Detectability and intra-fraction motion of individual elective lymph nodes in head and neck cancer patients on the Magnetic Resonance Image guided linear accelerator

Abstract

Background and purpose: Individual elective lymph node irradiation instead of elective neck irradiation is a new concept for head-and-neck cancer (HNC) patients developed for the Magnetic Resonance Image guided linear accelerator (MR-linac). To prepare this, the detectability, volume changes and intra-fraction motion of elective lymph nodes on the MR-linac was assessed.

Materials and methods: A total of 15 HNC patients underwent diagnostic pre-treatment MRI. Additionally, two MR-linac scans were obtained with a 10-minute time difference in the first week of radiation treatment. Elective lymph node contours inside lymph node levels (Ib-V) were segmented on the pre-treatment MRI and the MR-linac scans and compared on number and maximal transversal diameter. Intra-fraction motion of elective lymph nodes on the MR-linac was estimated using Center of Mass (COM) distances and incremental isotropic expansion of lymph node segmentations.

Results: Of all 679 detected lymph nodes on the pre-treatment MRI, eight lymph nodes were not detectable on the first MR-linac scan and 16 new lymph nodes were detected. Lymph node diameters between the pre-treatment MRI scan and the MR-linac scan varied from -0.19 to + 0.13 mm. COM distances varied from 1.2 to 1.7 mm and lymph node contours had to be expanded with 3 mm.

Conclusions: Nearly all elective lymph nodes were detectable on the 1.5T MR-linac scan with no major changes in target volumes compared to the pre-treatment MRI. Simulated intra-fraction motion during the MR-linac scans was smaller than the 5-mm margin that will be used in the first elective lymph node radiation treatment.

Keywords: Elective neck irradiation; Lymph nodes; Magnetic Resonance Imaging; Radiotherapy; Squamous cell carcinoma of head and neck.

Fig. 1

Patient and coil setup during the MRI scans. Patients were fixated in a thermoplastic mask. The diagnostic MRI scanner incorporates two flexible coils placed on both sides of the neck and an anterior and posterior 16-channel coil (left). The MRI scanner of the MR-linac contains a 4-channel posterior coil in the tabletop and a 4-channel anterior coil (right).

Fig. 2

The lymph node count per patient in the pre-treatment MRI scan (blue) per lymph node level and size category (<5, 5–10 and > 10 mm), compared to new (green) and non-detectable lymph nodes (red) identified on the first MR-linac scan. Boxes represent the first, second (median) and third quartile whereas the whiskers represent the 5/95 percentile of all values. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

The difference in maximum transverse diameter of lymph nodes between the pre-treatment scan and the first MR-linac scan per lymph node level (Ib–V) and maximal transverse diameter (<5mm, 5–10 mm and > 10 mm). Boxes represent the first, second (median) and third quartile whereas the whiskers represent the 5/95 percentile of all values.

Fig. 4

Center Of Mass (COM) differences of lymph nodes between the first and second MR-linac scan per lymph node level (Ib–V). COM differences are given in x  = left–right (light brown), y = anterior-posterior (green) and z = feed-head (pink) directions as well as the shortest distance between both center of masses (orange). Boxes represent the first, second and third quartile whereas the whiskers represent the 5/95 percentile of all values. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

Percentage of covered elective lymph nodes in the second MR-linac scan if the segmented volumes of the first MR-linac scan were isotropically expanded with incremental steps of 1 mm. 95 % of the lymph node volumes had to be within the expanded volume to be considered covered. Lymph nodes are divided per lymph node level Ib (light blue), II (red), III (purple), IV (green) and V (dark blue).