International Journal of
Radiation Oncology biology
Surface Dose Investigation of the Flattening Filter-Free Photon Beams Yuenan Wang, Ph.D.,* Mohammad K. Khan, M.D., Ph.D.,y Joseph Y. Ting, Ph.D.,* and Stephen B. Easterling, M.S.* *Melbourne Cancer Center, Melbourne, FL; and yRadiation Oncology, Winship Cancer Center, Emory University, Atlanta, GA Received Sep 2, 2011, and in revised form Nov 15, 2011. Accepted for publication Dec 18, 2011
Summary Entrance doses of 6X and 10X flattened and flattening filter-free (FFF) photons were measured using an ultrasmall parallel plate ionization chamber in a solid water phantom at field sizes ranging from 2 2 cm2 to 10 10 cm2 created by multileaf collimator. A linear relationship between surface doses and field sizes was observed for both flattened and FFF photons. Entrance doses increased exponentially with buildup depth. This study demonstrated that buildup doses of FFF photons are modestly larger than those of flattened photons for field sizes smaller than 10 cm. However, the difference is not substantial and may be clinically insignificant.
Purpose: Flattening filter-free (FFF) x-rays can provide more efficient use of photons and a significant increase of dose rate compared with conventional flattened x-rays, features that are especially beneficial for stereotactic radiosurgery (SRS) and stereotactic body radiotherapy (SBRT). The available data on the entrance doses of the FFF photon beams remain limited. The purpose of this study was to investigate the entrance dose of FFF photons in the buildup region and to compare it with that of conventional flattened photons. Methods and Materials: A Varian TrueBeam linear accelerator has been in full clinical operation with 6-MV and 10-MV FFF and flattened x-ray photons. Entrance dose at the surface was measured using a parallel plate ionization chamber in a solid water phantom with buildup depth Z 0w15 mm for 6X and 0w25 mm for 10X. Different field size (FS) patterns were created in the Eclipse Treatment Planning System by multileaf collimator (MLC) rather than jaws (FS Z 2 2, 3 3, 4 4, 6 6, and 10 10 cm2 by MLC and jaw size Z 2.2 2.2, 3.2 3.2, 4.2 4.2, 6 6, and 10 10 cm2). The smallest FS was about four times larger than the ion chamber dimension. All buildup dose measurements were normalized to FS Z 10 10 cm2 at the depth of dose maximum (dmax). Results: Good repeatability was demonstrated and surface dose increased linearly with FS for both flattened and FFF photons. The entrance dose of the FFF photons was modestly larger than that of the corresponding flattened photons for both 6X and 10X for different FS ranging from 2 2 cm2 to 10 10 cm2. Conclusions: The FFF photons have a higher entrance dose than that of the corresponding flattened photons for FS smaller than 10 10 cm2. However, the difference is not substantial and may be clinically insignificant. Ó 2012 Elsevier Inc. Keywords: Flattening filter free, Surface dose, Buildup region, Skin dose
Reprint requests to: Yuenan Wang, Ph.D., Melbourne Cancer Center, 1130 S Hickory St., Melbourne, FL 32901. Tel: (321) 409-1956 x7666; Fax: (321) 409-1253; E-mail: [email protected]
Int J Radiation Oncol Biol Phys, Vol. 83, No. 2, pp. e281ee285, 2012 0360-3016/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.ijrobp.2011.12.064
Conflict of interest: none.
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International Journal of Radiation Oncology Biology Physics
important because knowledge of the buildup effect can facilitate preservation of skin sparing or the delivery of an adequate dose to superficial target volumes. Human skin can show visible radiation damage, and severe skin reactions have been reported for some IMRT and SBRT cases (14, 15). Therefore, dose in the buildup region for the FFF photons needs to be further evaluated. Because the dose gradient in the buildup region is very steep, the size of the dosimeter along the beam direction should be as small as possible. The cylindric ionization chamber commonly used in radiation measurement is not suitable for skin dose quantification. A plane parallel ionization chamber with a sensitive volume has been proposed to be appropriate to perform skin dose measurements in the buildup region by Rawlinson et al. (16), Gerbi and Khan (17), and Attix (18). The purpose of this study was to evaluate the surface dose and doses in the buildup region for the FFF x-rays.
Recently, there has been an increasing interest in the use of flattening filter-free (FFF) x-ray photons. When the flattening filter is removed from the x-ray beam path, photon production should be far more efficient and dose rate should increase substantially at the treatment target, which is especially beneficial for high-dose-perfraction delivery techniques such as stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) (1). The FFF mode for SBRT lung cancer treatment can reduce beam-on time significantly (2) and can facilitate breath-hold or respiratory gating for more precise management of organ motion (3). In addition, the FFF x-rays have been proposed to offer dosimetric advantages such as reduced peripheral doses and decreased out-of-field scattered dose (1, 4e6). It has been suggested that more vendors are expected to develop dedicated treatment units with the FFF mode in the future because of the convincing arguments of removing the flattening filter (1). The nonflat x-rays or FFF photon beams have become an emerging technology in radiotherapy. Many studies have been conducted on FFF x-rays in a wide range of physics and clinical perspectives recently. For example, SBRT for lung cancer treatment using FFF photon beams is feasible and reduces beam-on time substantially, which can ease the management of organ movement (3). The treatment planning quality using FFF beams has been reported to be dosimetrically equivalent to that of flat x-rays, and beam-on time is reduced by more than 50% for both intensity-modulated radiation therapy (IMRT) and RapidArc (2). Applying nonflat beams to whole-breast irradiation is also proposed to be feasible (7). Small field dosimetry using 6X nonflat beams for high-resolution SRS and SBRT delivery has been investigated, and the output factors for small fields (0.5w10 cm) at the depth of dose maximum (dmax) of FFF photons are reported to be slightly higher than those of flattened photons (8). Georg et al. have reviewed the energy spectra, monitor chamber design, backscattering, and beam steering effects of the unflattened beams (1). The dosimetric characteristics of the FFF beams on TrueBeam have been successfully modeled using an anisotropic analytic algorithm (9). In addition, previous measurements and Monte Carlo simulations have shown that the nonflat x-rays have dosimetric advantages such as reduced head scatter and lower outof-field dose (4e6, 10, 11). It has also been reported that operating an FFF linear accelerator generally has a reduced neutron production (12). The nonflat beams with these favorable features may reduce unwanted exposure and risk of secondary cancers. The FFF beams contain more low-energy components and have softer energy spectra than do the corresponding flattened beams, which can lead to increased dose in the buildup region (13). Meanwhile, the FFF photons have lower head scatter because the flattening filter is removed from a linear accelerator’s gantry head, which may decrease the dose in the buildup region (13). In the buildup region where longitudinal electronic disequilibrium exists, these two competing factors will determine the buildup dosimetric characteristics of the FFF photons. The first factor can shift dmax toward the surface, given that the mean energy of primary photons is reduced in the FFF beams. However, the second factor will increase dmax because of the lower head scatter contamination of the FFF photons. The dose in the buildup region for the FFF photons thus becomes interesting and may differ from that for the conventional flattened photons. The surface dose or skin dose, as a part of patient quality assurance in external-beam radiation therapy, is still clinically
Methods and Materials A Varian TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, CA) has been in full clinical operation with 6-MV and 10-MV flattened and FFF photons since July 2010 in our institution (19). Based on our commissioning data with the standard setup of source-to-surface distance (SSD) Z 100 cm and field size (FS) Z 10 10 cm2, dmax is about 1.5 cm for 6X flat and FFF beams, and dmax is about 2.5 cm for 10X flat and FFF beams. The dose rates for 6X flat, 6X FFF, 10X flat, and 10X FFF x-rays are 600, 1400, 600, and 2400 MU/min, respectively. Entrance dose was measured using a 0.02 cm3 parallel plate ionization chamber (PTW Type 23342, PTW, Germany) with a thin flat window thickness of 0.03 mm (membrane material: polyethylene), a window area density of 2.76 mg/cm2, a plate separation of 1 mm, and a collecting diameter of w5 mm. A solid water phantom was set up with SSD of 100 cm. The buildup depths for 6X flat and FFF beams were 0, 1, 2, 4, 5, 7, 10, 15 mm, and for 10X flat and FFF beams were 0, 1, 2, 4, 5, 7, 10, 15, 20 and 25 mm. Different FS were created in the Eclipse Treatment Planning System (Varian Medical Systems) by multileaf collimator (MLC) rather than collimator jaws (FS Z 2 2, 3 3, 4 4, 6 6, and 10 10 cm2 by MLC and the corresponding jaw size Z 2.2 2.2, 3.2 3.2, 4 4.2, 6 6, and 10 10 cm2). Using MLC to define FS may be more clinically meaningful because the FFF mode is an IMRT, SRS, or SBRT dedicated technology, which uses MLC rather than jaws to determine treatment fields. The smallest FS (FS Z 2 2 cm2) is four times bigger than the parallel plate ionization chamber’s diameter (w5 mm) to assure the accuracy of the measurement. The buildup region has not obtained electron equilibrium longitudinally. For FS 4 4 cm2, electron equilibrium may not be obtained longitudinally and laterally. Therefore, our measurements require the parallel plate ionization chamber to have a very thin window thickness and a much smaller dimension laterally compared with the MLC fields. The plane parallel chamber used during this study has an ultrathin entrance window (0.03 mm), which can create negligible disturbance. The lateral dimension of the chamber is also small (w5 mm) compared with the smallest FS. The small sensitive volume (0.02 cm3) of the parallel plate ionization chamber can provide appropriate relative dose measurements in the buildup region. A PC Electrometer (Sun Nuclear Corp, Melbourne, FL) was used as the electrometer. All measurements were normalized to the standard FS: Z 10 10 cm2 at dmax (dmax Z 1.5 cm for 6X
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Surface dose for nonflat x-rays e283 dose within the buildup region was linearly increased with the FS (correlation coefficient R2w1) for both the flat and FFF beams (Fig. 1). The slope was about 1% per cm2 for both 6X and 10X flat and FFF photon beams. Good repeatability of the measurements was observed. The fractional surface dose, which was surface dose (D0) at any FS divided by Dmax at a 10 10 cm2 field, was observed to be greater for the FFF beams than for the corresponding flattened beams for both 6X and 10X. This was consistent with the larger beam output factors at the phantom surface for the FFF photons compared with the flattened photons for FS 10 cm. Because the surface dose increased linearly with the FS, surface dose at was obtained by extrapolation and found to be 16.4% for 6X flat, 22.8% for 6X FFF, 10.2% for 10X flat, and 15.7% for 10X FFF beams.
Doses at different buildup depths Fig. 1. Relative surface dose (D0/Dmax) increases linearly with the field size (w1%/cm2) for both 6X and 10X flat and FFF photon beams (error bar Z standard deviation). The surface output factors for field sizes 2 2 w 10 10 cm2 show 6X FFF > 6X Flat > 10X FFF > 10X Flat, which have zero-field-size surface doses of 22.8%, 16.4%, 15.7%, and 10.2%, respectively. FFF Z flattening filter-free. and 2.5 cm for 10X). No overresponse corrections or bias corrections were conducted for the ionization chamber (20) because only the relative buildup doses for the flat and FFF photons were of interest in this study. The relative or fractional doses were compared between flat and FFF beam to evaluate their buildup dosimetric characteristics. At least three repeated measurements were conducted for each beam energy, buildup depth, and FS to determine the uncertainty values.
Results Surface dose vs. field size Good repeatability was demonstrated. All the results were mean values, and the standard deviations were less than 2%. The surface
The buildup dose of the 6X FFF photons was modestly larger than that of the flattened photons for buildup depths of 0wdmax at FS of 2 2 w 10 10 cm2 (Fig. 2). However, the difference was not substantial (Table 1). For example, at a buildup depth of 5 mm, the buildup doses for 6X flat ranged 85% to 87%, which was not significantly different from 90% to 91% for 6X FFF photons. For 6X flattened x-rays with a FS of 10 10 cm2, the fractional dose increased from 27% to 94% in the first 7-mm buildup depth. In comparison, for 6X FFF x-rays, the fractional dose increased from 33% to 96% in the first 7-mm buildup depth. Therefore, a 6-mm bolus may be adequate for both 6X flattened and FFF photons to obtain 90% buildup dose on the skin in the application of whole-breast irradiation. Similarly, the buildup dose of the 10X FFF photons was slightly larger than that of the flattened photons for different buildup depths at FS of 2 2 w 10 10 cm2, shown in Fig. 3. Still, the difference was not substantial (Table 2). For instance, at a depth of 5 mm for FS Z 2 2 w 10 10 cm2, the buildup doses for 10X flat and 10X FFF photons ranged from 67% to 72% and 75% to 78%, respectively. In the first 2 mm, for 10X flattened x-rays at a FS of 10 10 cm2, the fractional dose increased from 20% to 49%. Similarly, for 10X FFF x-rays with a FS of 10 10 cm2, the fractional dose increased from 24% to 57%. A 1-cm bolus may be adequate for 10X
Fig. 2. (a) Buildup dose of 6X FFF photons is modestly larger than the flattened photons for different buildup depths 0wdmax at field sizes of 2 2 w 10 10 cm2. All data are normalized to dmax z 15 mm at a 10 10 cm2 field. (b) Buildup dose difference between 6X FFF and flattened photons. FS Z field size; FFF Z flattening filter-free.
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e284 Wang et al.
Table 1 Summary of buildup doses (mean value standard deviation) for 6 flat and flattening filter-free x-ray beams for field sizes Z 2 2 w 10 10 cm2 Buildup (mm) 0 1 2 4 5 7 10 15
6X flat x-rays: field size (cm) 22 0.18 0.47 0.63 0.81 0.86 0.93 0.98 1.00
0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.00
33 0.20 0.47 0.62 0.81 0.85 0.93 0.98 1.00
0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01
44 0.21 0.48 0.63 0.81 0.85 0.93 0.98 1.00
0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01
6X flattening filter-free x-rays: field size (cm)
66 0.23 0.49 0.64 0.81 0.86 0.93 0.98 1.00
0.02 0.01 0.02 0.02 0.02 0.02 0.01 0.01
10 10 0.27 0.53 0.67 0.83 0.87 0.94 0.98 1.00
flattened and FFF photons to obtain larger than 90% buildup dose on the skin.
Discussion To our knowledge, this is the first study using small fields formed by MLC for surface dose quantification in the buildup region between the FFF x-ray photons and the conventional flattened x-ray photons (21). The FFF photons have a modestly higher skin dose than do the corresponding conventional flattened x-ray photons for both 6X and 10X. However, the difference is not substantial and may not be clinically important. Even though previous studies have comprehensively described FFF beam dosimetric characteristics, the dose in the buildup region nevertheless is well worth quantifying, and its clinical impact is worth pursuing. For high-energy photons used in conventional radiotherapy, skin dose may be less of a concern because of the skin-sparing effect, which allows high-energy photons to be delivered to deep-seated tumors without exceeding the tolerance of the skin. However, for the unconventional hypofractionated delivery where the fractional dose is extremely high, an acute skin reaction at the chest wall can occur (14). Therefore, it is clinically worth while to analyze and compare the buildup dose characteristics for the flattened and FFF photons.
0.02 0.01 0.01 0.01 0.01 0.01 0.00 0.01
0.25 0.58 0.72 0.87 0.91 0.97 1.00 1.00
0.02 0.02 0.01 0.02 0.02 0.02 0.01 0.01
33 0.26 0.57 0.71 0.87 0.90 0.96 0.99 1.00
0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.00
44 0.27 0.58 0.72 0.87 0.90 0.96 0.99 1.00
0.02 0.02 0.01 0.01 0.01 0.02 0.01 0.00
66 0.29 0.59 0.72 0.87 0.90 0.96 0.99 1.00
0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01
10 10 0.33 0.61 0.74 0.88 0.91 0.96 0.99 1.00
0.02 0.01 0.01 0.02 0.02 0.02 0.00 0.00
Our study using MLC rather than jaws to create radiation fields for buildup dose quantifications is clinically meaningful because IMRT or SBRT fields are formed by MLC rather than jaws. Our results are consistent with those of the previous studies conducted on the surface dose for the FFF photons (6, 10, 11), where unflattened beams produced a higher surface dose for small FS compared with the flattened beams. For example, Cashmore (11) and Kragl et al. (10) have found that the buildup dose increased with the depth in solid water, and the surface dose of the FFF beam was slightly larger than the flattened beam using the Elekta Precise Linac, which is consistent with our results obtained using the Varian TrueBeam Linac. Vassiliev et al. (6) have reported the buildup dose measurement using the Varian Clinac 21EX at a depth of 0.3 cm of 6-MV flat and FFF x-rays, where the surface dose ratios of FFF to flat beams were 1.2 0.1 for a 4 4 cm2 field and 1.16 0.08 for a 10 10 cm2 field. The corresponding results in our study using the Varian TrueBeam Linac were 1.14 0.1 and 1.10 0.1, respectively (Table 1). Furthermore, we use MLC rather than jaws to form the radiation fields for buildup dose measurements, which can be more applied to the IMRT, SRS, and SBRT methods. The surface dose generated by jaws is different from that formed by MLC because jaws and MLC have different scatters at the treatment target. Because the FFF mode is primarily IMRT dedicated, where segments are formed by MLC rather than by jaws, FS patterns determined by MLC have more clinical application. The different FSs were arranged within a 10 10 cm2
Fig. 3. (a) Buildup dose of 10X FFF photons is modestly larger than the flattened photons for different buildup depths 0wdmax at field sizes of 2 2 w 10 10 cm2. All data are normalized to dmax z 25 mm at a 10 10 cm2 field. (b) Buildup dose difference between 10X FFF and flattened photons. FS Z field size; FFF Z flattening filter-free.
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Table 2 Summary of buildup doses (mean value standard deviation) for 10X flat and flattening filter-free x-ray beams for field sizes Z 2 2 w 10 10 cm2 Buildup (mm) 0 1 2 4 5 7 10 15 20 25
10X flat x-rays: field size (cm) 22 0.13 0.31 0.44 0.63 0.69 0.80 0.89 0.98 1.00 1.00
0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01
33 0.13 0.30 0.43 0.61 0.67 0.78 0.87 0.96 1.00 1.00
0.02 0.01 0.02 0.00 0.01 0.01 0.01 0.01 0.01 0.00
44 0.14 0.31 0.43 0.62 0.67 0.77 0.87 0.96 0.99 1.00
0.02 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.00
10X flattening filter-free x-rays: field size (cm)
66 0.16 0.33 0.45 0.63 0.68 0.79 0.87 0.96 1.00 1.00
0.02 0.00 0.00 0.01 0.01 0.00 0.01 0.01 0.01 0.01
10 10 0.20 0.37 0.49 0.66 0.72 0.81 0.90 0.97 1.00 1.00
field, in a manner analogous to that frequently used in IMRT, SRS, and SBRT treatments. There are several limitations in this study. First, we focused particularly on a FS ranging from 2 2 to 10 10 cm2. The FS is frequently used to form segments in IMRT or SBRT delivery. However, we did not cover FS larger than 10 10 cm2 or smaller than 2 2 cm2. The parallel plate ionization chamber with ultrasmall volume used in our study may not be suitable for FS less than 2 2 cm2. Second, we were only interested in the relative dose in the buildup region to compare the difference between flattened and unflattened photon beams, where all the radiation measurements were normalized to a standard 10 10 cm2 at dmax. There was no absolute surface dose quantification of the unflattened beams in this study. However, such measurements with certain correction methods are feasible (16, 17, 20). Nevertheless, this study using MLC to create different FS to evaluate the relative buildup dose of the unflattened photons has provided reliable and clinically relevant measurements to determine the difference in entrance doses between the conventional flattened and FFF beams.
Conclusions The FFF photons have a modestly higher surface dose in the buildup region than the corresponding flattened photons for FS 10 10 cm2. However, the difference is not substantial and may be clinically insignificant. Knowledge of dosimetric characteristics in the buildup region allows increased assurance for the implementation of FFF beam for IMRT and SRS.
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0.02 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.00 0.01
0.18 0.42 0.55 0.73 0.78 0.86 0.94 1.00 1.01 1.00
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.01 0.01
33 0.18 0.41 0.54 0.71 0.76 0.84 0.92 0.99 1.00 1.00
0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.01 0.01 0.01
44 0.19 0.41 0.54 0.71 0.75 0.84 0.91 0.98 1.00 1.00
0.01 0.01 0.01 0.00 0.01 0.00 0.01 0.01 0.01 0.01
66 0.20 0.43 0.55 0.71 0.76 0.84 0.91 0.98 1.00 1.00
0.02 0.02 0.01 0.00 0.01 0.01 0.00 0.01 0.00 0.01
10 10 0.24 0.45 0.57 0.73 0.77 0.85 0.92 0.98 1.00 1.00
0.02 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.00 0.00
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