Segmentation Protocol:

Overview:  

After the raw DICOM files are loaded into Slicer, all files are thickened to 3 mm axial slices by using the Legacy Resample Scalar Volumes function. All segmentation occurs on 3mm axial slices using the segmentation editor. Our protocol requires the creation of 2 masks, a “raw” region of interest (ROI) and a vessel and borders mask (VB) in order to produce the final CT volumes (CTV). The CTV is produced by using the “logical operators” tool within Slicer to subtract the VB from the ROI. The ROI generally corresponds with the boundaries of lymph node levels II, III, and IV, with minor modifications, most significantly at the superior boundary of IIA and the inferior boundary of IV, as these regions are less accessible in cervical lymph node dissection (LND) due to the digastric muscle and the origin of the sternocleidomastoid (SCM) respectively. For patients undergoing bilateral LND, each side should be segmented and measured independently. The boundaries of the target volume are described as clean straight lines or gradually sweeping curves, but there are two important exceptions. Whenever an identifiable lymph node is on the borderline of the target volume, the node is included. Likewise, whenever there is tumor tissue on the borderline of a target volume, that tumor tissue should also be included.

Marking the Outer Bounds: Segmentation began with the creation of a new mask for capturing vessels and outer boundaries (VB-mask) which will ultimately be subtracted, and not included in the final CTV. Before beginning segmentation, check at the bottom of the segmentation editor to ensure the settings are set to “allow overlap”. This setting allows separate masks to overlap with one another, and is an essential step. Next, a paintbrush tool is used to draw a large x on the most inferior slice that includes the inferior border of the transverse process of C1 ipsilateral to the surgical side. This represents an out-of-bounds marker, below which the lymphatic tissue of interest resides. Next, the inferior boundary is marked by scrolling inferiorly and identifying whichever occurred first: the disappearance of the thyroid isthmus, or the clavicle crossing anterior to the internal jugular vein (IJV). Whichever occurrs more superiorly becomes the inferior bound. An X is placed on the axial slice below the thyroid isthmus or the most superior slice containing anterior-posterior (AP) overlap between the IJV and clavicle.  All future segmentation occurred between these two marked axial slices.

Vessel Segmentation of IJV and Major Tributaries: We use the VB-mask to manually segment the IJV as accurately as possible in approximately 10 minutes or less. Using the level tracing tool we are able to accurately segment the IJV in most slices with one click of the mouse. In cases where the level tracing tool fails to accurately capture the outer margin of the IJV, we fine-tuned the vessel volume using the paintbrush and/or eraser tools. Large tributaries of the ­IJV such as the facial vein and superior thyroid vein are segmented as well if they are large and readily identifiable. We avoid segmenting minor vessels (<~4mm diameter), as they are often resected intraoperatively or contain insignificant volume and are not worth the time spent on segmentation.

Segmentation of the common carotid artery is optional because the carotid does not lie within the ROI or CTV and thus volume does not need to be require accurate calculation for subtraction. However,  the carotid is a key landmark for defining the ROI/CTV in each axial slice, therefore visual demarcation of the carotid can facilitate the segmentation process. We use a small paintbrush tool, in the VB mask, to mark the carotid every few slices, to clarify the path of the carotid, and mark its bifurcation. Smaller branches off of the carotid are not marked as they are not significant landmarks for segmentation.

Identifying and Marking the Submandibular Gland (SMG): When the SMG is present within an axial slice (within IIA), the anterior margin of the region of interest becomes the posterior border of the SMG. We grossly mark the posterior margin of the SMG in a separate mask (pink below), which serves as a visual reminder to advance the anterior margin of the ROI during segmentation.

Segmentation of “Raw” Region of Interest (ROI): The “raw” ROI) represents the area of lymphatic tissue to be targeted for resection intraoperatively. This volume is “raw” in that it includes large vessels (mostly IJV) which must be subtracted to produce final CT-Volume. Because the shape of the ROI varies in each nodal station, we describe each region separately, as though the reader is traveling inferiorly. However, in practice, we do not segment in this order, but rather began with whichever axial slices are the most anatomically simple, as this helps guide decisions made in more difficult slices later on.

Level II: Station IIB is posterior and therefore most superior, and lies deep to the SCM but superficial to the scalene muscles.  Therefore, while segmenting in this level, we use the medial surface of the SCM as the lateral border of the ROI. The medial border of the ROI is the internal carotid artery and scalene muscle. We did not include any tissue deep to or between the scalenes. Furthermore, we do not include any tissue that deep to the IJV above the location where the digastric crosses the IJV. 

When the submandibular gland (SMG) is present: In the upper-majority of Level IIA, starting at the posterior edge of the SCM, we drew a line using the “draw” tool that travels directly medially, until scalene muscles or any interior muscles of the neck are contacted, then deflect the line in the anterior direction and hug the lateral margin of any scalenes while approaching the lateral aspect of the carotid artery. We continued this line around the lateral margin of the carotid, before aiming straight for the postero-medial, aspect of the SMG. We then followed along the posterior margin of the SMG. Then turned sharply to head directly toward the anterior-most point of the SCM. We then trace the interior margin of the SCM back to the most posterior point of the SCM.

Below the SMG: In the inferior portion of level II, level III, and the superior portion of level IV, the medial margin of the SCM is the lateral margin of the target volume. A similar path is drawn as the path described previously, when the SMG is present, starting with the posterior corner of the SCM, heading medially toward and along the outside of the deeper neck muscles and eventually rounding the superficial (anterolateral) margin of the carotid artery. However, at this point instead of heading toward the SMG (which is no longer present) we turned directly anterior, then, after reaching a point in line with the most anterior margin of the SCM, we turned 90* and traced laterally toward the anterior margin of the SCM. We complete the shape by tracing the medial face of the SCM back to its posterior point.

Lower Level IV: In the lower portions of Level IV, the SCM is mostly anterior to the IJV, which has now become the posterior to the most posterior aspect of the SCM (see picture below). This requires re-defining the posterior margin of the space to be a combination of two lines, each a tangent off the IJV/carotid, one in an anterolateral direction, and the other projecting anteromedially. We conceptualize this space as a triangle where the base is anterior, along the deep aspect of the SCM and the apex is the common carotid.

Final Spot Checks: Before producing the official CT-volume via the subtraction process, we scroll through all axial slices to spot-check and ensure all segmentations look accurate, and use the paintbrush or draw tool to refine the ROI or VB segments. We also use the 3D-viewer to ensure there are no extraneous markings in the ROI or VB segments, and that every axial slice had been segmented.

Optional Techniques Used for Best Practice: When areas of anatomical uncertainty are encounter, we found it helpful to mark these areas in a separate mask, which is helpful for reassessing these areas in the 3D-viewer or in consultation with a colleague.

Identification and clarification of the SCM: Complete segmentation of the SCM is not recommended, but the anterior and posterior (AP) points of the SCM are critical landmarks for segmentation that can be obscured due to infiltration of tumor tissue. In these cases, demarcation of the AP points of the SCM in a separate mask is helpful.