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| Today's post features Dixie, who doesn't have any medical conditions but enjoys sitting up on the couch and slowly destroying paper bags. |
It is always fascinating
hearing how specialists of this calibre organise their thoughts. Staging, of
course, is the determination of the anatomical spread of cancer and imaging is
one part of it (as is physical examination, but also occasionally surgery). It
is different from grading, which is performed by pathologists.
So once you have detected
a tumour, why perform staging? In the most basic sense it is to determine
tumour size, the extent of invasion and margins, involvement of regional lymph
nodes and identification of metastases.
Much of this is achieved
through imaging, which is also important for surgical planning. Fortunately
some tissues (fascial planes, tendons, cartilage and bone) are more resistant
to invasion by neoplastic cells than others but even benign tumours recur with
incomplete resection so we want to achieve clean margins the first time.
Knowing where to cut is vital!
Dr Hoffman discussed the
pros and cons of different imaging modalities commonly used in the staging of
veterinary oncology patients.
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| Just like a work of art...Dixie strikes yet another thoughtful pose. |
Advantages: provides
global information, is relatively inexpensive, especially useful for imaging
the appendicular skeleton.
Disadvantages: superimposition
of structures, may not be read in detail (Dr Hoffman said that our eyes tend to
really see an area of the radiograph about the size of a 20 cent piece…so if we
don’t cast our eyes over the whole film we’re missing a lot), increasingly
replaced with CT and MRI (especially for the head and axial skeleton).
Interestingly, for a
metastases hunt, Dr Hoffman suggested that sometimes four views of the thorax
are helpful: both laterals plus ventrodorsal and dorsoventral projections.
While most publications recommend three views, Dr Hoffman has worked in some
centres where four thoracic views is the standard.
There are two potential
exceptions to the three (or four) radiograph requirement: lymphoma (usually
thoracic lymph nodes, pleural effusion and pulmonary infiltration can be
appreciated on two lateral views) and mast cell tumours which rarely
metastasize to the lungs.
Of course, evaluating the
patient for concurrent disease is important for staging and prognostication,
but according to Dr Hoffman abdominal ultrasound would be a better investment
of resources for dogs with MCTs than thoracic ultrasound.
Sonography
Advantages: excellent for
evaluation of soft tissue and fluids, provides guidance for fine needle
aspiration and biopsy, useful for monitoring response to treatment (e.g. tumour
size), often won’t need a general anaesthetic. Ultrasound is excellent when the
operator is methodical, thorough and experienced.
Weaknesses: operator
dependent (you will miss the lesion if you don’t point the transducer at it),
provides local (not global) information, 3-dimensional interpretation is in the
operator’s mind.
Sonography is less helpful
in the case of osteosarcoma (OSA) as these generally metastasize to the lungs,
however extraskeletal OSAs occur and can be detected on abdominal ultrasound.
Dr Hoffman spent a lot of
time discussing what can and cannot be inferred from imaging. As veterinarians
we are frequently asked to take a best guess at what a lump is likely to be –
but owners will often use this information to make life-or-death decisions.
Nodules on the liver or
spleen are common but Dr Hoffman said that many animals have died because these
are found on ultrasound. Such nodules may represent metastases, but they may
also be benign lesions.
Without a biopsy it is
difficult to make an assessment. One might suspect that these are neoplastic
but further evidence should be sought.
She cited Aristotle: “It
is the mark of an educated mind to be able to entertain a thought without
accepting it”.
In other words, consider
neoplasia as a differential, even the most likely differential if it fits the
clinical picture, but don’t seal the deal without further evidence.
Splenic masses are a case
in point. We tend to fear that every splenic mass represents a haemangiosarcoma,
but this accounts for only 1/3 of splenic masses. Another 1/3 are
non-neoplastic, and the remaining 1/3 are tumours of some other kind, 50% of
which are malignant. Which means that 50 per cent of splenic masses are
curable!
Computed tomography
Advantages: avoids the
problem of superimposition by taking imaging “slices” of the subject, and
utilises various algorithms to optimise contrast, provides 3-D image for
surgical planning and may help avoid abortive surgical attempts, very sensitive
in detecting lung lesions (CT can detect lesions of 1mm in diameter, radiology
can only detect lesions from 4-7mm in diameter - though the prognostic
significance remains unknown since most literature on prognosis involves data
based on radiographic assessment for metastases). CT is superior when compared
to MRI in detecting the size of length of OSA
Disadvantages: costly,
available in fewer centres, requires general anaesthesia.
MRI
Advantages: provides cross
sectional imaging, excellent to illustrate tumour margins and metastases,
sensitive for detecting inflammation.
Disadvantages: slower than
CT, costly, available in fewer centres, requires general anaesthesia.
Scintigraphy
Advantages: excellent
screening for lesion detection (especially skeletal metastases and thyroid
cancer), permits functional studies.
Disadvantages: low
resolution, available in fewer centres.
Her take home advice was
this: “People want to know what a tumour is based on the gray scale on the
screen. The pixels aren’t cells and while we need a gross structural idea of
where the tumour has gone we also need to know the cell line and the characteristics
of that cell line.”
