MRI-only based Radiotherapy | Line Winther Waring, Marie Elgaard Korsholm Nielsen
| | 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 work ow.
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 ful l 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 ful lled since the data is approximately normal distributed with constant variances.
Results: For the HN- , sarcoma- and pelvic patients the statistical analysis show non signifi cant difference in the investigated DVH points. For the prostate patients the statistical analysis of the target volumes show that the MRIu differs signi ficant 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 signifi cant 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. | | Type | Master's thesis [Academic thesis] | | Year | 2012 | | Publisher | Technical University of Denmark, DTU Informatics, E-mail: reception@imm.dtu.dk | | Address | Asmussens Alle, Building 305, DK-2800 Kgs. Lyngby, Denmark | | Series | IMM-M.Sc.-2012-23 | | Note | Supervised by Associate Professor Rasmus Reinhold Paulsen, rrp@imm.dtu.dk, DTU Informatics | | Electronic version(s) | [pdf] | | Publication link | http://www.imm.dtu.dk/English.aspx | | BibTeX data | [bibtex] | | IMM Group(s) | Image Analysis & Computer Graphics |
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