Feasibility of MR-only proton dose calculations for prostate cancer radiotherapy using a commercial pseudo-CT generation method

Abstract

A magnetic resonance (MR)-only radiotherapy workflow can reduce cost, radiation exposure and uncertainties introduced by CT-MRI registration. A crucial prerequisite is generating the so called pseudo-CT (pCT) images for accurate dose calculation and planning. Many pCT generation methods have been proposed in the scope of photon radiotherapy. This work aims at verifying for the first time whether a commercially available photon-oriented pCT generation method can be employed for accurate intensity-modulated proton therapy (IMPT) dose calculation. A retrospective study was conducted on ten prostate cancer patients. For pCT generation from MR images, a commercial solution for creating bulk-assigned pCTs, called MR for Attenuation Correction (MRCAT), was employed. The assigned pseudo-Hounsfield Unit (HU) values were adapted to yield an increased agreement to the reference CT in terms of proton range. Internal air cavities were copied from the CT to minimise inter-scan differences. CT- and MRCAT-based dose calculations for opposing beam IMPT plans were compared by gamma analysis and evaluation of clinically relevant target and organ at risk dose volume histogram (DVH) parameters. The proton range in beam's eye view (BEV) was compared using single field uniform dose (SFUD) plans. On average, a [Formula: see text] mm) gamma pass rate of 98.4% was obtained using a [Formula: see text] dose threshold after adaptation of the pseudo-HU values. Mean differences between CT- and MRCAT-based dose in the DVH parameters were below 1 Gy ([Formula: see text]). The median proton range difference was [Formula: see text] mm, with on average 96% of all BEV dose profiles showing a range agreement better than 3 mm. Results suggest that accurate MR-based proton dose calculation using an automatic commercial bulk-assignment pCT generation method, originally designed for photon radiotherapy, is feasible following adaptation of the assigned pseudo-HU values.

Synthetic CT reconstruction using a deep spatial pyramid convolutional framework for MR‐only breast radiotherapy

“…As a result, the corresponding CT and MRI simulation scans are made largely incompatible, necessitating intensive manual intervention in the form of contouring for bulk density overrides for the purpose of dose calculations or image pairing in this case. Techniques for propagating intestinal gas between modalities may be applied to mitigate one facet of this problem . Additionally, style‐transfer methods exemplified by CycleGAN that use unpaired CT and MR images to learn the mapping between CT and MR “styles” may be applied to the problem of sCT generation in an effort to avoid the requirement of multi‐modality image registration and the challenges that come with curating paired image data …”

“…Additionally, atlas‐based approaches struggle in cases where the incoming patient's anatomy differs from that represented by the atlas due to missing tissues, air cavities, or surgical implants . Alternatively, voxel‐based approaches involve the assignment of CT numbers through a number of methods, including segmentation with standard or specialized MRI sequences, statistical methods, and learning‐based approaches . The simplest and most widely used voxel‐based approaches use MRI voxel intensities from standard sequences to segment general tissue classes (air, fat, soft tissue, etc.)…”

MRI-guidance for motion management in external beam radiotherapy: current status and future challenges

“…Various techniques exist to approximate electron density data from MRI, such as bulk-density, atlas-based or machine learning solutions (Edmund and Nyholm 2017). These were investigated in great detail for 3D MRI imaging, particularly in relatively homogenous sites such as the brain and pelvis, and MRI-based planning was successfully integrated in clinical workflows (Edmund andNyholm 2017, Johnstone et al 2017), with some early work also carried out for proton therapy (Maspero et al 2017). On the other hand, synthetic-CT generation in sites affected by respiratory motion (e.g.…”

Proton beam behavior in a parallel configured MRI‐proton therapy hybrid: Effects of time‐varying gradient magnetic fields

“…As these beams are composed of charged particles, the effect of the magnetic resonance imaging's (MRI) magnetic fields on the proton trajectories and resulting dose distributions need to be explored. While fewer than MR‐guided x‐ray radiotherapy, there have been publications that explore the feasibility of an MR‐guided proton system . Recently, there have been some publications that explore numerical algorithms to predict proton trajectory and dose deposition in magnetic field without utilizing time‐consuming Monte Carlo techniques .…”

“…While fewer than MR-guided x-ray radiotherapy, there have been publications that explore the feasibility of an MR-guided proton system. [13][14][15][16][17][18][19][20][21] Recently, there have been some publications that explore numerical algorithms to predict proton trajectory and dose deposition in magnetic field without utilizing time-consuming Monte Carlo techniques. [22][23][24] In contrast, this work focuses not on the algorithm, but on the characterization of beam distortions due to the time-varying magnetic fields produced by the MRI's gradient coils which are essential for image generation.…”