diff --git a/dicom-viewer/index.html b/dicom-viewer/index.html
index 6f56333..d252579 100644
--- a/dicom-viewer/index.html
+++ b/dicom-viewer/index.html
@@ -30,7 +30,7 @@
   ></div>
 
 
-<!-- Example A: array of named stacks (simple) -->
+<!-- 
 <div
   class="dicom-viewer-root"
   id="viewer_named_simple"
@@ -54,7 +54,6 @@
   style="width:600px;height:400px;"
 ></div>
 
-<!-- Example B: nested cases with multiple stacks per case -->
 <div
   class="dicom-viewer-root"
   id="viewer_named_cases"
@@ -93,7 +92,7 @@
     }
   ]'
   style="width:600px;height:400px;margin-top:12px;"
-></div>
+></div> -->
 <!--   <div id="root2" class="dicom-viewer-root" style="max-height:500px; height:500px; overflow:auto;"
   data-images="[&quot;http://localhost:8000/media/atlas/dicom/4596ea7654b7a5074d2c3f40039c556b11fb01ae.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/b2fa301bc731d15be3e000402c9a79c114d17aa5.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/66310ef801fbecb37c5ced160c450dfd5ecc6d2d.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/63d2d117bf256a76a61190066f91eda6c4d67c8f.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/181a3575ea7fb8bf8f6449ff61e6e369a3247454.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/7c5c0538339266ac8897fa21c481b2f61172068e.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/1681410681308af7111af6644fed598fde304b96.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/4adcd3d7a64a9c59b530f730ce0d991bf7f27813.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/671e6004c7fb402a627a2282bbf304d7b4dcc12b.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/8d18ac56914b9de72b8aecced09654ec4b2c9bb7.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/74ce708a90dff0636fc4bc914fe47e8b6d9320bd.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/a96b6c9900570e076edf849a0ae3b52734ed2650.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/0183718d9336458478ec42130395d78e01492bd6.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/52eee968bbce5590274346eed43f1837f9daba11.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/961449f88197566c4643fcfd20f48ea4e810490b.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/1a16ad5b25e11739eadbcb8ea461072b100113a0.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/b89ddcc3f499059d2dedb0028a2bb1ce4abf5574.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/c1379c03a4e449c2fa88784d601cb766af907739.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/4fb532d5003c59a73f7e4248f365b1a8660ea710.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/e861b0de042a1f213cbccc995679aacd29c7ef53.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/1a854282afe264c3afff6a6e287cd646b2193a0c.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/f59d6d9bc33160abad3af8e2b246179b3574d884.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/04fadab0ed8b3f1925e601c4f9cfb792c95626c2.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/d45d0b12fb650e65daeeee4e5fa3b21c988597f6.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/e19c47b7787ac45dbc225319dfa143dc2c1fdf1f.dcm&quot;, &quot;http://localhost:8000/media/atlas/dicom/dccabf3103a5be254a45457ce2d8d9ce9cb19a6a.dcm&quot;]"
   data-auto-cache-stack="true"
diff --git a/dicom-viewer/src/App.tsx b/dicom-viewer/src/App.tsx
index cdfaa4c..ba5e561 100644
--- a/dicom-viewer/src/App.tsx
+++ b/dicom-viewer/src/App.tsx
@@ -220,6 +220,38 @@ function App({ container_id, imageStacks, autoCacheStack, annotationJson, viewer
     () => Array(MAX_VIEWPORTS).fill(0)
   );
 
+  // Client-side generated thumbnails cache: stackIndex -> dataURL
+  const [stackThumbnails, setStackThumbnails] = useState<Record<number, string>>({});
+  const thumbRequestedRef = useRef<Record<number, boolean>>({});
+
+  // Helper to find a canvas element under a given node (recursive)
+  function findCanvasIn(node: ParentNode | null): HTMLCanvasElement | null {
+    if (!node) return null;
+    try {
+      if ((node as Element).tagName && (node as Element).tagName.toLowerCase() === 'canvas') {
+        return node as unknown as HTMLCanvasElement;
+      }
+    } catch (e) {
+      // ignore
+    }
+    try {
+      const q = (node as Element).querySelector && (node as Element).querySelector('canvas');
+      if (q) return q as HTMLCanvasElement;
+    } catch (e) {
+      // ignore
+    }
+    try {
+      const children = (node as Element).children || [];
+      for (let i = 0; i < children.length; i++) {
+        const found = findCanvasIn(children[i]);
+        if (found) return found;
+      }
+    } catch (e) {
+      // ignore
+    }
+    return null;
+  }
+
   const [stackOrderModified, setStackOrderModified] = useState(false);
   const [loadingState, setLoadingState] = useState<null | "annotations" | "viewerState">(null);
   // Load annotations and viewer state from props if provided
@@ -398,6 +430,147 @@ function App({ container_id, imageStacks, autoCacheStack, annotationJson, viewer
     });
   };
 
+  const generateThumbnail = useCallback(async (stackIdx: number, chosenImageId: string | null) => {
+    if (!chosenImageId) {
+      console.warn('generateThumbnail: no chosenImageId for stack', stackIdx);
+      return;
+    }
+    // avoid regenerating
+    if (stackThumbnails[stackIdx]) {
+      // already have it
+      return;
+    }
+    if (thumbRequestedRef.current[stackIdx]) {
+      console.debug('generateThumbnail: already requested for', stackIdx);
+      return;
+    }
+
+    const renderingEngine = renderingEngineRef.current;
+    if (!renderingEngine) {
+      console.warn('generateThumbnail: no renderingEngine yet for', stackIdx);
+      return;
+    }
+    // mark requested once we are sure the rendering engine exists
+    thumbRequestedRef.current[stackIdx] = true;
+    console.debug('generateThumbnail: starting for', stackIdx, chosenImageId);
+
+    const viewportId = `THUMB_${container_id || 'main'}_${stackIdx}`;
+    const el = document.createElement('div');
+    el.style.width = '160px';
+    el.style.height = '160px';
+    el.style.position = 'absolute';
+    el.style.left = '-9999px';
+    el.style.top = '0';
+    document.body.appendChild(el);
+
+    try {
+      renderingEngine.enableElement({ viewportId, type: Enums.ViewportType.STACK, element: el });
+      const vp = renderingEngine.getViewport(viewportId);
+      if (!vp) {
+        console.warn('generateThumbnail: no viewport returned after enableElement for', viewportId);
+        return;
+      }
+      // Use single-image stack for thumbnail
+      try {
+        vp.setStack([chosenImageId]);
+      } catch (err) {
+        console.warn('generateThumbnail: vp.setStack failed', err);
+      }
+
+      // Wait for IMAGE_RENDERED event on the temporary viewport element or timeout
+      const canvasReady = await new Promise<boolean>(resolve => {
+        let resolved = false;
+        const timeout = setTimeout(() => {
+          if (!resolved) {
+            resolved = true;
+            console.warn('generateThumbnail: IMAGE_RENDERED timeout for', viewportId);
+            resolve(false);
+          }
+        }, 2000);
+
+        const onRendered = () => {
+          if (resolved) return;
+          resolved = true;
+          clearTimeout(timeout);
+          console.debug('generateThumbnail: IMAGE_RENDERED received for', viewportId);
+          resolve(true);
+        };
+
+        try {
+          if (vp && vp.element) {
+            try {
+              vp.element.addEventListener(IMAGE_RENDERED, onRendered, { once: true } as any);
+            } catch (err) {
+              console.warn('generateThumbnail: failed to add IMAGE_RENDERED listener', err);
+            }
+          }
+          // Trigger a render in case it doesn't auto-render.
+          // vp.render may return undefined (not a Promise), so guard before calling .catch()
+          if (typeof vp.render === 'function') {
+            try {
+              const res = vp.render();
+              if (res && typeof (res as any).then === 'function') {
+                (res as any).catch(() => undefined);
+              }
+            } catch (err) {
+              // ignore render errors here
+            }
+          }
+        } catch (err) {
+          clearTimeout(timeout);
+          console.warn('generateThumbnail: error while waiting for render', err);
+          resolve(false);
+        }
+      });
+
+      // Find canvas inside the temporary element or viewport element. Try multiple locations.
+      let canvas = findCanvasIn(el) || (vp && vp.element ? findCanvasIn(vp.element) : null);
+      if (!canvas) {
+        console.debug('generateThumbnail: no canvas found immediately, waiting briefly and retrying for', viewportId, 'canvasReady=', canvasReady);
+        await new Promise(r => setTimeout(r, 150));
+        canvas = findCanvasIn(el) || (vp && vp.element ? findCanvasIn(vp.element) : null);
+      }
+
+      if (canvas) {
+        try {
+          const dataUrl = canvas.toDataURL('image/png');
+          setStackThumbnails(prev => ({ ...prev, [stackIdx]: dataUrl }));
+          console.debug('generateThumbnail: thumbnail captured for stack', stackIdx);
+        } catch (err) {
+          console.warn('generateThumbnail: toDataURL failed', err);
+          // Mark as broken so UI shows placeholder instead of attempting to load raw DICOM URL
+          setStackThumbnails(prev => ({ ...prev, [stackIdx]: 'BROKEN' }));
+        }
+      } else {
+        console.warn('generateThumbnail: no canvas found after render for', viewportId, 'canvasReady=', canvasReady, 'vp.element=', !!(vp && vp.element));
+        // Mark as broken so we don't attempt to use raw .dcm URL as <img> src
+        setStackThumbnails(prev => ({ ...prev, [stackIdx]: 'BROKEN' }));
+      }
+    } catch (err) {
+      // log and mark broken so UI shows placeholder
+      console.warn('Thumbnail generation failed for', chosenImageId, err);
+      try { setStackThumbnails(prev => ({ ...prev, [stackIdx]: 'BROKEN' })); } catch (e) { /* ignore */ }
+    } finally {
+      try { renderingEngine.disableElement(viewportId); } catch (e) { /* ignore */ }
+      if (el && el.parentNode) el.parentNode.removeChild(el);
+    }
+  }, [container_id, stackThumbnails]);
+
+  // Kick off thumbnail generation when stacks change
+  useEffect(() => {
+    if (!availableStacks || availableStacks.length === 0) return;
+    const renderingEngine = renderingEngineRef.current;
+    if (!renderingEngine) return; // wait until rendering engine exists
+    availableStacks.forEach((stack, idx) => {
+      const imageIds = Array.isArray(stack.imageIds) ? stack.imageIds : [];
+      const midIndex = Math.floor(imageIds.length / 2);
+      const chosen = imageIds[midIndex] || imageIds[0] || null;
+      if (!stackThumbnails[idx] && !thumbRequestedRef.current[idx]) {
+        generateThumbnail(idx, chosen as string | null);
+      }
+    });
+  }, [availableStacks, generateThumbnail, stackThumbnails]);
+
   // Wrap setLoadedImageIds to also cache DICOM metadata
   const setLoadedImageIdsAndCacheMeta = async (imageIds: string[]) => {
     console.log("Setting loaded imageIds:", imageIds);
@@ -937,6 +1110,23 @@ function App({ container_id, imageStacks, autoCacheStack, annotationJson, viewer
       if (!renderingEngine) {
         renderingEngine = new RenderingEngine(renderingEngineId);
         renderingEngineRef.current = renderingEngine;
+        // If stacks were already available before the engine was ready, kick off thumbnails now
+        try {
+          if (availableStacks && availableStacks.length > 0) {
+            availableStacks.forEach((stack, idx) => {
+              const imageIds = Array.isArray(stack.imageIds) ? stack.imageIds : [];
+              const midIndex = Math.floor(imageIds.length / 2);
+              const chosen = imageIds[midIndex] || imageIds[0] || null;
+              if (chosen && !stackThumbnails[idx] && !thumbRequestedRef.current[idx]) {
+                // fire-and-forget
+                // eslint-disable-next-line @typescript-eslint/no-floating-promises
+                generateThumbnail(idx, chosen as string | null);
+              }
+            });
+          }
+        } catch (e) {
+          // ignore
+        }
       }
 
       // --- In your setup function, use the correct tool group per viewport ---
@@ -3518,7 +3708,8 @@ function App({ container_id, imageStacks, autoCacheStack, annotationJson, viewer
                       // Prefer the middle image for a representative thumbnail, fallback to first
                       const midIndex = Math.floor(imageIds.length / 2);
                       const chosenImage = imageIds[midIndex] || imageIds[0];
-                      const thumbnailUrl = chosenImage ? (chosenImage.startsWith('wadouri:') ? chosenImage.slice(7) : chosenImage) : null;
+                      // Remove the wadouri: scheme if present. Use replace to avoid off-by-one slice errors.
+                      const thumbnailUrl = chosenImage ? chosenImage.replace(/^wadouri:/, '') : null;
 
                       return (
                         <div
@@ -3542,11 +3733,27 @@ function App({ container_id, imageStacks, autoCacheStack, annotationJson, viewer
                           }}
                         >
                           <div style={{ display: 'flex', alignItems: 'center', gap: 8 }}>
-                            {thumbnailUrl ? (
-                              <img src={thumbnailUrl} alt={label} style={{ width: 56, height: 56, objectFit: 'cover', borderRadius: 4, background: '#111' }} />
-                            ) : (
-                              <div style={{ width: 56, height: 56, borderRadius: 4, background: '#111', display: 'flex', alignItems: 'center', justifyContent: 'center', color: '#777' }}>No Img</div>
-                            )}
+                            {(() => {
+                              const generated = stackThumbnails[idx];
+                              // If generation explicitly failed, treat as missing so placeholder is shown
+                              const generatedValid = generated && generated !== 'BROKEN' ? generated : null;
+                              const src = generatedValid || thumbnailUrl || null;
+                              if (!src) {
+                                return (<div style={{ width: 56, height: 56, borderRadius: 4, background: '#111', display: 'flex', alignItems: 'center', justifyContent: 'center', color: '#777' }}>No Img</div>);
+                              }
+                              return (
+                                <img
+                                  src={src}
+                                  alt={label}
+                                  style={{ width: 56, height: 56, objectFit: 'cover', borderRadius: 4, background: '#111' }}
+                                  onError={(e) => {
+                                    console.warn('Thumbnail <img> failed to load for stack', idx, src);
+                                    // Mark thumbnail as broken so we render the placeholder instead
+                                    setStackThumbnails(prev => ({ ...prev, [idx]: 'BROKEN' }));
+                                  }}
+                                />
+                              );
+                            })()}
                             <div style={{ display: 'flex', flexDirection: 'column' }}>
                               <div style={{ fontWeight: 600 }}>{label}</div>
                               <div style={{ fontSize: 12, opacity: 0.7 }}>{count} img{count === 1 ? '' : 's'}</div>