Discussion

There are a number of published cases of false positive non-echoplanar imaging findings that include malignancy (external auditory canal squamous cell carcinoma), structures containing fluid (e.g. abscess cavity, cyst or cholesterol granuloma, or encephalocele), iatrogenic causes (e.g. cartilage or bone grafts; bone dust, pâté or powder; silastic sheet and dental brace artefact) and incidental findings (e.g. non-specific inflammation, cerumen and tympanosclerosis).^{Reference Lingam, Nash, Majithia, Kalan and Singh3,Reference Muhonen, Mahboubi, Moshtaghi, Sahyouni, Ghavami and Maducdoc5}

Lingam et al. described methods to negate pitfalls in specificity, such as multi-disciplinary discussion of questionable high diffusion-weighted imaging signals between otologists and radiologists in order to corroborate information such as the operative approach, location and type of materials used, confidence of successful clearance, extent of disease, current clinical status of the ear, and examination findings, which are useful factors for determining the risk of recidivism. They also suggested that additional imaging, such as further MRI sequences, apparent diffusion co-efficient values and fusion with high-resolution CT images, is a good adjunct to diffusion-weighted imaging and apparent diffusion co-efficient mapping to interrogate potentially spurious results.^{Reference Lingam, Nash, Majithia, Kalan and Singh3}

Diffusion-weighted imaging relies upon the measurement of Brownian motion of water molecules because diffusion restriction occurs as a result of limited motion of water molecules within tissues. Cellular tissue contains fluid, giving it a lower diffusion co-efficient in comparison with non-cellular structures and therefore producing high signal intensity in material such as keratin.^{Reference Muhonen, Mahboubi, Moshtaghi, Sahyouni, Ghavami and Maducdoc5} Apparent diffusion co-efficient mapping quantifies the diffusion of water molecules in tissue and can be used to cross-examine areas of diffusion restriction, distinguishing between true restriction and T2-weighted shine-through.^{Reference Muhonen, Mahboubi, Moshtaghi, Sahyouni, Ghavami and Maducdoc5}

Interestingly, in the case of patient 1, despite multiple modalities of imaging, the cartilage filler was not identifiable as a false positive finding. The area of diffusion restriction from the cartilage filler corresponded to the same point on the apparent diffusion co-efficient map and was consistent with the high-resolution CT temporal bone imaging. Patient 2 had also undergone high-resolution CT of the temporal bone in addition to non-echoplanar imaging MRI, and again, even with additional imaging techniques, the false positive finding was not discernible through imaging alone. We ask whether re-look surgery could be avoided with repeat scanning in patients who have diffusion restriction in the presence of features that favour a false positive finding over recidivous cholesteatoma.

The timing of post-operative surveillance scanning remains a point of debate in terms of both length of follow up and frequency of imaging. Steens et al. found that 14 of 45 (31 per cent) of post-operative patients with a normal clinical assessment and no disease on their first follow-up diffusion-weighted imaging had positive or equivocal findings on repeat scanning 6 months later, and 5 of 45 (11 per cent) had cholesteatoma confirmed during re-look surgery.^{Reference Steens, Venderink, Kunst, Meijer and Mylanus6}

Muhonen et al. discussed the only other four reported cases of false positive non-echoplanar diffusion-weighted imaging results associated with cartilage grafts. The authors were confident in all cases that complete cholesteatoma clearance was achieved during the initial surgical procedures. One patient underwent revision surgery with no evidence of disease encountered. The other three patients were assessed with clinical examination and repeat imaging with stable or resolved scan findings and no cholesteatoma recurrence on follow up at 52–84 months. On the basis of their experiences, they recommended further non-echoplanar diffusion-weighted imaging to be undertaken at 6 to 12 months where scan findings are suspicious for a false positive.^{Reference Muhonen, Mahboubi, Moshtaghi, Sahyouni, Ghavami and Maducdoc5}

Features that may raise concern for false positive diffusion restriction are reduced size and intensity of the lesion on repeat imaging and presence of diffusion restriction away from the initial cholesteatoma location. Conversely, non-resolving lesions, those that increase in size or intensity, and development of symptoms or clinical signs suggestive of recidivism are features that warrant consideration of a further procedure.^{Reference Muhonen, Mahboubi, Moshtaghi, Sahyouni, Ghavami and Maducdoc5}

We acknowledge that it is difficult to provide definitive advice because there is limited evidence in the medical literature to support the validity of our recommendations; however, we suggest careful documentation of the cholesteatoma location and confidence of clearance in the operation notes. The location of any bone, cartilage or silastic used for grafting should also be noted clearly. In cases where clinical suspicion of recidivism is low, findings of diffusion restriction should be discussed in detail between the otologist and the radiologist. If the clinical and radiological features favour a false positive finding, further surveillance scanning with non-echoplanar diffusion-weighted imaging at 6–12 months may be appropriate because both Muhonen et al. and Steens et al. have published cases demonstrating resolution of abnormal diffusion restriction in subsequent re-imaging.^{Reference Muhonen, Mahboubi, Moshtaghi, Sahyouni, Ghavami and Maducdoc5,Reference Steens, Venderink, Kunst, Meijer and Mylanus6} The caveat to this recommendation is to ensure that close clinical follow up is observed, maintaining a high suspicion for cholesteatoma recurrence and a low threshold for consideration of second look surgery. We advise that the presence of adverse features, such as clinical signs of cholesteatoma, presence of diffusion restriction at the location of the original site and evidence of progression on serial imaging are indications for revision surgery.