MRI-based IMRT planning for MR-linac: comparison between CT- and MRI-based plans for pancreatic and prostate cancers

Abstract

The treatment planning in radiation therapy (RT) can be arranged to combine benefits of computed tomography (CT) and magnetic resonance imaging (MRI) together to maintain dose calculation accuracy and improved target delineation. Our aim is study the dosimetric impact of uniform relative electron density assignment on IMRT treatment planning with additional consideration given to the effect of a 1.5 T transverse magnetic field (TMF) in MR-Linac. A series of intensity modulated RT (IMRT) plans were generated for two representative tumor sites, pancreas and prostate, using CT and MRI datasets. Representative CT-based IMRT plans were generated to assess the impact of different electron density (ED) assignment on plan quality using CT without the presence of a 1.5 T TMF. The relative ED (rED) values used were taken from the ICRU report 46. Four types of rED assignment in the organs at risk (OARs), the planning target volumes (PTV) and in the non-specified tissue (NST) were considered. Dose was recalculated (no optimization) using a Monaco 5.09.07a research planning system employing Monte Carlo calculations with an option to include TMF. To investigate the dosimetric effect of different rED assignment, the dose-volume parameters (DVPs) obtained from these specific rED plans were compared to those obtained from the original plans based on CT. Overall, we found that uniform rED assignment results in differences in DVPs within 3% for the PTV and 5% for OAR. The presence of 1.5 T TMF on IMRT DVPs resulted in differences that were generally within 3% of the Gold St for both the pancreas and prostate. The combination of uniform rED assignment and TMF produced differences in DVPs that were within 4-5% of the Gold St. Larger differences in DVPs were observed for OARs on T2-based plans. The effects of using different rED assignments and the presence of 1.5 T TMF for pancreas and prostate IMRT plans are generally within 3% and 5% of PTV and OAR Gold St values. There are noticeable dosimetric differences between the CT- and MRI-based IMRT plans caused by a combination of anatomical changes between the two image acquisition times, uniform rED assignment and 1.5 T TMF.

Quantification of confounding factors in MRI-based dose calculations as applied to prostate IMRT

“…To estimate tissue attenuation properties from MRI, several methods have been proposed in the last few decades (Lee et al 2003, Chen et al 2004, Schmidt and Payne 2015, Prior et al 2016. More recently, MRI vendors have launched solutions as well , Siversson et al 2015.…”

Properties of the anisotropy of dose contributions: A planning study on prostate cases

“…Anisotropy profiles in a cross‐section through the isocenter show curves with similar scaling law throughout the PTV, to a certain range independent from the treatment technique. The calculations for the fluence presented by Brahme and also for the anisotropy performed by Bratengeier were based on rotational symmetric target volumes. Thus, deviations from this scaling in “real” patient data (cf.…”

“…Thus, the previously considered ideas for ad hoc plan adaptation are particularly interesting for the next generation of linear accelerators, the MR‐linac. Various fluence‐based concepts for adaptive planning have been developed . On the other side, the current state of MR‐linac technology does not yet offer dynamic arcs and thus excludes VMAT.…”

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

“…The large dosimetric differences suggest that the uniform rED assignment for target and normal lung tissues may not be a feasible approach to MRI‐based treatment planning, particularly if changes in the electron density of the tumor and normal lung tissues are indicative of treatment response. We used simulated MRI‐based plans to avoid the effect of significant anatomy difference between the MRI and CT acquisitions …”

“…A major limitation in the exclusive use of MRI for treatment planning is the lack of a relationship between the MRI signal intensity ( I MRI ) and electron density (ED), which is necessary for dose calculations. One method to address this issue is to assign bulk ED or relative ED (rED) to each organ, tissue or target necessary for the planning a specific value that is assumed uniform throughout the delineated structure . The specific values can either be derived from the International Commission on Radiation Units and Measurements (ICRU) Report 46 or set to a value chosen by the planner to be appropriate for that organ or tissue .…”