Evaluation of a simplified optimizer for MR-guided adaptive RT in case of pancreatic cancer

Abstract

Purpose: Magnetic resonance-guided adaptive radiotherapy (MRgART) is considered a promising resource for pancreatic cancer, as it allows to online modify the dose distribution according to daily anatomy. This study aims to compare the dosimetric performance of a simplified optimizer implemented on a MR-Linac treatment planning system (TPS) with those obtained using an advanced optimizer implemented on a conventional Linac.

Methods: Twenty patients affected by locally advanced pancreatic cancer (LAPC) were considered. Gross tumor volume (GTV) and surrounding organ at risks (OARs) were contoured on the average 4DCT scan. Planning target volume was generated from GTV by adding an isotropic 3 mm margin and excluding overlap areas with OARs. Treatment plans were generated by using the simple optimizer for the MR-Linac in intensity-modulated radiation therapy (IMRT) and the advanced optimizer for conventional Linac in IMRT and volumetric modulated arc therapy (VMAT) technique. Prescription dose was 40 Gy in five fractions. The dosimetric comparison was performed on target coverage, dosimetric indicators, and low dose diffusion.

Results: The simplified optimizer of MR-Linac generated clinically acceptable plans in 80% and optimal plans in 55% of cases. The number of clinically acceptable plans obtained using the advanced optimizer of the conventional Linac with IMRT was the same of MR-Linac, but the percentage of optimal plans was higher (65%). Using the VMAT technique, it is possible to obtain clinically acceptable plan in 95% and optimal plans in 90% of cases. The advanced optimizer combined with VMAT technique ensures higher target dose homogeneity and minor diffusion of low doses, but its actual optimization time is not suitable for MRgART.

Conclusion: Simplified optimization solutions implemented in the MR-Linac TPS allows to elaborate in most of cases treatment plans dosimetrically comparable with those obtained by using an advanced optimizer. A superior treatment plan quality is possible using the VMAT technique that could represent a breakthrough for the MRgART if the modern advancements will lead to shorter optimization times.

Keywords: IMRT; MR-guided Radiotherapy; online adaptive radiotherapy; pancreatic cancer; plan optimization.

Figure 1

Schematic representation of the results obtained in terms of clinical evaluation for the MR‐Linac in IMRT modality (MR‐Linac) and the traditional Linac in IMRT (IMRT) and VMAT (VMAT) modality. A plan was considered as “clinically acceptable” if the V95% of CTV ≥ 99.5% and V95% of PTV was comprised between 95% and 98%. A plan was labeled as “optimal” if CTV V95% ≥ 99.5% and PTV V95% ≥ 98%. All the other cases were considered as not clinically acceptable plans.

Figure 2

Results in terms of P‐values for Wilcoxon signed‐rank test obtained for target structures and dosimetric indicators (upper part) and for OARs and treated volumes (lower part). Comparisons were performed between MR‐Linac and IMRT plans obtained with traditional Linac, MR‐Linac, and VMAT plans on traditional Linac, and between the two delivery modalities of traditional Linac.

Figure 3

Bar plot (a and b) and box plot (c) showing the values of target coverage expressed as D98% CTV and V95% PTV for MR‐Linac, IMRT and VMAT for conventional Linac.

Figure 4

Bar plot (a and b) and box plot (c) showing low dose diffusion expressed as cubic centimeters of V20Gy and V10Gy for IMRT MR‐Linac, IMRT and VMAT and conventional Linac.

Figure 5

Example of dose distribution for a patient in the case of IMRT with conventional Linac (a), IMRT with MR‐Linac (b), and VMAT with conventional Linac (c). The pink isodose is the 95% of prescription dose (38 Gy), the cyan line represents the 20 Gy isodose, the green line is the 10 Gy isodose.