MRI visibility and displacement of elective lymph nodes during radiotherapy in head and neck cancer patients

Abstract

Background and purposeTo decrease the impact of radiotherapy to healthy tissues in the head and neck region, we propose to restrict the elective neck irradiation to elective lymph nodes at risk of containing micro metastases instead of the larger lymph node volumes. To assess whether this new concept is achievable in the clinic, we determined the number, volume changes and displacement of elective lymph nodes during the course of radiotherapy.Materials and methodsMRI scans of 10 head and neck cancer (HNC) patients were acquired before radiotherapy and in week 2, 3, 4 and 5 during radiotherapy. The weekly delineations of elective lymph nodes inside the lymph node levels (Ib/II/III/IVa/V) were rigidly registered and analyzed regarding number and volume. The displacement of elective lymph nodes was determined by center of mass (COM) distances, vector-based analysis and the isotropic contour expansion of the lymph nodes of the pre-treatment scan or the scan of the previous week in order to geographically cover 95% of the lymph nodes in the scans of the other weeks.ResultsOn average, 31 elective lymph nodes in levels Ib-V on each side of the neck were determined. This number remained constant throughout radiotherapy in most lymph node levels. The volume of the elective lymph nodes reduced significantly in all weeks, up to 50% in week 5, compared to the pre-treatment scan. The largest median COM displacements were seen in level V, for example 5.2 mm in week 5 compared to the pre-treatment scan. The displacement of elective lymph nodes was mainly in cranial direction. Geographical coverage was obtained when the lymph node volumes were expanded with 7 mm in case the pre-treatment scan was used and 6.5 mm in case the scan of the previous week was used.ConclusionElective lymph nodes of HNC patients remained visible on MRI and decreased in size during radiotherapy. The displacement of elective lymph nodes differ per lymph node level and were mainly directed cranially. Weekly adaptation does not seem to improve coverage of elective lymph nodes. Based on our findings we expect elective lymph node irradiation is achievable in the clinic.

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

“…The number of detected elective lymph nodes in this study was similar to a previous study that examined the MRI visibility and displacement of lymph nodes during radiotherapy in HNC patients (level Ib: 5 vs. 5, level II: 18 vs. 21, level III: 8 vs. 11, level IV: 2 vs. 7 and level V: 6 vs. 8) [15] . Pathology studies that investigated neck dissections in HNSCC patients reported a lymph node yield varying from 34 to 46 [18] , [19] , [20] , [21] on each side of the neck, which is higher than the number of lymph nodes seen in the present study.…”

“…However, the differences in diameter of lymph nodes in level II were small and therefore not deemed clinical relevant. It is known that lymph nodes decrease in size during radiotherapy treatment [15] as lymphatic tissue is sensitive to radiation [22] . However, the results of this study indicate that this effect is not already visible in the first week of treatment.…”

“…After completing the conventional pre-treatment scanning protocol, patients were also scanned on the MR-linac in the same mask in the first week of radiotherapy treatment. The MR-linac scan was acquired within the first week of radiotherapy treatment since we were not interested in volume changes of lymph nodes due to the effect of radiotherapy [15] . The MR-linac incorporates a magnetic field strength of 1.5T with a 4–channel posterior coil in the tabletop and a 4-channel anterior coil ( Fig.…”

“…In a previous study we proved that large dose reductions (>5 Gy) could be achieved with elective lymph node irradiation for several Organs At Risk (OARs) such as the submandibular glands, carotid arteries and thyroid [14] . In another study, the MRI visibility and displacement of elective lymph nodes during radiotherapy in HNC patients was described [15] , in order to estimate safe inter-fraction radiotherapy margins for this new treatment concept. The first patients receiving elective lymph node irradiation will be treated on the MR-linac according to the protocol that was developed in the UMC-Utrecht [16] .…”