Technical Note: Is bulk electron density assignment appropriate for MRI-only based treatment planning for lung cancer?

Purpose: MRI-based treatment planning in radiation therapy (RT) is prohibitive, in part, due to the lack of electron density (ED) information within the image. The dosimetric differences between MRI- and CT-based planning for intensity modulated RT (IMRT) of lung cancer were investigated to assess the appropriateness of bulk ED assignment.

Methods: Planning CTs acquired for six representative lung cancer patients were used to generate bulk ED IMRT plans. To avoid the effect of anatomic differences between CT and MRI, "simulated MRI-based plans" were generated by forcing the relative ED (rED) to water on CT-delineated structures using organ specific values from the ICRU Report 46 and using the mean rED value of the internal target volume (ITV) from the planning CT. The "simulated MRI-based plans" were generated using a research planning system (Monaco v5.09.07a, Elekta, AB) and employing Monte Carlo dose calculation. The following dose-volume-parameters (DVPs) were collected from both the "simulated MRI-based plans" and the original planning CT: D₉₅ , the dose delivered to 95% of the ITV & planning target volume (PTV), D₅ and V₅ , the volume of normal lung irradiated ≥5 Gy. The percent point difference and relative dose difference were used for comparison with the CT based plan for V₅ and D₉₅ respectively. A total of five plans per patient were generated; three with the ITV rED (rED_ITV ) = 1.06, 1.0 and the mean value from the planning CT while the lung rED (rED_lung ) was fixed at the ICRU value of 0.26 and two with rED_lung = 0.1 and 0.5 while the rED_ITV was fixed to the mean value from the planning CT.

Results: Noticeable differences in the ITV and PTV DVPs were observed. Variations of the normal lung V₅ can be as large as 9.6%. In some instances, varying the rED_ITV between rED_mean and 1.06 resulted in D₉₅ increases ranging from 3.9% to 6.3%. Bulk rED assignment on normal lung affected the DVPs of the ITV and PTV by 4.0-9.8% and 0.3-19.6% respectively. Dose volume histograms were presented for representative cases where the variations in the DVPs were found to be very large or very small.

Conclusions: The commonly used bulk rED assignment in MRI-only based planning may not be appropriate for lung cancer. A voxel based method, e.g., synthetic CT generated from MRI data, is likely required for dosimetrically accurate MR-based planning for lung cancer.