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Absorbed dose in the build-up region in modern megavoltage photon radiotherapy

(2016)
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Abstract
In megavoltage photon radiotherapy, accurate knowledge of the absorbed dose in the build-up region is essential in the clinical management of skin toxicity and superficial target coverage. These build-up doses strongly depend on a wide range of treatment parameters and evolve, consequently, in close relation with treatment technology. Commercial treatment planning systems and multiple dosimetric techniques, however, fail to accurately predict buildup doses. In this respect, this dissertation investigated the impact of recent evolutions in megavoltage photon radiotherapy on the absorbed dose and calculation accuracy in the build-up region. To that purpose, we first compared and optimized the performance of two transmission scanners for radiochromic film dosimetry in the high dose gradient of the build-up region. The Nikon Coolscan 9000ED scanner (Nikon Co., Tokyo, Japan) was identified as the preferential tool for high-gradient dosimetry, mainly due to its high sensitivity resulting in more contrast in the digitized image. The main limitation of the Nikon digitizer is its film size restriction to 6.2x20 cm². Secondly, we focussed on flattening filter-free linear accelerators, which currently face swift implementation into clinical practice because of their increased dose rate and reduction in head scatter compared to conventional designs. Our study demonstrated that build-up doses in flattening filter-free beams provided by standard linear accelerators slightly exceed build-up doses in conventional beams, indicating that the shift of the X-ray spectrum to lower energies in flattening filter-free mode is nearly compensated by the reduction of electron contamination and head scatter. In literature, the reduction in electron contamination in flattening-filter free mode is often anticipated to improve build-up calculation accuracy, as treatment planning systems generally use simple electron contamination models. For the XVMC code implemented in the Monaco treatment planning system however, considerable differences between build-up measurements and calculations were detected in both flattened and unflattened beams, emphasizing the importance of other factors that negatively affect build-up calculation accuracy. Thirdly, this dissertation investigated the dosimetric impact of patient positioning devices, which are increasingly used to ensure set-up accuracy while delivering highly conformal dose distributions. The presence of the components of the AIO patient positioning device (Orfit Industries, Wijnegem, Belgium) was found to cause considerable and clinically relevant build-up dose increases. Integration of the device CT data into treatment planning allowed to accurately model the most important attenuation effects, but failed to accurately predict build-up doses. Finally, we investigated the potential of modern volumetric arc techniques (VMAT) to intentionally reduce the superficial dose to the hair follicles and prevent temporary alopecia in whole brain radiotherapy. Compared to the standard opposed fields techniques, VMAT was shown to considerably reduce the subcutaneously absorbed dose (20.5% on average), mainly on the top and the back of the skull (41.8% on average). Accordingly, the mean hair loss in the study subjects in a prospective phase II trial was the least pronounced on the top and the back of the skull.

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Chicago
De Puysseleyr, Annemieke. 2016. “Absorbed Dose in the Build-up Region in Modern Megavoltage Photon Radiotherapy”. Ghent, Belgium: Ghent University. Faculty of Medicine and Health Sciences.
APA
De Puysseleyr, A. (2016). Absorbed dose in the build-up region in modern megavoltage photon radiotherapy. Ghent University. Faculty of Medicine and Health Sciences, Ghent, Belgium.
Vancouver
1.
De Puysseleyr A. Absorbed dose in the build-up region in modern megavoltage photon radiotherapy. [Ghent, Belgium]: Ghent University. Faculty of Medicine and Health Sciences; 2016.
MLA
De Puysseleyr, Annemieke. “Absorbed Dose in the Build-up Region in Modern Megavoltage Photon Radiotherapy.” 2016 : n. pag. Print.
@phdthesis{7099984,
  abstract     = {In megavoltage photon radiotherapy, accurate knowledge of the absorbed dose in the build-up region is essential in the clinical management of skin toxicity and superficial target coverage. These build-up doses strongly depend on a wide range of treatment parameters and evolve, consequently, in close relation with treatment technology. Commercial treatment planning systems and multiple dosimetric techniques, however, fail to accurately predict buildup doses. In this respect, this dissertation investigated the impact of recent evolutions in megavoltage photon radiotherapy on the absorbed dose and calculation accuracy in the build-up region. 
To that purpose, we first compared and optimized the performance of two transmission scanners for radiochromic film dosimetry in the high dose gradient of the build-up region. The Nikon Coolscan 9000ED scanner (Nikon Co., Tokyo, Japan) was identified as the preferential tool for high-gradient dosimetry, mainly due to its high sensitivity resulting in more contrast in the digitized image. The main limitation of the Nikon digitizer is its film size restriction to 6.2x20 cm{\texttwosuperior}. 
Secondly, we focussed on flattening filter-free linear accelerators, which currently face swift implementation into clinical practice because of their increased dose rate and reduction in head scatter compared to conventional designs. Our study demonstrated that build-up doses in flattening filter-free beams provided by standard linear accelerators slightly exceed build-up doses in conventional beams, indicating that the shift of the X-ray spectrum to lower energies in flattening filter-free mode is nearly compensated by the reduction of electron contamination and head scatter. In literature, the reduction in electron contamination in flattening-filter free mode is often anticipated to improve build-up calculation accuracy, as treatment planning systems generally use simple electron contamination models. For the XVMC code implemented in the Monaco treatment planning system however, considerable differences between build-up measurements and calculations were detected in both flattened and unflattened beams, emphasizing the importance of other factors that negatively affect build-up calculation accuracy. 
Thirdly, this dissertation investigated the dosimetric impact of patient positioning devices, which are increasingly used to ensure set-up accuracy while delivering highly conformal dose distributions. The presence of the components of the AIO patient positioning device (Orfit Industries, Wijnegem, Belgium) was found to cause considerable and clinically relevant build-up dose increases. Integration of the device CT data into treatment planning allowed to accurately model the most important attenuation effects, but failed to accurately predict build-up doses. 
Finally, we investigated the potential of modern volumetric arc techniques (VMAT) to intentionally reduce the superficial dose to the hair follicles and prevent temporary alopecia in whole brain radiotherapy. Compared to the standard opposed fields techniques, VMAT was shown to considerably reduce the subcutaneously absorbed dose (20.5\% on average), mainly on the top and the back of the skull (41.8\% on average). Accordingly, the mean hair loss in the study subjects in a prospective phase II trial was the least pronounced on the top and the back of the skull.},
  author       = {De Puysseleyr, Annemieke},
  language     = {eng},
  pages        = {193},
  publisher    = {Ghent University. Faculty of Medicine and Health Sciences},
  school       = {Ghent University},
  title        = {Absorbed dose in the build-up region in modern megavoltage photon radiotherapy},
  year         = {2016},
}