Plan-class Specific Reference Field 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.

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.

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.

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.

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.