JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 4, NUMBER 1, WINTER 2003


Surface and buildup dose characteristics for 6, 10, and 18
MV photons from an Elekta Precise linear accelerator
       Eric E. Klein,* Jacqueline Esthappan, and Zuofeng Li
       Division of Physics, Department of Radiation Oncology, Washington University School of
       Medicine, Campus Box 8224, 660 S. Euclid Ave., St. Louis, Missouri 63110-1093

        Received 31 July 2002; accepted for publication 20 September 2002

       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, re-
       spectively, 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 de-
       creased 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.        DOI: 10.1120/1.1520113


       PACS number s : 87.53. j, 87.66. a


       Key words: buildup region, surface dose, Elekta, radiotherapy.



INTRODUCTION

Understanding head scatter characteristics of photon beams is important to properly commission
and test treatment planning TP algorithms. Most modern commercial TP systems TPS 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 exit regions have been published for
vintage Varian1­ and Siemens5 linacs, and for modern day Varian6
                  4 ­9                                                     or Elekta10 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-

1 1526-9914Õ2003Õ4,,1...Õ1Õ7Õ$17.00 © 2003 Am. Coll. Med. Phys. 1

2         Klein, Esthappan, and Li: Surface and buildup dose characteristics . . .                                    2




FIG. 1. Photon delivery system for Elekta Precise linacs. a 6 MV system that is unique by virtue of the x-low flattening
filter relatively distal from the primary filter. b Expanded view of the 18 MV system unique by virtue of the relatively
proximal steel difference filter.




tion. In addition, support devices such as treatment tables or polyurethene immobilization plat-
forms can also influence surface dose.2,8,9,11,12 The Elekta Precise linear acccelerators Elekta,
Norcross, GA have unique beam flattening filter configurations for each photon beam 6, 10, and
18 MV 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 FS , source-to-
skin distance SSD , and attenuating media.



METHODS AND MATERIALS

    The Elekta Precise linacs in our clinic deliver 6, 10, and 18 MV photons with PDD1 of 68.3%,         0
73.1%, and 79.5%, respectively. Measurements were performed with a parallel plate PP ioniza-
tion chamber PS-033, Capintec, Ramsey, NJ possessing an entry window thickness of
0.5 mg/ cm2, a plate separation of 2 mm, and a collecting diameter of 16.2 mm. For each mea-
surement point, the relative ionization was acquired by dividing the charge collected at depth, via
a modified Keithley electrometer Modified K602, CNMC Co., Nashville, TN , 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,

      i

                                                      M M
                                                                      M ,                                            1
                                                           2

where M         and M       are the collected positive and negative charges, respectively. The uncorrected
percent depth ionization PDI is calculated from the uncorrected bias averaged ionization read-
ings.

      ii

                                                               M d
                                                      PDI               .                                            2
                                                             M d max

The PDI was then corrected to PDD by accounting for chamber Capintec Parallel-Plate charac-
teristics according to the Gerbi method.


Journal of Applied Clinical Medical Physics, Vol. 4, No. 1, Winter 2003

3       Klein, Esthappan, and Li: Surface and buildup dose characteristics . . .                                       3


    iii

                                          P D D P D I         0,E l e      ( d / d max),                              3

where
        ( 0,E ) energy dependent chamber corrections,

                                      0,E 1.666 1.982IR                            C 15.8 ,

where
               IR     ionization ratio ( 6 MV 0.675, 10 MV 0.728, 18 MV 0.783)
               C    sidewall-collector distance ( mm) 6
                     constant of 5.5.

   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 SSD . In addition, surface doses were measured with the Elekta Uni-
versal wedge and a 9 mm lexan block tray in place. Data were compared with a similar vintage
Varian linac.


RESULTS

A. Surface dose characteristics

   At short SSDs ( 100 cm) the surface and buildup dose characteristics were similar to previ-
ously published data. The data are reported in fractional depth dose data PDD/100 , as this was
the reporting method in prior publications. The fractional depth dose FDD at surface ( FDD0) for
open 6, 10, and 18 MV photon beams was 0.171, 0.159, and 0.199, respectively, for a
15 15 cm2, 100 cm SSD field. The 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 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 source ; 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
( 1.16% / cm) . The graph in Fig. 2 depicts this trend.


C. Dependence on SSD

   The 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
``x-ray'' flattening filter for 6 MV. The graph in Fig. 3 summarizes these results.


TABLE I. Elekta precise fractional surface dose: 15 15 cm2 field size, 100 cm SSD. Values in parentheses are for a Varian
2100C accelerator.

Absorber/Energy                                  6 MV                      10 MV                      18 MV

Open                                          0.171 .205                     0.159                 0.199 0.215
Block Tray                                    0.200 .226                     0.200                 0.256 0.223
Universal Wedge                               0.107 .179                     0.124                 0.176 0.136
Varian 45° wedge



Journal of Applied Clinical Medical Physics, Vol. 4, No. 1, Winter 2003

4       Klein, Esthappan, and Li: Surface and buildup dose characteristics . . .                                         4




FIG. 2. Fractional surface dose as a function of energy and field size at a constant SSD of 100 cm, without a block tray in
place.




FIG. 3. Fractional surface dose as a function of energy and SSD with a constant collimating field size of 15 15 cm, and
without a block tray in place.



D. Buildup data

   Figures 4 graphically represent the buildup curves for each of the photon energies. The buildup
curves are normalized to the respective d            max  FDD values.
   The 15 15 cm2 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 ( FDD0) 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


Journal of Applied Clinical Medical Physics, Vol. 4, No. 1, Winter 2003

5        Klein, Esthappan, and Li: Surface and buildup dose characteristics . . .                                       5




FIG. 4. a Fractional depth dose as a function of depth and field size at a constant for 6 MV photons, without a block tray
in place. b Fractional depth dose as a function of depth and field size at a constant for 10 MV photons, without a block
tray in place. c Fractional depth dose as a function of depth and field size at a constant for 18 MV photons, without a
block tray in place.


Journal of Applied Clinical Medical Physics, Vol. 4, No. 1, Winter 2003

6       Klein, Esthappan, and Li: Surface and buildup dose characteristics . . .                                         6




FIG. 5. Fractional depth dose as a function of depth and energy for a 15 15 cm field size, without a block tray in place.




MV dependence of FDD0 on SSD. The constant FDD0 , 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 increase , but was consistent compared with prior reports for 18 MV photons 6% abso-
lute increase . This is due to the relatively thick 9 mm 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.

*Email address: klein@radonc.wustl.edu
 Email address: esthappan@radonc.wustl.edu
 Email address: zuofeng@radonc.wustl.edu
 1C. W. Coffey, J. L. Beach, D. J. Thompson, and M. Mendiondo, ``X-ray beam characteristics of the Varian Clinac-6/100
  linear accelerator,'' Med. Phys. 7, 716 ­722 1980 .
 2G. Krithivas and S. H. Rao, ``A study of the characteristics of radiation contaminants within a clinically useful photon
  beam,'' Med. Phys. 12, 764 ­768 1985 .
 3J. A. Purdy, ``Buildup/surface dose and exit dose measurements for a 6-MV linear accelerator,'' Med. Phys. 13, 259­262
   1986 .
 4D. E. Velkey, D. J. Manson, J. A. Purdy, and G. D. Oliver, ``Build-up region of megavoltage photon radiation sources,''
  Med. Phys. 2, 14 ­19 1975 .
 5J. L. Horton, ``Dosimetry of the Siemens Mevatron 67 linear accelerator,'' Int. J. Radiat. Oncol., Biol., Phys. 9,
  1217­1233 1983 .
 6M. J. Butson, T. Cheung, P. K. Yu, and B. Webb, ``Variations in skin dose associated with linac bed materials at 6 MV
  x-ray energy,'' Phys. Med. Biol. 47, N25­30 2002 .
 7J. Carl and A. Vestergaard, ``Skin damage probabilities using fixation materials in high-energy photon beams,''
  Radiother. Oncol. 55, 191­198 2000 .
 8E. E. Klein and J. A. Purdy, ``Entrance and exit depth doses for the Clinac 2100C,'' Int. J. Radiat. Oncol., Biol., Phys.
  27, 429­ 435 1993 .
 9A. Lamb and S. Blake, ``Investigation and modeling of the surface dose from linear accelerator produced 6 and 10 MV
  photon beams,'' Phys. Med. Biol. 43, 1133­1146 1998 .
10A. R. Hounsell and J. M. Wilkinson, ``Electron contamination and build-up doses in conformal radiotherapy fields,''
  Phys. Med. Biol. 44, 43­55 1999 .


Journal of Applied Clinical Medical Physics, Vol. 4, No. 1, Winter 2003

7       Klein, Esthappan, and Li: Surface and buildup dose characteristics . . .                                        7


11D. E. Mellenberg, ``Dose behind various immobilization and beam-modifying devices,'' Int. J. Radiat. Oncol., Biol.,
  Phys. 32, 1193 1995 .
12D. P. Fontenla, J. J. Napoli, M. Hunt, D. Fass, B. McCormick, and G. J. Kutcher, ``Effects of beam modifiers and
  immobilization devices on the dose in the build-up region,'' Int. J. Radiat. Oncol., Biol., Phys. 30, 211­119 1994 .
13B. J. Gerbi and F. M. Khan, ``Measurement of dose in the buildup region using fixed-separation plane-parallel ionization
  chambers,'' Med. Phys. 17, 17­26 1990 .




Journal of Applied Clinical Medical Physics, Vol. 4, No. 1, Winter 2003

