The 2021 Kidney and Kidney Tumor Segmentation Challenge

The 2021 Kidney and Kidney Tumor Segmentation challenge (abbreviated KiTS21) is a competition in which teams compete to develop the best system for automatic semantic segmentation of renal tumors and surrounding anatomy.

Kidney cancer is one of the most common malignancies in adults around the world, and its incidence is thought to be increasing [1]. Fortunately, most kidney tumors are discovered early while they’re still localized and operable. However, there are important questions concerning management of localized kidney tumors that remain unanswered [2], and metastatic renal cancer remains almost uniformly fatal [3].

Kidney tumors are notorious for their conspicuous appearance in computed tomography (CT) imaging, and this has enabled important work by radiologists and surgeons to study the relationship between tumor size, shape, and appearance and its prospects for treatment [4,5,6]. It’s laborious work, however, and it relies on assessments that are often subjective and imprecise.

Automatic segmentation of renal tumors and surrounding anatomy (Fig. 1) is a promising tool for addressing these limitations: Segmentation-based assessments are objective and necessarily well-defined, and automation eliminates all effort save for the click of a button. Expanding on the 2019 Kidney Tumor Segmentation Challenge [7], KiTS21 aims to accelerate the development of reliable tools to address this need, while also serving as a high-quality benchmark for competing approaches to segmentation methods generally.

An ITK-SNAP 3D rendering of a segmented case

Fig. 1: A rendering showing the regions that participants are asked to segment, including kidneys, ureters, arteries, veins, cysts (not shown), and tumors.

An axial slice of a segmented case

Fig. 2: An example of a segmented axial slice.

Timeline All dates 2021 🎉

Mar 1 - Jul 1 Annotation, Release, and Refinement of Training Data
Aug 9 Deadline for Intention to Submit & Required Paper
Aug 16 - 30 Submissions Accepted
Sep 1 Results Announced
Sep 27 or Oct 1 Satellite Event at MICCAI 2021

How to Participate

  1. Download the data
  2. Build and train a model
  3. Describe your approach with a short paper
  4. Submit your paper for review by August 9, 2021
  5. Submit your model for evaluation by August 30, 2021
  6. Attend the satellite event at MICCAI 2021 (optional)

The Data

The KiTS21 cohort includes patients who underwent partial or radical nephrectomy for suspected renal malignancy between 2010 and 2020 at either an M Health Fairview or Cleveland Clinic medical center. A retrospective review of these cases was conducted to identify all patients who had undergone a contrast-enhanced preoperative CT scan that includes the entirety of all kidneys.

Each case's most recent corticomedullary preoperative scan was (or will be) independently segmented three times for each instance of the following semantic classes.

  1. Kidney: Defined in the same way as the kidney class from KiTS19
  2. Tumor: Defined in the same way as the tumor class from KiTS19
  3. Cyst: Kidney masses radiologically (or pathologically, if available) determined to be cysts.
  4. Ureter: The ureters from the point that they leave the kidney up until the first (most superior) axial slice that depicts the aortic bifurcation.
  5. Arteries: The renal arteries and abdominal aorta between the celiac trunk and the aortic bifurcation.
  6. Veins: The renal veins and inferior vena cava between the celiac trunk and the aortic bifurcation.

We are hard at work on collecting and annotating data for KiTS21. At this point it's difficult to predict the total number of patients that KiTS21 will include. We had originally aimed to segment 800 cases, but unfortunately the COVID19 global pandemic has delayed our progress. We plan to continue collecting and annotating training cases right up until the training set is "frozen" on July 1, 2021. After this point, we will continue to collect and annotate cases for the test set until submissions begin in the middle of August. Training set annotation progress can be tracked using the Browse feature, described in detail in the section below.

Data Annotation Process

In an effort to be as transparent as possible, we've decided to perform our training set annotations in full view of the public. This is the primary reason for creating an independent website for KiTS21 on top of our entry, which will be used only to manage the submission process.

You may have noticed a link on the top-right of this page labeled "Browse". This will take you to a list of the KiTS21 training cases, where each case has indicators for its status in the annotation process (e.g., see Fig. 3). The meaning of each symbol is as follows:

  • circle Awaiting upstream annotation. For example, delinations cannot be started until guidance is accepted.
  • circle Annotation in progress.
  • check Submitted, but waiting for downstream annotation to finish before queueing for review. For example, localization and guidance must be reviewed at the same time, so localizations must wait for guidance to be submitted before going out for review.
  • rules In need of review.
  • error Rejected by review, in need of revisions.
  • check_circle Accepted by review.

When you click on an icon, you will be taken to an instance of the ULabel Annotation Tool where you can see the raw annotations made by our annotation team. Only logged-in members of the annotation team can submit their changes to the server, but you may make edits and save them locally. The annotation team's progress is synced with the KiTS21 GitHub repository about once per week.

It's important to note the distinction between what we call "annotations" and what we call "segmentations". We use "annotations" to refer to the raw vectorized interactions that the user generates during an annotation session. A "segmentation," on the other hand, refers to the rasterized output of a postprocessing script that uses "annotations" to define regions of interest.

We placed members of our annotation team into three categories:

  • Experts: Attending radiologists and urologic cancer surgeons
  • Trainees: Medical students, undergraduates planning to study medicine, and a lowly Computer Science PhD Student. All trainees received several hours of training from the experts
  • Laypeople: People who received no training other than a brief sheet with instructions

Broadly, our annotation process is as follows.

  1. Trainees place 3D bounding-boxes around each region of interest
  2. Trainees annotate axial slices within these bounding boxes with "guidance pin annotations" (shown in green in Fig. 4)
  3. Experts review the bounding boxes and guidance pins for accuracy and completeness, leaving comments if necessary
  4. Trainees revise guidance pins in cases where they were rejected by the experts
  5. 3-4 is repeated as many times as is necessary until all guidance is accepted by the experts
  6. The guidance for each region is sent individually to three laypeople for "contour annotations" (shown in bright green in Fig. 5)
  7. Contours are reviewed by trainees based on whether they adhere to the expert-approved guidance
  8. Contour annotations are postprocessed to generate segmentations (shown in Fig. 1)
  9. Segmentations are reviewed by trainees and subjected to minor revisions as needed

Our postprocessing script uses thresholds and fairly simple heuristic-based geometric algorithms. Its source code is available on the KiTS21 GitHub repository under /annotation/

An example of a card on the browse page

Fig. 3: An example of a case in progress as it's listed on the browse page. The icon on the top right indicates the status of region detection, and its current value signals that it needs review. Similar icons are used to show the status of each region individually.

An example of guidance pins for a tumor

Fig. 4: "Guidance pins" that were placed by medical students to show intention to annotate a tumor for an eventual worker on mechanical turk.

An example of a contour based on guidance pins

Fig. 5: Contour drawn by a crowdworker based on the guidance pins drawn by a medical student. The "excess" sinus fat included in the contour is removed automatically via a radiodensity threshold during postprocessing.

How to Win

Put simply, the model that produces the "best" segmentations of kidneys, tumors, cysts, arteries, veins, and ureters for the patients in the test set will be declared the winner. Unfortunately "best" can be difficult to define [8].

Before metrics are discussed, we need to discuss the regions that they will be applied to. A common choice in multiclass segmentation is to simply compute the metric for each semantic class and take the average. For this challenge, we don't think this is the best approach. To illustrate why, consider a case in the test set where a single kidney holds both a tumor and a cyst. If a submission has a very high-quality kidney segmentation, but struggles to differentiate kidney voxels from those belonging to the masses, we believe this deserves a high score for the kidney region, but low scores for each mass. Similarly, suppose the masses were segmented nicely, but the system confuses the cyst with the tumor. We don't think it is ideal to penalize the submission twice here (one for the tumor region, once for the cyst region) when in fact it has done a very good job segmenting masses. To address this, we use what we call "Hierarchical Evaluation Classes" (HECs). In an HEC, classes that are considered subsets of another class are combined with that class for the purposes of computing a metric for the superset. For KiTS21, the following HECs will be used.

  • Kidney and Masses: Kidney + Tumor + Cyst
  • Kidney Mass: Tumor + Cyst
  • Tumor: Tumor only
  • Ureter and Vessels: Ureter + Arteries + Veins
  • Vessels: Arteries + Veins
  • Arteries: Arteries only

In 2019, we used a simple Sørensen-Dice ranking with "Kidney and Tumor", and "Tumor" HECs. The decision to use Dice alone was made to prioritize simplicity and ease of interpretation. We still believe that these things are important, but we also recognize that Dice scores have their limitations. One limitation was the outsized influence that small tumors had on the rankings. This was because segmentation errors are overwhelmingly on the borders of regions, and small regions have a higher ratio of border voxels to interior voxels, leading to lower values for volumetric overlap scores like Dice.

The test set is being annotated using an identical workflow as that used for the training set, and so it too will have several segmentations per case. We've decided to take advantage of this in order to address some of the limitations of a single-reference Dice approach. Our planned approach is similar to that of Heimann et al. for liver segmentation in the 2007 MICCAI Challenge Workshop [9]. In particular, we will be computing gauged scores for each predicted region, which are adjusted according to the average error observed between human raters in that region.

Formally, let be the error between the prediction and reference, where the subscripts represent:

  • : The case number of the scan in the test set that these regions were pulled from.
  • : The index of the hierarchical validation class that these regions are meant to represent.
  • : The index of the set of raters who created the reference.
  • : The metric used to compute error (1-6 corresponding to the table with six metrics below).

Similarly, let represent the average error between rater and the remaining two raters. Submissions will be ranked according to a total score

The intent of this transformation is to normalize scores so that predictions with roughly equal quality to that of our trainees will achieve a score of . Note that negative scores are possible where error is at least 10 times as high as it is between trainees on average. The six metrics to be used (tentatively) for this computation are as follows:

1 - Dice A common measure of discrepancy in volumetric overlap. If we let be the set of predicted voxels and be the set of reference voxels,
1 - IoU Sometimes referred to as 1 - Jaccard. Another measure of discrepancy in volumetric overlap, with a greater emphasis on cases with high disagreement.
Symmetric Relative Volume Difference A measure of difference in total predicted volume, but slightly modified from traditional Relative Volume Difference.
Absolute Volume Difference Another measure of difference in total predicted volume, but without normalizing for the size of the reference. Using our notation from above,
Average Symmetric Surface Distance The average distance between each prediction boundary voxel to the nearest boundary voxel of the reference, and vice versa. Boundary voxels are defined as any voxel in a region that has a voxel outside of that region in its 18-neighborhood. Let and represent the sets of boundary voxels on the prediction and reference sets respectively.
RMS Symmetric Surface Distance Similar to average symmetric surface distance, but with greater emphasis on boundary areas with large disagreement.

In the occasional cases where either a reference HEC or a predicted HEC is empty but not both, the metrics will be computed as follows:

  1. will be set to 1.
  2. will be set to 1.
  3. will be zero if the prediction is empty. If an erroneous prediction is made, the gauge error will be set to 1 divided by the proportion of instances for which the annotators made the same mistake on that region during step 1 of the annotation process (and the mistake was discovered during review).
  4. will be set to 1 times the volume, and the gauge error will be computed by setting to the average volume of regions in that class.
  5. will be set to 10cm (the max possible).
  6. will be set to 10cm (the max possible).

The above treatment of the volume-difference-based scores was derived such that human annotators would be expected to achieve a total score of roughly 90 on cases with empty references, since that is also the intention with nonempty cases. It's also somewhat intuitive since it consists of a flat penalty for the false positive/negative plus an additional term that increases with the size of the region that was missed or erroneously predicted. In cases where both the predicted and reference HEC are empty, all errors will be set to zero.

The code that will be used to compute these metrics on the test set will be available on the GitHub repository under /evaluation/ (in preparation).

Annotation Team

  • Experts
    • Christopher Weight, MD. Urologist at the Cleveland Clinic.
    • Susan Austin, MD. Radiologist at the Mayo Clinic.
    • Mark Austin, MD. Radiologist at the Mayo Clinic.
  • Trainees
    • Ed Walczak. Surgical Resident at the University of Minnesota.
    • Sean McSweeney. Medical Student at the University of Minnesota.
    • Ranveer Vasdev. Medical Student at the University of Minnesota
    • Chris Hornung. Medical Student at the University of Minnesota.
    • Jamee Schoephoerster. Medical Student at the University of Minnesota.
    • Bailey Abernathy. Medical Student at the University of Minnesota.
    • David Wu. Medical Student at the University of Minnesota.
    • Safa Abdulkadir. Medical Student at the University of Minnesota.
    • Ben Byun. Medical Student at the University of Minnesota.
    • Justice Spriggs. Medical Student at the University of Minnesota.
    • Griffin Struyk. Medical Student at the University of Minnesota.
    • Alexandra Austin. Medical Student at the University of Minnesota.
    • Ben Simpson. Medical Student at the University of Minnesota.
    • Michael Hagstrom. Medical Student at the University of Minnesota.
    • Sierra Virnig. Medical Student at Rocky Vista College of Osteopathic Medicine.
    • John French. Medical Student at the University of Missouri.
    • Nitin Venkatesh. Undergraduate at the University of Minnesota.
    • Sarah Chan. Undergraduate at Brigham Young University.
    • Keenan Moore. Undergraduate at Carleton College.
    • Anna Jacobsen. Undergraduate at the University of Utah.
    • Nicholas Heller. PhD Student at the University of Minnesota.
  • Laypeople


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