Treatment planning for MR-guided SBRT of pancreatic tumors on a 1.5 T MR-Linac: A global consensus protocol

Abstract

Background and purpose: Treatment planning for MR-guided stereotactic body radiotherapy (SBRT) for pancreatic tumors can be challenging, leading to a wide variation of protocols and practices. This study aimed to harmonize treatment planning by developing a consensus planning protocol for MR-guided pancreas SBRT on a 1.5 T MR-Linac.

Materials and methods: A consortium was founded of thirteen centers that treat pancreatic tumors on a 1.5 T MR-Linac. A phased planning exercise was conducted in which centers iteratively created treatment plans for two cases of pancreatic cancer. Each phase was followed by a meeting where the instructions for the next phase were determined. After three phases, a consensus protocol was reached.

Results: In the benchmarking phase (phase I), substantial variation between the SBRT protocols became apparent (for example, the gross tumor volume (GTV) D_99% ranged between 36.8 - 53.7 Gy for case 1, 22.6 - 35.5 Gy for case 2). The next phase involved planning according to the same basic dosimetric objectives, constraints, and planning margins (phase II), which led to a large degree of harmonization (GTV D_99% range: 47.9-53.6 Gy for case 1, 33.9-36.6 Gy for case 2). In phase III, the final consensus protocol was formulated in a treatment planning system template and again used for treatment planning. This not only resulted in further dosimetric harmonization (GTV D_99% range: 48.2-50.9 Gy for case 1, 33.5-36.0 Gy for case 2) but also in less variation of estimated treatment delivery times.

Conclusion: A global consensus protocol has been developed for treatment planning for MR-guided pancreatic SBRT on a 1.5 T MR-Linac. Aside from harmonizing the large variation in the current clinical practice, this protocol can provide a starting point for centers that are planning to treat pancreatic tumors on MR-Linac systems.

Keywords: Consensus protocol; MR-guided SBRT; Pancreatic cancer; Treatment planning.

Fig. 1

3D T₂w MRI data of case 1 (a) and case 2 (b), on which the planning contours are projected (red = GTV, cyan = duodenum, green = small bowel). Note that not all contours are shown and that treatment planning itself was performed on CT imaging. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Dose-volume histograms (DVHs) of the GTV (red) and duodenum (cyan) from all thirteen centers. The DVHs are extracted from the treatment plans of case 1 (top row) and case 2 (bottom row), as created in phases I-III (columns). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Boxplots of the distribution of the most critical DVH parameters of the GTV, duodenum, small bowel and stomach, in phases I-III. The whiskers of the boxplot extend to the minimum and maximum of the values not considered outliers (values further than 1.5 times the interquartile range from the box). The outliers are indicated by the red plus signs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4

The dose distributions for a) case 1 and b) case 2 from all thirteen centers in phase I (variation in baseline practice). Red = GTV, cyan = duodenum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)