Plan-class Specific Reference Research Articles

On the implementation of the plan-class specific reference field using multidimensional clustering of plan features and alternative strategies for improved dosimetry in modulated clinical linear accelerator treatments.

The plan-class specific reference field concept could theoretically improve the calibration of radiation detectors in a beam environment much closer to clinical deliveries than existing broad beam dosimetry protocols. Due to a lack of quantitative guidelines and representative data, however, the pcsr field concept has not yet been widely implemented. This work utilizes quantitative plan complexity metrics from modulated clinical treatments in order to investigate the establishment of potential plan classes using two different clustering methodologies. The utility of these potential plan clusters is then further explored by analyzing their relevance to actual dosimetric correction factors. Two clinical databases containing several hundred modulated plans originally delivered on two Varian linear accelerators were analyzed using 21 plan complexity metrics. In the first approach, each database's plans were further subdivided into groups based on the anatomic site of treatment and then compared to one another using a series of nonparametric statistical tests. In the second approach, objective clustering algorithms were used to seek potential plan clusters in the multidimensional complexity-metric space. Concurrently, beam- and detector-specific dosimetric corrections for a subset of the modulated clinical plans were determined using Monte Carlo for three different ionization chambers. The distributions of the dosimetric correction factors were compared to the derived plan clusters to see which plan clusters, if any, could help predict the correction factor magnitudes. Ultimately, a simplified volume averaging metric (SVAM) is shown to be much more relevant to the total dosimetric correction factor than the established plan clusters. Plan groups based on the site of treatment did not show noticeable distinction from one another in the context of the metrics investigated. An objective clustering algorithm was able to discriminate volumetric modulated arc therapy (VMAT) plans from step-and-shoot intensity-modulated radiation therapy plans with an accuracy of 90.8%, but no clusters were found to exist at any level more specific than delivery modality. Monte Carlo determined correction factors for the modulated plans ranged from 0.970 to 1.104, 0.983 to 1.027, and 0.986 to 1.009 for the A12, A1SL, and A26 chambers, respectively, and were highly variable even within the treatment modality plan clusters. The magnitudes of these correction factors were explained almost entirely by volume averaging with SVAM demonstrating positive correlation with all Monte Carlo established total correction factors. Plan complexity metrics do provide some quantitative basis for the investigation of plan clusters, but an objective clustering algorithm demonstrated that quantifiable differences could only be found between VMAT and step-and-shoot beams delivered on the same treatment machine. The inherent variability of the Monte Carlo determined correction factors could not be explained solely by the modality of the treatment but were instead almost entirely dependent upon the volume averaging correction, which itself depends on the detector position within the dose distribution, dose gradients, and other factors. Considering the continued difficulty of determining a relevant plan metric to base plan clusters on, case-by-case corrections may instead obviate the need for the pcsr field concept in the future.

Plan-Class Specific Reference Quality Assurance for Volumetric Modulated Arc Therapy

Background: There have been much efforts to develop the proper and realistic machine Quality Assurance (QA) reflecting on real Volumetric Modulated Arc Therapy (VMAT) plan. In this work we propose and test a special VMAT plan of plan-class specific (pcsr) QA, as a machine QA so that it might be a good solution to supplement weak point of present machine QA to make it more realistic for VMAT treatment. Materials and Methods: We divided human body into 5 treatment sites: brain, head and neck, chest, abdomen, and pelvis. One plan for each treatment site was selected from real VMAT cases and contours were mapped into the computational human phantom where the same plan as real VMAT plan was created and called plan-class specific reference (pcsr) QA plan. We delivered this pcsr QA plan on a daily basis over the full research period and tracked how much MLC movement and dosimetric error occurred in regular delivery. Several real patients under treatments were also tracked to test the usefulness of pcsr QA through comparisons between them. We used dynalog file viewer (DFV) and Dynalog file to analyze position and speed of individual MLC leaf. The gamma pass rate from portal dosimetry for different gamma criteria was analyzed to evaluate analyze dosimetric accuracy. Results and Discussion: The maxRMS of MLC position error for all plans were all within the tolerance limit of < 0.35 cm and the positional variation of maxPEs for both pcsr and real plans were observed very stable over the research session. Daily variations of maxRMS of MLC speed error and gamma pass rate for real VMAT plans were observed very comparable to those in their pcsr plans in good acceptable fluctuation. Conclusion: We believe that the newly proposed pcsr QA would be useful and helpful to predict the mid-term quality of real VMAT treatment delivery.

Small composite field correction factors for the CyberKnife radiosurgery system: clinical and PCSR plans

A formalism has been proposed for small and non-standard photon fields in which correction factors are used to correct dosimeter response in small fields (indiviual or composite) relative to that in a larger machine-specific reference (MSR) field. For clinical plans consisting of several fields, a plan-class specific reference (PCSR) plan can also be defined, serving as an intermediate calibration field between the MSR and clinical plans within a certain plan-class. In this work, the formalism was applied in the calculation of for 21 clinical plans delivered by the radiosurgery system, each plan employing one or two of the smallest diameter collimators: 5 mm, 7.5 mm, and 10 mm. Three detectors were considered: the Exradin A16 and A26 micro chambers, and the W1 plastic scintillator. The clinical plans were grouped into 7 plan-classes according to commonly shared characteristics. The suitability of using a PCSR plan to represent the detector response of each plan within the plan-class was investigated. Total and intermediate correction factors were calculated using the Monte Carlo user code. The corrections for the micro chambers were large, primarily due to the presence of the low-density air cavity and the volume averaging effect. The correction for the scintillator was found to be close to unity for most plans, indicating that this detector may be used to measure small clinical plan correction factors in any plan except for those using the 5 mm collimator. The PCSR plan was shown to be applicable to plan-classes comprising isocentric plans only, with plan-classes divided according to collimator size. For non-isocentric plans, the variation of as a function of the point of measurement within a single plan, as well as the high inter-plan-class variability of the correction factor, precludes the use of a PCSR plan.

Beam monitor calibration in scanned light-ion beams.

To propose a formalism for the reference dosimetry of scanned light-ion beams consistent with IAEA TRS-398 and Alfonso et al. [Med. Phys. 35, 5179-5186 (2008)]. To identify machine-specific reference (msr) fields and plan-class specific reference (pcsr) fields consistent with the definitions given by Alfonso et al. To review the literature of beam monitor calibration in scanned beams using three different methods in terms of this common formalism. Four types of msr fields are identified as those that are meant to calibrate the beam monitor for scanned beams with particular energies. Two types of pcsr fields are identified as those that are meant to apply one or more tuning factors to the entire delivery chain. The formalism establishes the energy-dependent relation between the number of particles incident on the phantom surface and the beam monitor reading and distinguishes three routes to determine the beam monitor calibration function: (i) the use of a calibrated reference ionization chamber in a single-layer scanned beam, (ii) the use of a cross-calibrated large-area parallel plate ionization chamber in a single-energy beamlet, and (iii) the use of a calibrated reference ionization chamber in a box field to adjust a calibration curve obtained by a Faraday cup or an ionization chamber. Examples of all three methods and comparisons between them from the literature are analysed. The formalism can form the basis of future dosimetry recommendations for scanned particle beams and the analysis of the literature data in terms of this formalism can form the basis of data compilations for the application of the dosimetry procedures.

On the use of presage detectors for the dosimetry of helical tomotherapy small fields

Introduction The use of small fields in modern radiotherapy techniques has stretched the importance of verifying the delivered 3D dose distributions characterized by steep dose gradients and complex shapes. In the absence of an ideal dosimeter a multi-detector approach is essential. Purpose To apply 3D PRESAGE dosimetry for the verification of helical TomoTherapy dose distributions and assess possible corrections of the A1SL ion-chamber response for a head-and-neck plan. Materials and methods A plan-class specific reference field (fpcsr), as recommended by TG-148, and a typical head-and-neck irradiation plan were delivered to a PMMA phantom incorporating appropriate inserts for A1SL ion-chamber, film and PRESAGE dosimeters. Irradiation of fpcsr permitted the evaluation of the corresponding k(Qpcsr, fpcsr; Qref, fref) correction factor for the A1SL chamber, through comparison to PRESAGE measurements. PRESAGE dosimetry results for the head-and-neck plan, were compared to TPS calculations and corresponding measurements from 1D/2D dosimeters commonly employed for TomoTherapy QA purposes. Results Although within experimental uncertainties A1SL chamber was found to overestimate the dose at fpcsr by almost 2%, in agreement with published results using alanine dosimeters. Agreement within uncertainties was also observed between planned and presage-measured dose distribution for the head-and-neck plan, exhibiting local gamma index passing rates >90%. When placed within a homogeneous dose distribution, the response of A1SL seemed to be correction free. Conclusion Besides water equivalence and exquisite spatial resolution, necessitated for small field dosimetry and determination of appropriate correction factors, PRESAGE dosimeters offer the advantage of 3D dose verification. Disclosure Research supported by an IKY fellowship of excellence for postdoctoral research in Greece–Siemens Program.

Sci‐Sat AM: Radiation Dosimetry and Practical Therapy Solutions ‐ 12: Suitability of plan class specific reference fields for estimating dosimeter correction factors for small clinical CyberKnife fields

Purpose:The purpose of this work is to investigate the utility of plan class specific reference (PCSR) fields for predicting dosimeter response within isocentric and non‐isocentric composite clinical fields using the smallest fields employed by the CyberKnife radiosurgery system.Methods:Monte Carlo dosimeter response correction factors (CFs) were calculated for a plastic scintillator and microchamber dosimeter in 21 clinical fields and 9 candidate plan‐class PCSR fields which employ the 5, 7.5 and 10 mm diameter collimators. Measurements were performed in 5 PCSR fields to confirm the predicted relative response of detectors in the same field.Results:Ratios of corrected measured dose in the PCSR fields agree to within 1% of unity. Calculated CFs for isocentric fields agree within 1.5% of those for PCSR fields. Large and variable microchamber CFs are required for non‐isocentric fields, with differences as high as 5% between different clinical fields in the same plan class and 4% within the same field depending on the point of measurement. Non‐isocentric PCSR fields constructed to have relatively homogenous dose over a region larger than the detector have very different ion chamber CFs from clinical fields. The plastic scintillator detector has much more consistent response within each plan class but still require 3–4% corrections in some fields.Conclusions:While the PCSR field concept is useful for small isocentric fields, this approach may not be appropriate for non‐isocentric clinical fields which exhibit large and variable ion chamber CFs which differ significantly from CFs for homogenous field PCSRs.

Establishment of the Commissioning Procedure for Modifying the Beam Model of Helical Tomotherapy.

Although much evidence about the helical tomotherapy system are available, there is not a document about the procedure of quality assurance (QA) for changing the beam model. This study establishes the commissioning procedure for modifying the beam model of helical tomotherapy. Firstly, some intensity-modulated radiotherapy (IMRT) plans were created, and compared them with the calculated dose and the measured dose. Secondly, the absorbed doses to water in the machine-specific reference field and the plan-class specific reference field with a protocol in Japan; Standard Dosimetry of Absorbed Dose to Water in External Beam Radiotherapy (Standard Dosimetry 12) were measured. Thirdly, we reconfirmed patient-specific quality assurance. The recommended commissioning procedure after the change of the beam model was shown through three verification processes. This report would be helpful for not only changing the beam model of helical tomotherapy but also introducing Standard Dosimetry 12 to a clinic.

Correction factors for ionization chamber dosimetry in CyberKnife: Machine‐specific, plan‐class, and clinical fields

The aim of this work is the application of the formalism for ionization chamber reference dosimetry of small and nonstandard fields [R. Alfonso, P. Andreo, R. Capote, M. S. Huq, W. Kilby, P. Kjäll, T. R. Mackie, H. Palmans, K. Rosser, J. Seuntjens, W. Ullrich, and S. Vatnitsky, "A new formalism for reference dosimetry of small and nonstandard fields," Med. Phys. 35, 5179-5186 (2008)] to the CyberKnife robotic radiosurgery system. Correction factors for intermediate calibration fields, a machine-specific reference field (msr) and two plan-class specific reference fields (pcsr), have been studied. Furthermore, the applicability of the new formalism to clinical dosimetry has been analyzed through the investigation of two clinical treatments. PTW31014 and Scanditronix-Wellhofer CC13 ionization chamber measurements were performed for the fields under investigation. Absorbed dose to water was determined using alanine reference dosimetry, and experimental correction factors were calculated from alanine to ionization chamber readings ratios. In addition, correction factors were calculated for the intermediate calibration fields and one of the clinical treatment fields using the Monte Carlo method and these were compared with the experimental values. Overall correction factors deviating from unity by approximately 2% were obtained from both measurements and simulations, with values below and above unity for the studied intermediate calibration fields and clinical fields for the ionization chambers under consideration. Monte Carlo simulations yielded correction factors comparable with those obtained from measurements for the machine-specific reference field, although differences from 1% to 3.3% were observed between measured and calculated correction factors for the composite intermediate calibration fields. Dose distribution inhomogeneities are thought to be responsible for such discrepancies. The differences found between overall correction factors associated with the proposed intermediate calibration fields and the clinical fields under investigation show that depending on the clinical field and the detector used, either a machine-specific reference field or a plan-class specific reference field is more representative for the clinical field. Given the experimental and numerical uncertainties and the small number of clinical fields considered in this study the significance of these observations is limited and it remains unclear for the CyberKnife if there would be a significant gain in using a pcsr field rather than a msr field as reference field for relative dosimetry.

Quality correction factors of composite IMRT beam deliveries: Theoretical considerations

In the scope of intensity modulated radiation therapy (IMRT) dosimetry using ionization chambers, quality correction factors of plan-class-specific reference (PCSR) fields are theoretically investigated. The symmetry of the problem is studied to provide recommendable criteria for composite beam deliveries where correction factors are minimal and also to establish a theoretical limit for PCSR delivery k(Q) factors. The concept of virtual symmetric collapsed (VSC) beam, being associated to a given modulated composite delivery, is defined in the scope of this investigation. Under symmetrical measurement conditions, any composite delivery has the property of having a k(Q) factor identical to its associated VSC beam. Using this concept of VSC, a fundamental property of IMRT k(Q) factors is demonstrated in the form of a theorem. The sensitivity to the conditions required by the theorem is thoroughly examined. The theorem states that if a composite modulated beam delivery produces a uniform dose distribution in a volume V(cyl) which is symmetric with the cylindrical delivery and all beams fulfills two conditions in V(cyl): (1) the dose modulation function is unchanged along the beam axis, and (2) the dose gradient in the beam direction is constant for a given lateral position; then its associated VSC beam produces no lateral dose gradient in V(cyl), no matter what beam modulation or gantry angles are being used. The examination of the conditions required by the theorem lead to the following results. The effect of the depth-dose gradient not being perfectly constant with depth on the VSC beam lateral dose gradient is found negligible. The effect of the dose modulation function being degraded with depth on the VSC beam lateral dose gradient is found to be only related to scatter and beam hardening, as the theorem holds also for diverging beams. The use of the symmetry of the problem in the present paper leads to a valuable theorem showing that k(Q) factors of composite IMRT beam deliveries are close to unity under specific conditions. The theoretical limit k(Q(pcsr),Q(msr) ) (f(pcsr),f(msr) )=1 is determined based on the property of PCSR deliveries to provide a uniform dose in the target volume. The present approach explains recent experimental observations and proposes ideal conditions for IMRT reference dosimetry. The result of this study could potentially serve as a theoretical basis for reference dosimetry of composite IMRT beam deliveries or for routine IMRT quality assurance.

On charged particle equilibrium violation in external photon fields

In a recent paper by Bouchard et al. [Med. Phys. 36(10), 4654-4663 (2009)], a theoretical model of quality correction factors for idealistic so-called plan-class specific reference (PCSR) fields was proposed. The reasoning was founded on the definition of PCSR fields made earlier by Alfonso et al. [Med. Phys. 35(11), 5179-5186 (2008)], requiring the beam to achieve charged particle equilibrium (CPE), in a time-averaged sense, in the reference medium. The relation obtained by Bouchard et al. was derived using Fano's theorem (1954) which states that if CPE is established in a given medium, the dose is independent of point-to-point density variations. A potential misconception on the achievability of the condition required by Fano (1954) might be responsible for false practical conclusions, both in the definition of PCSR fields as well as the theoretical model of quality correction factor. In this paper, the practical achievability of CPE in external beams is treated in detail. The fact that this condition is not achievable in single or composite deliveries is illustrated by an intuitive method and is also formally demonstrated. Fano's theorem is not applicable in external beam radiation dosimetry without (virtually) removing attenuation effects, and therefore, the relation conditionally defined by Bouchard et al. (2009) cannot be valid in practice. A definition of PCSR fields in the recent formalism for nonstandard beams proposed by Alfonso et al. (2008) should be modified, revising the criterion of CPE condition. The authors propose reconsidering the terminology used to describe standard and nonstandard beams. The authors argue that quality correction factors of intensity modulated radiation therapy PCSR fields (i.e., k(Q(pcsr),Q) (f(pcsr),f(ref) )) could be unity under ideal conditions, but it is concluded that further investigation is necessary to confirm that hypothesis.

Reference dosimetry for helical tomotherapy: Practical implementation and a multicenter validation

The aim of this study was to implement a protocol for reference dosimetry in tomotherapy and to validate the beam output measurements with an independent dosimetry system. Beam output was measured at the reference depth of 10 cm in water for the following three cases: (1) a 5 × 10 cm(2) static machine specific reference field (MSR), (2) a rotational 5 × 10 cm(2) field without modulation and no tabletop in the beam, (3) a plan class specific reference (PCSR) field defined as a rotational homogeneous dose delivery to a cylindrical shaped target volume: plan with modulation and table-top movement. The formalism for reference dosimetry of small and nonstandard fields [Med.Phys.35: 5179-5186, 2008] and QA recommendations [Med.Phys.37: 4817-4853, 2010] were adopted in the dose measurement protocol. All ionization chamber measurements were verified independently using alanine∕EPR dosimetry. As a pilot study, the beam output was measured on tomotherapy Hi-art systems at three other centers and directly compared to the centers specifications and to alanine dosimetry. For the four centers, the mean static output at a depth of 10 cm in water and SAD = 85 cm, measured with an A1SL chamber following the TG-148 report was 6.238 Gy∕min ± 0.058 (1 SD); the rotational output was 6.255 Gy∕min ± 0.069 (1 SD). The dose stated by the center was found in good agreement with the measurements of the visiting team: D(center)∕D(visit) = 1.000 ± 0.003 (1 SD). The A1SL chamber measurements were all in good agreement with Alanine∕EPR dosimetry. Going from the static reference field to the rotational ∕ non modulated field the dose rate remains constant within 0.2% except for one center where a deviation of 1.3% was detected. Following the TG-148 report, beam output measurements in water at the reference depth using a local protocol, as developed at different centers, was verified. The measurements were found in good agreement with alanine∕EPR dosimetry. The presented methodology may provide a good concept for reference dosimetry.

Investigation of three radiation detectors for accurate measurement of absorbed dose in nonstandard fields

To establish accurate experimental dosimetry techniques for reference dose measurements in nonstandard composite fields. A cylindrical PMMA phantom filled with water was constructed, at the center of which reference absorbed dose to water for a head and neck IMRT delivery was measured. Based on the proposed new formalism for reference dosimetry of nonstandard fields [Alfonso et al., Med. Phys. 35, 5179-5186 (2008)], a candidate plan-class specific reference (pcsr) field for a typical head and neck IMRT delivery was created on the CT images of the phantom. The absorbed dose to water in the pcsr field normalized to that in a reference 10 x 10 cm2 field was measured using three radiation detectors: Gafchromic EBT films, a diamond detector, and a guarded liquid-filled ionization chamber developed in-house (GLIC-03). Pcsr correction factors [formula in text] were determined for five different types of air-filled ionization chambers (Exradin A12, NE-2571, Exradin A1SL, Exradin A14, and PinPoint 31006) in a fully rotated delivery and in a delivery with the same MLC settings and weights but from a single gantry angle (a collapsed delivery). The combined standard uncertainty in measuring the correction factor [formula in text] using the three dosimetry techniques was 0.3%. For all the air-filled ionization chambers and the pcsr field tested, the correction factor was not different from unity by more than +/- 0.8%. For the fully rotated delivery, the correction factors were in a narrow range of 0.9955-0.9986, while in the collapsed delivery, they were in a slightly broader range of 0.9922-1.0048. In the collapsed delivery, the Farmer-type chambers (Exradin A12 and NE2571) had very similar correction factors (0.9922 and 0.9931, respectively), whereas the correction factors for the smaller chambers showed more distinct chamber-type dependence. The authors have established three experimental dosimetry techniques that allow reference measurements of nonstandard field correction factors [formula in text] for air-filled ionization chambers at the 0.3% 1sigma uncertainty level. These techniques can be used to determine criteria for the selection of plan-class specific reference fields and ultimately improve clinical reference dosimetry of nonstandard fields.

Application of a new dosimetry formalism to volumetric modulated arc therapy (VMAT)

Volumetric modulated arc therapy (VMAT) offers a challenge to classical dosimetry protocols as the beams are dynamic in orientation and aperture shape and may include small apertures. The aim of this paper is to apply a formalism to VMAT beams that has recently been published by the International Atomic Energy Agency (IAEA) working party to improve the dosimetry for small and non-standard fields. We investigated three possible fields and assessed their suitability as plan class specific reference (pcsr) fields. The factors in the new dosimetry formalism were investigated: the conversion of dose to water from the conventional reference field to the pcsr and then from the pcsr to a treatment plan, using a PTW semiflex chamber, two Farmer chambers and an electron diode. Finally, the dose was compared for Alanine, the new formalism and calculated using Pinnacle3 (Philips Radiation Oncology Systems) for two typical clinical VMAT beams. Correction factors between the reference field and the pcsr determined with Alanine range from 0.1% to 2.3% for the three pcsr fields. Dose to water measured using the calibrated ionization chambers is less than 2% different to the dose calculated by Pinnacle3. VMAT planning and delivery procedures have been successfully implemented and a new dosimetry protocol has been investigated for this new technique. Calibration factors for pcsr fields are found to be up to 2.3% different when using the new formalism, compared to using a standard dosimetry protocol. Using the calibration factors determined in the pcsr fields, the ionization chambers and electron diode agree to within 1% with Alanine dosimetry for two clinical VMAT plans. Good agreements between calculations and measurements are found for these two plans when the new formalism is used.

TU-D-BRB-02: Advanced Techniques to Determine Plan-Class Specific Reference Field Correction Factors for Accurate Dosimetry of Nonstandard Beams

Purpose: To establish experimental reference dosimetry techniques for measuring plan-class specific reference field correction factors for the proposed new formalism for reference dosimetry in nonstandard fields. Methods and Materials: A Lucite cylindrical phantom filled with water was constructed in the center of which reference absorbed dose to water for an IMRT delivery was measured. A plan-class specific reference (pcsr) field for a typical head and neck IMRT delivery was created on CT images of the phantom. The absorbed dose in the pcsr field normalized to that in a 10×10 cm2 field was measured using three reference dosimeters: Gafchromic® EBT films, a diamond detector, and an in-house developed guarded liquid ionization chamber (GLIC-03). Pcsr correction factors were determined for five air-filled ionization chambers (Exradin A1SL, Exradin A12, Exradin A14, PinPoint® 31006, and NE2571) in a fully-rotated delivery and in a delivery from a single angle (collapsed delivery). Results: The relative dose measurement accuracy of the three dosimeters was 0.56%, 0.10%, and 0.29% for the film, diamond detector, and GLIC-03, respectively. The combined relative standard uncertainty in measuring using the three techniques was 0.3%. For all chambers and the pcsr field selected, was unity to within ±1% and in the range of 0.990–0.993 and 0.990–1.003 in the fully-rotated and collapsed deliveries, respectively. The correction factors were the same for the chambers in the fully-rotated delivery. In the collapsed delivery, the Farmer-type chambers (Exradin A12 and NE2571) had a larger but consistent correction factor (0.990 and 0.991, respectively). The correction factors for the smaller chambers were close to unity but showed chamber type dependence. Conclusions: Our techniques provide a potential to improve the dosimetric accuracy in suitable plan-class specific reference fields. The techniques of determining the correction factor will be extremely valuable for other nonstandard field deliveries.

A new formalism for reference dosimetry of small and nonstandard fields

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.