Surface and buildup dose characteristics for 6, 10, and 18 MV photons from an Elekta Precise linear accelerator

Understanding head scatter characteristics of photon beams is vital to properly commission treatment planning (TP) algorithms. Simultaneously, having definitive surface and buildup region dosimetry is important to optimize bolus. The Elekta Precise linacs have unique beam flattening filter configurations for each photon beam (6, 10, and 18 MV) in terms of material and location. We performed a comprehensive set of surface and buildup dose measurements with a thin window parallel‐plate (PP) chamber to examine effects of field size (FS), source‐to‐skin distance (SSD), and attenuating media. Relative ionization data were converted to fractional depth dose (FDD) after correcting for bias effects and using the Gerbi method to account for chamber characteristics. Data were compared with a similar vintage Varian linac. At short SSDs the surface and buildup dose characteristics were similar to published data for Varian and Elekta accelerators. The FDD at surface (FDD0) for 6, 10, and 18 MV photons was 0.171, 0.159, and 0.199, respectively, for a 15×15 cm2, 100 cm SSD field. A blocking tray increased FDD0 to 0.200, 0.200, and 0.256, while the universal wedge decreased FDD0 to 0.107, 0.124, and 0.176. FDD0 increased linearly with FS (~1.16%/cm). FDD0 decreased exponentially for 10 and 18 MV with increasing SSD. However, the 6 MV FDD0 actually increased slightly with increasing SSD. This is likely due to the unique distal flattening filter for 6 MV The measured buildup curves have been used to optimize TP calculations and guide bolus decisions. Overall the FDD0 and buildup doses were very similar to published data. Of interest were the relatively low 10 MV surface doses, and the 6 MV FDD0's dependence on SSD. © 2003 American College of Medical Physics. PACS number(s): 87.53.–j, 87.66.–a


INTRODUCTION
Understanding head scatter characteristics of photon beams is important to properly commission and test treatment planning ͑TP͒ algorithms. Most modern commercial TP systems ͑TPS͒ either use model-based or analytical methods to calculate dose distributions in the buildup region. In either case it is vital to measure buildup curves accurately in order to evaluate the accuracy of TPS calculations. Simultaneously, having definitive dosimetry at the surface and buildup region is important to optimize bolus thickness required to enhance surface dose, in clinical cases such as inflammatory breast disease. Finally, it is imperative to describe the effect of scattering ͑wedge, collimating jaws, etc.͒ and immobilization ͑polyurethane foam, treatment tables, etc.͒ media on surface dose.
Comprehensive data sets in the buildup and build-down ͑exit͒ regions have been published for vintage Varian 1-4 and Siemens 5 linacs, and for modern day Varian [6][7][8][9] or Elekta 10 linacs. In these publications, it is evident that subtle differences in the unique beam delivery systems can affect dose to the buildup region. These include the beam monitor chamber and flattening filter construc-tion. In addition, support devices such as treatment tables or polyurethene immobilization platforms can also influence surface dose. 2,8,9,11,12 The Elekta Precise linear acccelerators ͑Elekta, Norcross, GA͒ have unique beam flattening filter configurations for each photon beam ͑6, 10, and 18 MV͒ in terms of material and location. The filter configurations for the 6 and 18 MV photon beams are shown in Fig. 1.
We performed a comprehensive set of surface and buildup dose measurements on the Precise linacs with a thin window parallel-plate chamber to examine effects of field size ͑FS͒, source-toskin distance ͑SSD͒, and attenuating media.

METHODS AND MATERIALS
The Elekta Precise linacs in our clinic deliver 6, 10, and 18 MV photons with PDD 10 of 68.3%, 73.1%, and 79.5%, respectively. Measurements were performed with a parallel plate ͑PP͒ ionization chamber ͑PS-033, Capintec, Ramsey, NJ͒ possessing an entry window thickness of 0.5 mg/cm 2 , a plate separation of 2 mm, and a collecting diameter of 16.2 mm. For each measurement point, the relative ionization was acquired by dividing the charge collected at depth, via a modified Keithley electrometer ͑Modified K602, CNMC Co., Nashville, TN͒, by the charge at the depth of d max and then corrected to PDD by correcting for bias effects and using the Gerbi method to account for chamber characteristics. 13 This was accomplished in the following method: Bias Correction: All ionization readings were corrected by first accounting for bias effects where, where M ϩ and M Ϫ are the collected positive and negative charges, respectively. The uncorrected percent depth ionization ͑PDI͒ is calculated from the uncorrected bias averaged ionization readings. ͑ii͒ The PDI was then corrected to PDD by accounting for chamber ͑Capintec Parallel-Plate͒ characteristics according to the Gerbi method.  Buildup region data were measured for field sizes ranging from 5 to 40 cm, SSDs ranging from 80 to 140 cm, and for depths from surface to just beyond d max . Field sizes were defined by the collimator setting ͑100 cm SSD͒. In addition, surface doses were measured with the Elekta Universal wedge and a 9 mm lexan block tray in place. Data were compared with a similar vintage Varian linac.

A. Surface dose characteristics
At short SSDs (р100 cm) the surface and buildup dose characteristics were similar to previously published data. The data are reported in fractional depth dose data ͑PDD/100͒, as this was the reporting method in prior publications. The fractional depth dose ͑FDD͒ at surface (FDD 0 ) for open 6, 10, and 18 MV photon beams was 0.171, 0.159, and 0.199, respectively, for a 15ϫ15 cm 2 , 100 cm SSD field. The blocking tray increased FDD 0 to 0.200, 0.200, and 0.256, while the universal wedge decreased FDD 0 to 0.107, 0.124, and 0.176 for the 6, 10, and 18 MV photons, respectively. Table I summarizes this data along with a comparison to Varian data. The wedge data in each case were measured without the respective block trays in place. The Elekta wedge is the Universal wedge with its proximal surface located at 18.6 cm from the source; and the Elekta block tray is a 9 mm lexan tray, located at 64.7 cm from the source. The Varian wedge is a 45°lead wedge ͑located at 49.2 cm from the source͒; and the Varian block tray is a 6 mm lexan tray, located at 61.6 cm from the source.

B. Dependence on field size
We found that the surface dose, reported as FDD, increased nearly linearly with FS (ϳ1.16%/cm). The graph in Fig. 2 depicts this trend.

C. Dependence on SSD
The FDD 0 decreased exponentially for 10 and 18 MV with increasing SSD. However, the 6 MV FDD 0 actually increased slightly with increasing SSD. This is likely due to the unique distal ''x-ray'' flattening filter for 6 MV. The graph in Fig. 3 summarizes these results.  The 15ϫ15 cm 2 data for each energy were extracted for comparison. Once again data are normalized to each respective d max value. The results are displayed in Fig. 5.

DISCUSSION AND CONCLUSIONS
The measured buildup curves have been used to validate TP calculations and guide bolus decisions. Overall the FDD at surface (FDD 0 ) and buildup doses were very similar to published data. One point of interest is the relatively low 10 MV surface doses. This is most likely due to the two-tier flattening filter system employed for the 10 MV beam. Also of special interest was the 6 MV dependence of FDD 0 on SSD. The constant FDD 0 , independent of SSD, is likely a reflection of the distal location of the 6 MV flattening filter, placing the source of scattered electrons and photons further from the target and thereby creating a smaller solid angle of head scatter. In terms of dependence on attenuation devices, it is of interest that the influence of the block tray on surface was minimal for 6 MV beams compared with prior reports, with only a minimal increase ͑3% absolute increase͒, but was consistent compared with prior reports for 18 MV photons ͑6% absolute increase͒. This is due to the relatively thick ͑9 mm͒ lexan tray compared with other machines' 6 mm trays. This thick tray absorbs a greater percentage of low energy scattered electrons and photons for 6 MV photons compared to the 18 MV beam. This same scenario is even more evident for the steel universal wedge whereby the wedge acts as both an generator and absorber of head scatter. The result is a greater absorption for 6 MV compared with 18 MV photons. In general, the Precise linac delivers lower surface dose for 6 and 18 MV photons compared with a similar vintage Varian machine, with the exception of attenuated 18 MV photons.