Knowledge-based planning for intensity-modulated radiation therapy: A review of data-driven approaches

Abstract

Purpose: Intensity-Modulated Radiation Therapy (IMRT), including its variations (including IMRT, Volumetric Arc Therapy (VMAT), and Tomotherapy), is a widely used and critically important technology for cancer treatment. It is a knowledge-intensive technology due not only to its own technical complexity, but also to the inherently conflicting nature of maximizing tumor control while minimizing normal organ damage. As IMRT experience and especially the carefully designed clinical plan data are accumulated during the past two decades, a new set of methods commonly termed knowledge-based planning (KBP) have been developed that aim to improve the quality and efficiency of IMRT planning by learning from the database of past clinical plans. Some of this development has led to commercial products recently that allowed the investigation of KBP in numerous clinical applications. In this literature review, we will attempt to present a summary of published methods of knowledge-based approaches in IMRT and recent clinical validation results.

Methods: In March 2018, a literature search was conducted in the NIH Medline database using the PubMed interface to identify publications that describe methods and validations related to KBP in IMRT including variations such as VMAT and Tomotherapy. The search criteria were designed to have a broad scope to capture relevant results with high sensitivity. The authors filtered down the search results according to a predefined selection criteria by reviewing the titles and abstracts first and then by reviewing the full text. A few papers were added to the list based on the references of the reviewed papers. The final set of papers was reviewed and summarized here.

Results: The initial search yielded a total of 740 articles. A careful review of the titles, abstracts, and eventually the full text and then adding relevant articles from reviewing the references resulted in a final list of 73 articles published between 2011 and early 2018. These articles described methods for developing knowledge models for predicting such parameters as dosimetric and dose-volume points, voxel-level doses, and objective function weights that improve or automate IMRT planning for various cancer sites, addressing different clinical and quality assurance needs, and using a variety of machine learning approaches. A number of articles reported carefully designed clinical studies that assessed the performance of KBP models in realistic clinical applications. Overwhelming majority of the studies demonstrated the benefits of KBP in achieving comparable and often improved quality of IMRT planning while reducing planning time and plan quality variation.

Conclusions: The number of KBP-related studies has been steadily increasing since 2011 indicating a growing interest in applying this approach to clinical applications. Validation studies have generally shown KBP to produce plans with quality comparable to expert planners while reducing the time and efforts to generate plans. However, current studies are mostly retrospective and leverage relatively small datasets. Larger datasets collected through multi-institutional collaboration will enable the development of more advanced models to further improve the performance of KBP in complex clinical cases. Prospective studies will be an important next step toward widespread adoption of this exciting technology.

Keywords: IMRT; KBP; VMAT; IMRT planning; intensity-modulated radiation therapy; knowledge modeling; knowledge-based planning; machine learning; tomotherapy.

Figure 1

Flow diagram of article selection.

Figure 2

Trend of publications related to knowledge‐based planning. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 3

Prescribed dose‐volume constraints used for manual planning. (a) Rectum constraints; (b) Bladder constraints. Notice that in each case, the diagonal line (thick brown) is a reasonable first‐order approximation of the dose‐volume histogram curve. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 4

Visualization of knowledge‐based planning (KBP) method performance in rectum dose sparing. The thick diagonal line in black is the proxy dosevolume histogram (DVH) curve of clinical plans. The green and red DVH curves represent the approximated average performance of the re‐planned cases in nine KBP studies relative to the clinical plans. The green curves indicate case/atlas‐based methods while the red curves indicate model‐based methods. The thicker lines indicate studies with 30 or more sample cases. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 5

Visualization of knowledge‐based planning (KBP) method performance in bladder dose sparing. The thick diagonal line in black is the proxy dosevolume histogram (DVH) curve of clinical plans. The green and red DVH curves represent the approximated average performance of the re‐planned cases in nine KBP studies relative to the clinical plans. The green curves indicate case/atlas‐based methods while the red curves indicate model‐based methods. The thicker lines indicate studies with 30 or more sample cases. [Color figure can be viewed at wileyonlinelibrary.com]

Figure 6

The size of datasets used for training and validating knowledge models. The error bars indicate standard deviation. Note that the large deviations in 2016 and 2017 are due to one significantly larger dataset. [Color figure can be viewed at wileyonlinelibrary.com]