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The Importance of Hounsfield Units for IMRT and VMAT Plans of Nasopharynx Cancer
Y Akdeniz1*, G Ugurluer2 , I Kaptan1 , B Ispir1 , M Serin2 , (1) Acibadem Adana Hospital, Adana, Seyhan, (2) Acibadem University Faculty of Medicine, Adana, Seyhan,
Presentations
SU-I-GPD-J-111 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose: Hounsfield unit (HU) is a quantity used for provide information from CT scans and used in calculations for radiotherapy. In this study, the importance of HU for IMRT and VMAT in nasopharynx cancer (NPC) has been investigated.
Methods: The planning CT of 10 patients who has been diagnosed with NPC were assessed. VMAT and 7-field IMRT plans were done. Then plans were copied on CT images which have zero HU in body contour (Figure 1). By that way, modified VMAT (m-VMAT) and IMRT (m-IMRT) plans were created. Calculations were done by using Eclipse v13.6 TPS with AAA. The plans were compared in terms of PTV coverage and OAR protection. The dose distributions of most heterogeneous slice in terms of HU were compared by gamma analysis.
Results: The minimum doses that 1% of PTV received (D1%) were respectively 73.97±0.83 and 73.29±0.81 Gy for IMRT and m-IMRT; 74.85±0,84 and 74.17±0,91 Gy for VMAT and m-VMAT (ρ≤0.05). There was 4.4% difference between VMAT plans for D1% of brain stem. There was no significant difference between the plans for PTV and other OAR criteria in Table1 and the differences were less than 2%. IMRT also VMAT plans were examined with gamma analysis (3%/3mm). The pass rate results were 97.66±1.62 and 97.88±1.34. We found mean value of pass rate 94.49±2.35 and 93.81±1.97 by using 2%/2mm criteria for IMRT and VMAT plans respectively.
Conclusion: The contribution of HU to dose calculations did not make important difference for IMRT and VMAT plans even in heterogeneous regions of the body such as head and neck. Gamma analysis pass rates were in the range of acceptance criteria for clinical applications. However, for more accurate dose calculations HU data is required to use. Further more, the importance of HU for the low contrast regions like thorax should be investigated.
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