@MASTERSTHESIS\{IMM2012-06404,
    author       = "S. C. Claessen",
    title        = "Adaptive Radiotherapy - {CBCT} Based Dose Calculation",
    year         = "2012",
    school       = "Technical University of Denmark, {DTU} Informatics, {E-}mail: reception@imm.dtu.dk",
    address      = "Asmussens Alle, Building 305, {DK-}2800 Kgs. Lyngby, Denmark",
    type         = "",
    note         = "Supervisors: Claus Behrens, David Sj{\"{o}}str{\"{o}}m, Eva Emma Maria Sj{\"{o}}lin, and Rasmus R. Poulsen, rrp@imm.dtu.dk, {DTU} Informatics",
    url          = "http://www.imm.dtu.dk/English.aspx",
    abstract     = "Background: The aim of this report is to evaluate, if {CBCT} can be used for dose calculation. The advantage in using {CBCT} for dose calculation, is to be able to predict and assess the dose delivered to the patient on a daily basis. 

Materials \& Methods: The study was subdivided into three parts. Part A: Various scan parameters and phantom congurations were investigated for their impact on the Hounseld units (HUs) for Computed tomography (CT) and {CBCT}. For this investigation a {CIRS} water equivalent electron density phantom (DP) was used. The different configurations of the {DP} were used to obtain HUrelative electron density calibration curves. The {CT} curve from the {CT} scan obtained with clinic standard parameters (CTS) was used as reference. Part B: The default and the obtained calibration curves from the different {CBCT} protocols were selected for the dose calculation on the {CBCT} images. Using the Alderson phantom it was investigated which protocol and calibration curve was the most appropriate for the {CBCT} based dose calculation. Part C: The results from the Alderson (ARP) study were evaluated in a clinical perspective on an head and neck patient (H\&N) treated with intensity-modulated radiotherapy. In order to investigate if a site-specific calibration curve for different patient groups was needed different calibration curves were used for the head and neck patient. The calibration curves obtained from the pelvis configuration on {DP} were used to calculate the dose distribution on the pelvis patient treated with rapid arc. The intention was to investigate if {CBCT} can be used for dose calculation for this patient group.

Results: Part A: Compared to {CT} it was found, that the HUs from {CBCT} were more easily affected when changing the scan parameters. Voltage, {DP} size and insert arrangements affect the HUs the most. Part B: The {ARP} study showed a maximum dose difference of 0.6\% when comparing the use of the {CTS} and the default calibration curve. The most appropriate {CBCT} based dose calculation was achieved with the standard dose head protocol. A 0.3\% maximum dose difference between {CBCT} and {CT} was found. It was found that further investigation is needed in order to evaluate whether the {CBCT} or the {CT} calibration curve achieves the most accurate {CBCT} based dose calculation. Part C: For the H\&N patient a 1.7\% maximum dose difference between using the {CTS} and the default calibration curve was detected. A maximum dose difference between {CT} and {CBCT} was found to be 0.3\%. As for the {ARP} study, a further investigation is needed in order to evaluate whether the {CBCT} or the {CT} calibration curve achieves the most accurate {CBCT} based dose calculation. The results for cervix showed that further investigation was needed to conclude whether {CBCT} can be used for dose calculation on cervix patients. Further it was found that using a site-specific calibration curve for different patient groups achieved a {CBCT} based dose distribution most similar to the {CT} based dose distribution.

Conclusion: It is concluded that {CBCT} can be used for dose calculation on H\&N patients, but that further investigation is needed for the cervix patient. The standard protocol for {CBCT} is shown the to achieve a dose calculation similar to that of {CT} for the H\&N patient. Also, it is concluded that the use of site-specific calibration curve is needed in order to calculate an accurate dose distribution on {CBCT}."
}