Dosimetric and geometric evaluation of MRI as the only imaging modality for the radiotherapy treatment process of localized prostate cancer

Abstract: Introduction MRI provides better soft tissue contrast than CT and is an invaluable tool in the radiotherapy treatment process. The gold standard today is image fusion of MRI and CT data where targets and organs-at-risk are delineated on the MRI data and the dose calculation is performed on the CT data. It is appealing to exclude CT from the process in order to reduce cost and time and to eliminate the uncertainty inherited from the image registration. Since MRI data does not contain any electron density information, this property must be assigned to the data. The purpose of this study is to evaluate the use of MRI as the only modality for the entire radiotherapy treatment process of localized prostate cancer. A study of the dosimetric consequences of MRI based dose planning and the contrast properties of novel intra-prostatic marker materials in multimodality imaging was conducted. Materials and methods Ten patients were selected for the dosimetry study and were both MR- and CT-scanned using routine clinical imaging parameters. 3DCRT, IMRT and VMAT plans were generated for the CT data, unit density assigned CT-data, unit density assigned MRI data and bulk density assigned MRI data. The resulting dose distributions were compared by calculating the root-mean-square values (rms) for the volumes enclosed by isodose levels 60%, 70%, 80%, 90%, 95% and 100% normalized to the prescription dose and by generating topographic maps of the percentage dose- and the absolute dose difference. DVH"s were compared in order to study general tendencies in the dose distributions. The geometric distortion was studied with the help of a geometric distortion evaluation phantom provided by the vendor and the effect of using the vendors built in gradient distortion correction algo-rithm was studied utilizing both phantom measurements and scans of three healthy volunteers. The contrast properties of the marker implant materials was studied by inserting them in two pieces of ham followed by CT- and MRI scanning them using clinical routine protocols. Megavoltage and kilovoltage on-board imager systems capable of performing CBCT were also used to test the visibility of the markers at the linear accelerator. The best suitable marker was implanted into four volunteer patients in order to study the visibility in vivo with ana-tomical structures present. ResultsDose discrepancies > 2 % are found mainly in clinically insignificant areas in all patients regardless if bony areas is assigned a relevant CT-number or not. The dosimetric uncertainty is however significantly reduced (p < 0.05) inside the target volume for the 3DCRT plans (rms at V95% reduced from 1.4 ± 0.4 to 0.6 ± 0.2) but not for the VMAT plans (p ≈ 0.07) for the bulk density assigned MRI data compared to unit density assigned MRI data (rms at V95% reduced from 1.3 ± 0.6 to 1.1 ± 0.6). In contrary to the hypothesis, the dosimetric uncertainty is signifi-cantly increased (p < 0.05) inside the target volume for IMRT plans for bulk density assigned MRI data com-pared to unit density assigned MRI data (rms at V95% increased from 0.8 ± 0.3 to 1.2 ± 0.4). No apparent reason for this was found, but it is believed that the chosen CT-number of 403 HU is an overestimate of the true mean for this tissue and that the fluence optimized plans are more sensitive to the choice of CT-number. Large indi-vidual differences are found when studying local percentage dose differences and are mostly prominent for the VMAT treatment technique. The gradient distortion correction algorithm reduces the geometric distortion in the MR-images but residual distortion is present. The gold/nitinol material composition for marker implants showed acceptable visibility in all imaging modalities except the on-board MV-imaging system. ConclusionMRI-only based radiotherapy treatment planning and setup of prostate cancer patients is feasible. Incorporating a quality assurance program in order to reduce the geometric distortion in the MR-images even further as well as assigning relevant CT-numbers to bone tissues reduces the dosimetric uncertainty, however more work is re-quired in order to study these effects more thoroughly. The gold/nitinol material composition is recommended for use in multimodality imaging when CBCT or kV imaging is employed.