inappropriate detector was used to commission the 6 mm × 6 mm photon field defined by a microMLC (Novalis Tx, BrainLAB) leading to incorrect data being entered into the treatment planning system and to the mistreatment of 145 patients within a period of a year. The mistake was discovered by the manufacturer through a corroboration of the data measured worldwide by its users.
Small photon fields differ from conventional broad reference fields in their lateral dimensions causing the penumbrae at both sides of the field to overlap and most commonly used detectors to be large relative to the field size.
Three physical conditions, of which one, two or all are fulfilled, determine if an external photon beam is small: (i) there is a loss of charged particle equilibrium, (ii) partial occlusion of the primary photon source by the collimating devices takes place and (iii) the size of the detector compared to the beam dimensions is large.
In addition, dosimetric errors are larger than in conventional beams mostly due to two reasons: (i) the reference conditions recommended by conventional Codes of Practice cannot be realised in some machines and (ii) the measurement procedures for absorbed dose determination in small and composite fields is not standardized.
No ideal detector exists for small field dosimetry so it is recommended to use two or three different types of detector suitable for a particular measurement to improve assurance against unacceptable dose errors by means of redundancy in the results.
Introduction to References
There have been a number of publications over the years outlining the problems associated with using small fields in radiotherapy. These are too numerous to mention here but a summary was presented in 2010 by an IPEM working group.
Alfonso et al. addressed the problem of absolute small field dosimetry by presenting formalism for the dosimetry of small and composite fields. The concept of two new intermediate calibration fields was introduced: (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.
On-going IAEA/AAPM work on small field dosimetry
The IAEA and AAPM have formed a working group on small field dosimetry whose remit is to produce a Code of Practice based on the Alfonso formalism. As of February 2015, the Code is in draft form but it is expected to be published soon.