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A closer look at RapidArc® radiosurgery plans using very small fields

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 Phys. Med. Biol. 56 1853 (http://iopscience.iop.org/0031-9155/56/6/020) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

PHYSICS IN MEDICINE AND BIOLOGY

Phys. Med. Biol. 56 (2011) 1853–1863

doi:10.1088/0031-9155/56/6/020

R A closer look at RapidArc radiosurgery plans using very small fields

Lotte S Fog, Jens F B Rasmussen, Marianne Aznar, Flemming Kjær-Kristoffersen, Ivan R Vogelius, Svend Aage Engelholm and Jens Peter Bangsgaard Department of Radiation Oncology, 3994, Rigshospitalet, Blegdamsvej 9, 2100 Copenhagen, Denmark

Received 4 October 2010, in final form 14 January 2011 Published 1 March 2011 Online at stacks.iop.org/PMB/56/1853 Abstract R has become the treatment of choice for an increasing number of RapidArc treatment sites in many clinics. The extensive use of multiple subfields in R treatments presents unique challenges, especially for small targets RapidArc treated in few fractions. In this work, very small static fields and subsequently R and conventional plans for two targets (0.4 and 9.9 cm3) were RapidArc investigated. Doses from static fields 1–4 MLC leaves (0.25–1.00 cm) wide, and larger fields with 1–4 MLC leaves closed in their centres, were measured using the portal dosimeter-based QA system EPIQA (v 1.3) and gafchromic film. RapidArc and conventional plans for two tumours were then measured using EPIQA, gafchromic EBT2 film and the phantom-based QA system Delta4. Eclipse 8.6 and 8.9, grid spacings of 1.25 and 2.50 mm and a Varian HD linac were used. For static fields one MLC leaf wide, the dose was underestimated by Eclipse by as much as 53% (v 8.6, 2.5 mm grid). Eclipse underestimated the dose downstream from a few MLC leaves closed in the centre of a large MLC field by as much as 30%. Eclipse consistently overestimated the width of the penumbra by about 100%. For the conventional plans, there was good agreement between the calculated and measured dose for the 9.9 cm3 PTV, R but a 10% underdose was observed for the 0.4 cm3 PTV. For the RapidArc 3 plans, the measured dose for the 9.9 cm PTV was in good agreement with the calculated one. However, for the 0.4 cm3 PTV about 10% overdosing was detected (Eclipse v 8.6, 2.5 mm grid spacing). EPIQA data indicated that the measured dose profiles were overmodulated compared to the calculated one. The use of small subfields, typically a few MLC leaves wide, or larger fields with one or a few MLC leaves closed in its centre can result in significant errors in the dose calculation. The detector systems used vary in their ability to detect the discrepancies. Using a smaller grid size and newer version of

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Eclipse reduces the discrepancies observed in this work but does not eliminate them. S Online supplementary data available from stacks.iop.org/PMB/56/1853/mmedia

(Some figures in this article are in colour only in the electronic version) Introduction R RapidArc (RA, Varian Medical Systems Inc., Palo Alto, USA) has become established as a routine treatment for a range of cancers at several centres (Kjaer-Kristoffersen et al 2009, Lagerwaard et al 2009a, 2009b, Keane and Urrutia 2009, Nicolini et al 2009, Aznar et al R offers a significantly reduced treatment time (Kjaer-Kristoffersen et 2010). As RapidArc al 2009) compared to IMRT and conventional treatments, it has the potential to benefit both patients and clinics if used in stereotactic radiosurgery (SRS) treatments. Some reports on stereotactic RA plans for very small targets exist in the literature. Lagerwaard et al (2008, 2009a, 2009b) have carried out several planning and measurement studies (using film in a phantom) of RapidArc brain plans and found good agreement between the calculated and measured dose for a range of tumour sizes. Verbakel et al (2009) report on measurements for three stereotactic body radiotherapy plans (tumour volumes 12.4–60.1 cm3) and conclude that film dosimetry corresponded well with calculated dose distributions. Finally, in planning studies Clark et al (2010) recommend the use of RA for stereotaxy of multiple brain metastasis, while Bignardi et al (2009) find that RA offers dosimetric advantages over intensity modulated and conformal radiotherapy for stereotactic body irradiation. RapidArc plans are typically highly modulated (Nicolini et al 2008). The use of subfields 1–4 MLC leaves wide is a signature feature of RA plans (Fog et al 2010) (two examples of RapidArc plans, one a prostate plan which was delivered at Rigshospitalet and one the RapidArc plan generated for the 9.9 cm3 target described in this work, are available as supplementary data at stacks.iop.org/PMB/56/1853/mmedia). Subfields will be noticeable in a considerable fraction of the total control points used for a plan, even in simple cases (spherical targets, weak/absent constraints to OAR). In the plans used in this study, subfields were present in approximately 37% to 57% of the control points. The calculation of dose delivered by such small fields relies on accurate measurements thereof—and such measurements present unique challenges (Alfonso et al 2008, Sanches-Doblado et al 2007, Seutjens and Verhaegen 2003, Das et al 2007, Lydon 2005, Vlamynck et al 1999). The RapidArc user has little control over the production of such subfields, as no parameter controlling the smoothness of the dose modulation or the smallest ‘acceptable’ field size is available. A range of measurement systems are available for RA quality assurance. In this paper we present the results of quality assurance of RA plans for stereotactic targets using several such systems. The purpose of the study is to provide a comparison between the measured and calculated dose for RapidArc plans for two clinically relevant PTV sizes. Also, in order to investigate the accuracy of the treatment planning system (TPS) calculation of the small subfields, measurements were carried out of a range of small static fields, using a portal vision imager-based measurement system.

Materials and methods R (Varian Medical Systems Inc., Palo Alto, Dose planning was carried out in Eclipse TPS USA) versions 8.6 and 8.9, using two different grid sizes. In the optimization process, the

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only constraint was PTV coverage. No organs at risk were defined. In 2010, version 8.9 of the Eclipse TPS superseded the previous version (8.6). Since both versions are still in widespread clinical use, both were evaluated in this work. A calculation grid spacing of 2.5 mm is commonly used for clinical calculations (Court et al 2008), as smaller grid spacings may require impractically large computation times. For small targets, however, a grid spacing of 1.5 or 1.25 mm may be used. Both grid spacings were evaluated in this work. Every QA system offers a unique combination of strong and weak points. EPIQA (Nicolini et al 2008) was used in this work as it is fast, and has a very small detector-to-detector distance (0.4 mm). EPIQA is a recently developed QA system. Thus, to verify the EPIQA data with a well-established QA system, some of the measurements were also carried out with film (Anand et al 2010), which also provides high resolution measurements but is time consuming. Finally, in order to obtain measurements at a range of depths in a phantom, the Delta4 system was used. Delta4 measurements are quick compared with film, though slower than EPIQA, but do not provide as comprehensive a data set as the detector-to-detector distance is large (5 mm in the central detector area) compared with the MLC leaf width (2.5 mm in this work). Thus, three different QA systems were used in this work: • Gafchromic EBT2 film (Anand et al 2010). For the film measurements, the film was positioned in a solid water phantom positioned on the couch. The solid water phantom was 30 cm wide in the horizontal plane, and 15 cm thick. The film was positioned at the isocentre, at a depth of 1.5 cm in the phantom. The film was calibrated by placing the film at the depth dosemax in the solid water phantom (SSD 100 cm). The film was scanned and analysed using an EPSON Expression 10000 XL film scanner and PTWs Verisoft 4.0 software. • EPIQA (Nicolini et al 2008) (software version 1.3.2) which provided data acquired by the portal imaging system, thereby measuring a two-dimensional dose map which rotated with the gantry during irradiation. For the EPIQA measurements, the PVI was positioned with the detector plate at the isocentre. • Delta4 (Bedford et al 2009). This system is used routinely at Rigshospitalet to verify RA plans (software version 1.00.0064). Delta4 data and the film data actually represent different dose distributions. Both methods measure dose at a depth in a phantom, but with Delta4, the high dose region was positioned at a depth of 11 cm in the centre of the phantom. However, the film data were acquired with the film positioned at a depth of dmax (with the gantry at 0◦ ), such that effective path length to the high dose area in the phantom varied from dmax to 15 cm as the gantry rotated. The measurements were carried out using the 6 MV beam from a Varian/Brainlab linac (Novalis TX, 2300 iX, equipped with high definition MLC leaves (0.25 cm leaf width centrally, 0.5 cm width in the remaining field)). Static fields Plans consisting of 3 cm long fields, with a width of 1, 2, 3 or 4 MLC leaves (0.25–1.00 cm) at the isocentre, each delivering 50 MU, were created for the investigation of static field delivery. These fields shall be referred to as ‘1 OUT’, ‘2 OUT’, etc (figure 1(A)). Additional MLC leaves were opened 5 cm away from the small investigated field in order to facilitate the Eclipse calculations. Similarly, fields in which all MLC leaves were retracted except for 1, 2, 3 or 4 MLC leaves at the isocentre (‘1 IN’, ‘2 IN’, etc, see figure 1(B)) were evaluated. Measurements were done using EPIQA and gafchromic film.

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(B)

Figure 1. MLC configurations used for the ‘1 out’ (1A) and the ‘1 in’ (1B) measurements. In the ‘1 out’ configuration, the field which is 1 MLC leaf wide is positioned at the isocentre; however, in order for Eclipse to calculate the dose, additional MLC leaves were opened far from the narrow field. The open field is shown in white. The isocentre is marked by a black and white cross.

Patient plans SRS plans for two tumours (0.4 and 9.9 cm3) previously treated clinically after dose planning in iplan v 4.1.2 (iplan, Brainlab AG, Kapellenstr. 12, 85622 Feldkirchen, Germany) were investigated. In addition to the clinical plans, RA plans were made for these targets for the purpose of this study. Subfields were present in approximately 57% (0.4 cm3 PTV) and 37% (9.9 cm3 PTV) of the control points. All plans delivered 20 Gy in a single dose to the PTV. The clinical plans were measured with Delta4 and film only as EPIQA cannot be used to measure arc plans without MLC movements. The RA plans were measured with Delta4, film and EPIQA. Results Static fields with no gantry rotation Measured profiles perpendicular to the direction of the MLC travel through the isocentre are shown in figure 2. The dose in the field centre, and the penumbra (defined as the part of the dose distribution with 10–90% of maximum dose) widths are listed in tables 1 and 2. The non-central peaks in figures 2(A) and (D) are due to radiation transmitted through the closed leaf tips. The film and EPIQA data are in good agreement, with the largest difference in the centres of the fields being 10% for the ‘1 IN’ and ‘1 OUT’ fields, and less than 6% for the ‘4 OUT’ fields. Data calculated with Eclipse 8.6 with a grid spacing of 2.5 mm are different from those from Eclipse 8.9, and from Eclipse 8.6 with 1.25 mm grid spacing. Compared with the three other calculation methods, data from Eclipse 8.6 with a grid spacing of 2.5 mm indicates a maximum dose which is 36% smaller for the ‘1 OUT’ field, and a minimum dose 26% larger for the ‘1 IN’ field, and a penumbra width 36% or 1.6 mm greater.

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(A)

(C)

(B)

(D)

Figure 2. Profiles perpendicular to the MLC direction of travel for fields in which all the MLC leaves were closed, except for 1 (A) and 4 (B), and for large open MLC fields where 1 (C) or 4 (D) MLC leaves were closed in the field centres.

Table 1. Dose values in the centres of the ‘IN’ and ‘OUT’ fields, and penumbra values.

Maximum dose (Gy)

Penumbra width (cm) (10–19% of maximum dose)

1 MLC 2 MLC 3 MLC 4 MLC 1 MLC 2 MLC 3 MLC 4 MLC leaf leaves leaves leaves leaf leaves leaves leaves OUT OUT OUT OUT OUT OUT OUT OUT EPIQA Gafchromic film Eclipse 8.6, grid spacing 0.125 cm Eclipse 8.6, grid spacing 0.25 cm Eclipse 8.9, grid spacing 0.125 cm Eclipse 8.9, grid spacing 0.25 cm

0.30 0.27 0.21

0.41

0.42

0.34

0.14

0.41

0.44 0.45 0.44

0.20 0.27 0.45

0.38

0.42

0.67

0.25

0.16

0.46

0.50

0.34 0.37 0.58

0.60

0.65

0.22

0.33

0.41

0.44

0.48

0.46

0.50

0.58

0.23

0.32

0.41

0.43

0.45

0.45

0.50

0.58

There are systematic discrepancies between the calculated and measured dose distributions. For the small open fields, the calculations underestimate the measured dose by 51% for Eclipse 8.6 with 2.5 mm grid spacing, and about 23% for the other calculations.

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L S Fog et al Table 2. Maximum dose and penumbra of the profiles through the isocentre and perpendicular to the MLC direction of travel for fields 1, 2, 3 and 4 MLC leaves wide.

Minimum dose (Gy)

Penumbra (10–90% of maximum dose) (cm)

1 MLC 2 MLC 3 MLC 4 MLC 1 MLC 2 MLC 3 MLC 4 MLC leaf leaves leaves leaves leaf leaves leaves leaves IN IN IN IN IN IN IN IN EPIQA Gafchromic film Elipse 8.6, grid spacing 0.125 cm Eclipse 8.6, grid spacing 0.25 cm Elipse 8.9, grid spacing 0.125 cm Elipse 8.9, grid spacing 0.25 cm

0.23 0.25 0.23

0.20

0.17

0.23 0.27 0.44

0.25

0.19

0.12

0.17 0.16 0.09

0.44

0.44

0.28 0.25 0.50

0.14

0.29

0.20

0.12

0.09

0.60

0.46

0.60

0.65

0.23

0.14

0.12

0.09

0.44

0.44

0.44

0.50

0.23

0.15

0.12

0.09

0.44

0.44

0.43

0.50

This discrepancy decreases with increasing field size, and for the ‘4 OUT’ field the measured and calculated doses are not significantly different. For the ‘1 OUT’ profile, the peak dose from Eclipse version 8.9 with a 2.5 mm grid spacing (0.23 Gy) is in better agreement with measured data than the same Eclipse version with a 1.25 mm grid spacing (0.24 Gy). The difference between the peak doses calculated with Eclipse v 8.9 and the two different grid spacings is between 0 and 0.01 Gy for all the data sets, with no systematic trend of one grid spacing resulting in a greater dose than the other—thus, it is not considered significant. For the ‘1 IN’ data, the Eclipse 8.6, 2.5 mm grid spacing data underestimate the minimum measured dose by 22%, whereas the other calculations are in agreement with measured data. This discrepancy decreases with increasing field size, and for the ‘4 IN’ field the measured and calculated doses are not significantly different. The calculated penumbra is consistently greater than the measured: for the OUT data, this difference is 142% or 3.8 mm (Eclipse v 8.6, 2.5 mm spacing) and 88% or 2.3 mm (the other calculations).

The conventional SRS plans Cranio-caudal profiles measured with gafchromic film for both PTVs are shown in figures 3(A) and (B); Delta4 data are shown in figures 3(C) and (D). For the 9.9 cm3 PTV, there is good agreement between calculated data and measurements for both film and Delta4 data (figures 3(B) and (D)). For the 0.4 cm3 PTV, the film data indicate a maximum dose 10% smaller than that determined by the calculations (figure 3(A)). In the Delta4 data for the 0.4 cm3 PTV, only 2 data points in the profile are in the high dose region (>90% of the maximum measured dose); thus, the measurement of delivered maximum dose is uncertain.

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(A) (C)

(B) (D)

Figure 3. Cranio-caudal dose profile through the centre of the phantom used isocentre in the coronal plane for the conventional plan (arc 1). Film data for the 0.6 cm3 PTV (A) and the 9.9 cm3 PTV (B); Delta4 data for the 0.6 cm3 PTV (C) and the 9.9 cm3 PTV (D).

R The RapidArc plans

Cranio-caudal profiles measured with gafchromic film for both PTVs are shown in figures 4(A) and (B); Delta4 data are shown in figures 4(C) and (D). For the 9.9 cm3 PTV, there is good agreement between calculated data and measurements for both film and Delta4 data (figures 4(B) and (D)), with the exception of the edges of the profile, where the measured dose is about 10% greater than the calculated. For the 0.4 cm3 PTV, the film data indicate a dose which is about 20% greater than the calculated. The delta4 data indicate a dose which is about 20% greater than the calculated. A profile through the isocentre, perpendicular to the MLC direction of motion, as measured with EPIQA is shown in figure 5. There are discrepancies between the measured and calculated dose in the centre of the field, and outside the field immediately downstream from the centre of the field where the MLC leaf tips meet (figure 5). The profile data (figure 5) indicate that the measured profile is overmodulated compared to the calculated one: it contains ‘peaks’ and ‘valleys’ not present in the calculated profile. The calculated data from Eclipse v 8.6, 2.5 mm grid spacing show poorer agreement with the measured data than the calculations from Eclipse v 8.9, and Eclipse 8.6 with a grid spacing of 1.25 mm. The agreement between the measured and calculated doses for the 9.9 cm3 PTV is good. Discussion The QA systems used in this work are based on very different approaches. As a result, they may vary in their ability to detect very modulated dose distributions, and their spatial resolutions.

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(A) (C)

(B) (D)

Figure 4. Cranio-caudal dose profile through the centre of the phantom used isocentre in the coronal plane for the RapidArc plan (arc 1). Film data for the 0.4 cm3 PTV (A) and the 9.9 cm3 PTV (B); Delta4 data for the 0.4 cm3 PTV (C) and the 9.9 cm3 PTV (D).

Figure 5. EPIQA data, measured with the portal vision images, for the 0.4 cm3 PTV (calculations for Eclipse 8.9 with a grid spacing of 0.25 cm were not available). Profile through the isocentre, perpendicular to the MLC direction of travel. The MLC leaves are indicated with the vertical lines.

For example, since the detector-to-detector distance in the Delta4 phantom (5 mm at the phantom centre) is similar to the dimensions of the PTV, the Delta4 system only provides data for a few points within the PTV. Thus, the Delta4 system is not suitable for quality assurance for such small PTVs.

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With its high spatial resolution and geometrical positioning (because the PVI used to acquire data with EPIQA does not rotate relative to the gantry during the arc delivery, the MLC leaves are ‘projected’ onto the detector plane, allowing regions of the dose map to be mapped onto the MLC configuration), the EPIQA data facilitate a more in-depth investigation of the contributions to the dose map. The dose map measured with EPIQA is difficult to relate to the patient dose. Firstly if the collimator angle is a value different from 0 or 180◦ , the dose plane equivalent to that measured with EPIQA rotates inside the patient during an arc delivery. Secondly, EPIQA measures dose only at a shallow depth, while in a RA patient plan, the effective PTV depth will vary during the arc rotation. The discrepancy between the measured and calculated dose may vary with depth. For the conventional plan and the 0.4 cm3 PTV, the film data indicated a 10% underdose while the Delta4 data indicated a 3% overdose. Since the Delta4 and film data both agreed with the calculated dose for the 9.9 cm3 PTV, there is no obvious reason to suspect systematic errors in the measurements. Rather, it seems likely that for very small targets, the different effective measurement depths gave rise to different discrepancies. Both calculation and measurement of dose from small MLC fields are challenging, partly because of the lack of electronic equilibrium (Vlamynck et al 1999). The cause of the discrepancy between measured and calculated data for small static fields is not known; however, we believe that the agreement between the two independent measurement systems supports the validity of the measurements. The Eclipse TPS used for the calculations has been commissioned in accordance with the recommendations in the literature (Ling et al 2008, Korreman et al 2009). However, Eclipse is commissioned using measured profiles and depth dose curves from jaw defined fields. The effect of the MLCs on a jaw-defined field is described through the use of parameters like the intraleaf transmission, the dosimetric leaf gap (the MLC gap assumed by Eclipse for closed MLC leaves to describe the transmission through the rounded MLC leaf tips). The data suggest that the use of very small subfields (1–4 HD MLC leaves wide) in RA plans may be the cause of the significant discrepancies between the calculated and measured dose. RA fields are typically highly modulated (Nicolini and Fogliata 2006). The use of small subfields is a signature feature of RA plans, and the user has limited ability to exclude them from the MLC configuration. Serious consideration must be given to the acceptable discrepancies between the measured and calculated dose. SRS treatments differ from other external radiotherapy treatment in its tighter positioning tolerances, its use of multiple arcs used to create very steep dose gradients, and its single-fraction delivery of a very large dose (Schnell et al 1985). AAPM report no 54 lists several factors which contribute to a total achievable spatial accuracy of 3.7 mm in SRS treatments—the greatest of these is 1.7 mm, arising from the CT image resolution, with the second largest being 1.0 mm. Since the publication of Schnell et al (1985), the CT image resolution for SRS has improved significantly and is now less than 1 mm (Sanford et al 2000). Thus a spatial accuracy of 1 mm or less seems achievable and should be aimed for. The uncertainty in the dose calculation mentioned by the AAPM is 1–2% for a 2 mm calculation grid (Schnell et al 1985). There are several ways in which the user can potentially minimize the effect of the errors seen in this study. Firstly, to minimize the errors arising from unnecessary use of very small subfields, the user can limit the number of monitor units used for targets larger than about 1 cm in diameter. The recommendation at the authors’ clinic (Kjær-Kristoffersen and Fog 2010) is a maximum

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of 250 MU Gy−1. This has, however, not been proven to eliminate the errors seen, despite being logically related to the use of small subfields. Secondly, the user can ensure that the collimator is rotated away from 0 and 180◦ in RA plans, thus smearing out hot and cold spots directly downstream from particular MLCs (or groups of MLCs) during the arc rotation. Thirdly, of the four calculation methods investigated in this work (Eclipse 8.6 and 8.9 with grid spacings of 1.25 and 2.5 mm), Eclipse 8.6 with a grid spacing of 2.5 mm consistently resulted in poorer agreement with measured data than the other three. Hence, these settings should be avoided for small targets. The authors recommend caution when creating RA plans for small tumours using a 2.5 mm grid spacing in Eclipse 8.6. In spite of the reported attractions of RapidArc with respect to conventional stereotactic techniques, the user should bear in mind that such small target sizes are taking the TPS (which was not originally developed for stereotactic treatment) to its limits. Several areas of future work would be interesting and useful. An investigation into the smallest PTV size for which the dose can be accurately calculated in Eclipse would be very useful. An investigation of the calculational errors and their dependence on energy and PTV depth would be useful. Hopefully, new versions of the TPS will decrease the discrepancies seen in this study. Conclusion The use of small subfields, typically a few MLC leaves wide, or larger fields with one or a few MLC leaves closed in its centre can result in significant errors in the dose calculation. The detector systems used vary in their ability to detect the discrepancies. The results presented in this work suggest that Eclipse underestimates the penumbra width by 2 to 3.5 mm in small MLC fields. The accuracy of the calculated doses varies with version and calculation grid spacing. Measurements carried out for a 9.9 cm3 PTV indicated good agreement between calculated and measured doses, for both conventional and RapidArc plans. However, measured and calculated doses differed by up to 20% for a 0.4 cm3 PTV. EPIQA data indicate that the measured RapidArc data are more modulated than the calculated data. The agreement between calculated and measured doses increases with increasing Eclipse version number and decreasing calculation grid spacing. Thus, the use of very small subfields (1–3 MLC leaves wide) in RapidArc plans may lead to significant discrepancies between the calculated and measured dose. The use of small subfields is, however a signature feature of RapidArc plans, and the user has limited ability to exclude them from the MLC configuration. Using a smaller grid size and newer version of Eclipse reduces the discrepancies observed in this work but does not eliminate them. References Alfonso R et al 2008 A new formalism for reference dosimetry of small and nonstandard fields Med. Phys. 35 5179 Anand A et al 2010 Dose response and energy dependency of Gafchromic EBT2 film over a wide range of beam energies and modalities Med. Phys. 37 3265 Aznar M C et al 2010 Rotational radiotherapy for prostate cancer in clinical practice Radiother. Oncol. 97 480–4 Bedford J L et al 2009 Evaluation of the Delta4 phantom for IMRT and VMAT verification Phys. Med. Biol. 54 N167–76 Bignardi M et al 2009 Critical appraisal of volumetric modulated arc therapy in stereotactic body radiation therapy for metastases to abdominal lymph nodes Int. J. Radiat. Oncol. Biol. Phys. 75 1570–7

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Clark G M et al 2010 Feasibility of single-isocenter volumetric modulated arc radiosurgery for treatment of multiple brain metastasis Int. J. Radiat. Oncol. Biol. Phys. 76 296–301 Court L E et al 2008 Experimental evaluation of the accuracy of skin dose calculation for a commercial treatment planning system J. Appl. Clin. Med. Phys. 9 29–35 Das I J, Ding G X and Ahnesjø A 2007 Small fields: nonequilibrium radiation dosimetry Med. Phys. 35 206–15 Fog L S et al 2010 Size does matter: hot and cold spots in RapidArc plans using small fields Radiother. Oncol. 95 (Suppl. 1) s90–1 (Winner of the Young Scientists Poster Award) Keane J and Urrutia T 2009 Volumetric verification of RapidArc plans Med. Phys. 36 2565–5 Kjaer-Kristoffersen F et al 2009 RapidArc volumetric modulated therapy planning for prostate cancer patients Acta Oncol. 48 227–32 Kjær-Kristoffersen F and Fog L 2010 The best laid plans: a look at the RapidArc QA experience for prostate and brain patients at Rigshospitalet with an emphasis on learning how to make better plans and predicting when this will be difficult Astro Poster Presentation (Poster no 3372) Available through the ASTRO virtual poster library Korreman S, Medin J and Kjaer-Kristoffersen F 2009 Dosimetric verification of RapidArc treatment delivery Acta Oncol. 48 185–91 Lagerwaard F J et al 2008 Volumetric modulated arc therapy (RapidArc) for rapid, non-invasive stereotactic radiosurgery of multiple brain metastases Int. J. Radiat. Oncol. Biol. Phys. 72 S530 Lagerwaard FJ et al 2009a Whole-brain radiotherapy with simultaneous integrated boost to multiple brain metastases using volumetric modulated arc therapy Int. J. Radiat. Oncol. Biol. Phys. 75 253–9 Lagerwaard F J et al 2009b Volumetric modulated arc radiotherapy for vestibular schwannomas Int. J. Radiat. Oncol. Biol. Phys. 74 610–5 Ling H et al 2008 Commissioning and quality assurance of RapidArc radiotherapy delivery system Int. J. Radiat. Oncol. Biol. Phys. 72 575–81 Lydon J M 2005 Theoretical and experimental validation of treatment planning for narrow MLC defined photon fields Phys. Med. Biol. 50 2701–14 Nicolini G and Fogliata A 2006 GLAaS: an absolute dose calibration algorithm for an amorphous silicon portal imager. Applications to IMRT verifications Med. Phys. 33 2839–51 Nicolini G et al 2008 The GLAaS algorithm for portal dosimetry and quality assurance of RapidArc, an intensity modulated rotational therapy Radiat. Oncol. 3 1–10 Nicolini G et al 2009 Quality assurance of RapidArc treatments with portal dosimetry: multicentric clinical practice experience Swiss Soc. Radiobiol. Med. Phys. submitted http://www.sgsmp.ch/2009/12_Submitted Abstract.pdf Sanches-Doblado F et al 2007 Uncertainty estimation in intensity-modulated radiotherapy absolute dosimetric verification Int. J. Radiat. Biol. Phys. 68 301–10 Sanford L M et al 2000 Image localization for frameless stereotactic radiotherapy Int. J. Radiat. Oncol. Biol. Phys. 46 1291–9 Schnell M C et al 1985 Stereotactic radiosurgery. Report of Task Group 42 Radiation Therapy Committee AAPM Report no 54 (College Park, MD: The American Association of Physicists in Medicine) Seutjens J and Verhaegen F 2003 Comments on Ionization chamber dosimetry of small photon fields: a Monte Carlo study on stopping-power ratios for radiosurgery and IMRT beams Phys. Med. Biol. 48 l43–8 Verbakel W F A R et al 2009 Rapid delivery of stereotactic radiotherapy for peripheral lung tumors using volumetric intensity-modulated arcs Radiother. Oncol. 93 122–4 Vlamynck K D et al 1999 Dose measurements compared with Monte Carlo simulations of narrow 6 MV multileaf collimator shaped photon beams Med. Phys. 26 1874–83