Summary: Single slice regions of interest with no multi-slice context or interval/thickness information may need to be reported as area only, not volume. Explicit interval/thickness information can and should be encoded. Thickness should be distinguished from interval.
Given a Region of Interest (ROI), no matter how it is encoded (as contours or segmented pixels or whatever), one can compute its area, using the pixel spacing (size) information. If a single planar ROI (on one slice) is grouped with a bunch of siblings on contiguous slices, then one can produce a sum of the areas. And if one knows the (regular) spacing between the slices (reconstruction interval in CT/MR/PET parlance), one can compute a volume from the sum of the areas multiplied by the slice spacing. Often one does not treat the top and bottom slice specially, i.e., the ROI is regarded as occupying the entire slice interval. Alternatively, one could consider the top and bottom slices (or both slices) as only being partially occupied, and perhaps halve the contribution of the top and bottom slices.
The slice interval is distinct from the slice "thickness" (Slice Thickness (0018,0050)), since data may be acquired and reconstructed such that there is either a gap between slices, or slices overlap, and in such cases, using the thickness rather than the interval would not return a volume representative of the object represented by the ROI(s). The slice interval is rarely encoded explicitly, and even if it is, may be unreliable, so one should compute the interval from the distance along the normal to the common orientation (parallel slices) using the Image Position (Patient) origin offset and the Image Orientation (Patient) row and column vectors. The Spacing Between Slices (0018,0088) is only officially defined for the MR and NM objects, though one does see it in CT images occasionally. In the past, some vendors erroneously encoded the gap between slices rather than the distance between their centers in Spacing Between Slices (0018,0088), so be wary of it.
This all presupposes that one does indeed have sufficient spatial information about the ROI available, encoded in the appropriate attributes, which is the case for 2D contours defined relative to 3D slices (e.g., SR SCOORDS with referenced cross-sectional images), 3D contours (e.g., SR SCOORD3D or RT Structure Sets), and Segmentation objects encoded as image objects with plane orientation, position and spacing.
And it works nicely down to just two slices.
But what if one only has one lonely slice? Then there is no "interval" per se.
For 2D contours defined relative to 3D image slices one could consult the adjacent (unreferenced) image slices and deduce the slice interval and assume that was applicable to the contour too. But for 3D contours and segmentation objects that stand alone in 3D space, and may have no explicit reference to the images from which they were derived, if indeed there were any images and if indeed those images were not re-sampled during segmentation, then there may be no "interval" information available at all.
The RT Structure Set does handle this in the ROI Contour Module, by the provision of an (optional) Contour Slab Thickness (3006,0044) value, though it may interact with an the associated Contour Offset Vector (3006,0045) such that the plane of the coordinates is not the center of the slab. See PS 3.3 Section C.18.104.22.168.
The Segmentation object, by virtue of inclusion of the Pixel Measures Sequence (functional group macro), which defines the Pixel Spacing, also requires the presence of the Slice Thickness attribute, but only if Volumetric Properties (0008,9206) is VOLUME or SAMPLED. And wouldn't you know it, the Segmentation IOD does not require the presence of Volumetric Properties :( That said, it is possible to encode it, so ideally one should; the question arises as to what the "thickness" of a segmentation is, and whether one should slavishly copy the slice thickness from the source images that were segmented, or whether one should use the interval (computed if necessary), since arguably one is segmenting the volume, regardless of how it was sampled. We should probably consider whether or not to include Spacing Between Slices (0018,0088) in the Pixel Measures Sequence as well, and to refine their definitions to make this clear.
The SR SCOORD3D content item attributes do not include interval or thickness. That does not prevent one from encoding a numeric content item to associate with it, though no standard templates currently do. Either way, it would be desirable to standardize the convention. Codes are already defined in PS 3.16 for 112225, DCM, “Slice Thickness”) and (112226, DCM, “Spacing between slices”) (these are used in the Image Library entries for cross-sectional images in the CAD templates).
Anyhow, from a recipient's perspective, given no explicit information and no referenced images there is no other choice than to report only area. If an image is referenced, and its interval or thickness are available, then one may be tempted to use it, but if they are different, which should one use? Probably the interval, to be consistent with the general case of multiple slices.
From a sender's perspective, should one explicitly encode interval or thickness information in the RT Structure Set, SR SCOORD3D, and Segmentation objects, even though it is not required? This is probably a good move, especially for single slice ROIs, and should probably be something considered by the standard for inclusion as a CP.