@MASTERSTHESIS\{IMM2012-06279,
    author       = "L. W. Waring and M. E. K. Nielsen",
    title        = "MRI-only based Radiotherapy",
    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         = "Supervised by Associate Professor Rasmus Reinhold Paulsen, rrp@imm.dtu.dk, {DTU} Informatics",
    url          = "http://www.imm.dtu.dk/English.aspx",
    abstract     = "Background:
Radiotherapy is used for cancer treatment and the technique requires image information of the patient's anatomy. Treatment planning is based on a {CT,} since the scan among other things contains information of the electron densities in the tissues. The {MRI} provides a high soft tissue contrast compared to the {CT}. Multimodality imaging is therefore increasingly used in order to give a solid base for an accurate delineation of the tumour and the neighbouring organs. However, combining the different modalities introduce a systematic registration error. The aim of this study is to evaluate if {MRI-}only based radiotherapy is feasible. This is investigated in order to eliminate the systematic registration error and simplify the workow.
Materials \& Methods: The investigation is performed by evaluating the dosimetric differences of a treatment plan based on an {MRI} as compared to a {CT}. The comparison is performed on four diagnostics groups; 12 head and neck patients treated with static {IMRT,} 6 sarcoma (extremities only) patients treated with {3D} {CRT,} 21 prostate and 5 pelvic (not prostate) patients treated with {VMAT}. The data from each patient contains a {CT} (including a structure set), an {MRI} and a clinical approved treatment plan. The structure set from the
{CT} is transferred to the {MRI} along with a {CT-}based clinical treatment plan. The transferred structure set does not include a body outline, which is therefore manually delineated in the {MRI}. The dose calculations based on the {MRI} are evaluated with a homogeneous density assigned {MRI} (MRIu), and a heterogeneous density assigned {MRI} (MRIb). In the MRIu, the entire body is assigned a {HU} equal to water. In the MRIb the {CT} segmented bone is transferred to the {MRI} and assigned a {HU} calculated based on electron densities in {ICRU} report 46 [38]. For {HN} patients, a second approach to the heterogeneous density assigned {MRI} (MRIb,c) is investigated. In addition to the MRIb, the MRIb,c includes segmentation and density correction of air cavities. The differences in the dose distributions are investigated with the use of {DVH} points. The {DVH} points for the target volumes, {PTV} and {CTV,} are Dmedian, D98\% and D2\%, as recommended in the {ICRU} Report 83 [6]. The OARs are investigated with the {DVH} points recommended by {DAHANCA} [16] and the clinical guidelines used at Herlev Hospital. For the prostate patients the differences in the dose distributions are further investigated using a gamma evaluation. The gamma evaluation is performed on the {CT} and the MRIu as well as the {CT} and the MRIb. The gamma evaluations are compared based on the percentage of points that full the gamma criteria in the {PTV}. An one-way two-tailed {ANOVA} and a paired t-test are used to investigate the differences in the {DVH} points. The assumptions of an {ANOVA} are fullled since the data is approximately normal distributed with constant variances.
Results: For the {HN-} , sarcoma- and pelvic patients the statistical analysis show non significant difference in the investigated {DVH} points. For the prostate patients the statistical analysis of the target volumes show that the MRIu differs significant from both the {CT} and the MRIb. This indicates, that a bulk density correction is necessary for the prostate patients. Similar results were found in the gamma evaluation. The analysis of the {OAR} for the prostate patients did not show any significant difference.
Conclusion: {MRI-}only based {RT} is found to be a feasible alternative to the {CT-}based {RT}. The results of the statistical analysis and the shape of the {DVH} is taken into consideration in an overall assessment of the most suitable density correction for each diagnostic group."
}