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Technical Note: Is bulk electron density assignment appropriate for MRI ‐only based treatment planning for lung cancer?
Author(s) -
Prior Phil,
Chen Xinfeng,
Gore Elizabeth,
Johnstone Candice,
Li X. Allen
Publication year - 2017
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1002/mp.12267
Subject(s) - radiation treatment planning , nuclear medicine , medicine , lung cancer , radiation therapy , radiology
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 CT s 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 ( DVP s) were collected from both the “simulated MRI ‐based plans” and the original planning CT : D 95 , the dose delivered to 95% of the ITV & planning target volume ( PTV ), D 5 and V 5 , 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 V 5 and D 95 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 DVP s were observed. Variations of the normal lung V 5 can be as large as 9.6%. In some instances, varying the rED ITV between rED mean and 1.06 resulted in D 95 increases ranging from 3.9% to 6.3%. Bulk rED assignment on normal lung affected the DVP s 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 DVP s 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.