ESTRO 2024 - Abstract Book

S5529

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2024

extended the diagnostic MRI protocol with a SPACE sequence (Siemens healthcare, Erlangen, Germany) including the body outline of the patient. The MRI DICOM files were modified utilizing the pydicom package in Python to mimic a CT based planning workflow using the diagnostic MRIs and to allow import of the MRIs within the patient position verification system (XVI, Elekta, Stockholm, Sweden). Target volume delineation and treatment planning was directly performed on the diagnostic MRI in RayStation (RaySearch Laboratories AB, Stockholm, Sweden). The treatment plan consisted of an intensity modulated 3-field technique from the dorsal direction. Typical beam angles were 130, 180 and 230 degrees. Dose calculation was performed on a mass density map overlayed on the MRI. During the treatment delivery, the patient position was verified and corrected based on a match between the diagnostic MRI and the CBCT. Due to the palliative setting and our beam set-up we assumed a straightforward density override acceptable in terms of dose accuracy. For the lungs we used 0.26 g/cm3 and for all other tissues we used a density of 1.00 g/cm3. To assess the impact of the density overrides on the dose distribution, we analyzed ten patients previously treated for spine irradiation. Treatment quality was analyzed by comparing the V95% of the planning target volume doses between the CT-based clinical treatment plans and the dose calculated by using the same density overrides as in the MR-only workflow.

Potential errors within the MR-only workflow were identified in detail using a light Healthcare Failure Mode and Effect Analysis (HFMEA-light) using a risk score scale ranging from 1 (low risk) to 100 (high risk).

Results:

The clinical implementation of the MR-only workflow for palliative spine radiation therapy was successful after modification of the DICOM MRIs (i.e. Modality, SOPClassUID, RescaleType, Intercept, and Slope) , the MRI density override schedule and the HFMEA-light analysis. On average, the difference between the PTV V95% of the CT-based plans and the use of the density overrides was 0.50% (range: 0.04% – 1.20%). In total, we identified 19 process steps using the HFMEA-light. The HFMEA-light analysis indicated that there were no high-risk scores for potential failure modes that could result in serious consequences for patients or employees (range of average value: 1- 13). All process steps had low risk scores, with the highest scores being 13, for two specific process steps. These steps were considered not clinically relevant and were related to patient positioning verification and matching the diagnostic MRI with the CBCT prior to treatment.

Conclusion:

CBCT based palliative spine radiation therapy can be performed using a diagnostic MR-only workflow without the need for a simulation CT. The MR-only workflow can be performed safely using a clinically acceptable dose distribution.

Keywords: MR-only, palliative