| Ch 4 | Page 6 / 13 | |
| Cancer diagnosis |
Scintigraphies | |
Scintigraphy is a functional exploration: image quality is of less interest than the actual function explored.
However, cancer is not the only pathology which can induce scintigraphic abnormalities. Interpretation should therefore always be made with caution and should take into account the patient's clinical history.
Bone scintigraphy detects metastases before any radiological symptoms appear and could be referred to as a 'screening' procedure for bone metastases.
The infused tracer is a diphosphonate labelled with 99Technetium. Like phosphorus, it fixes where bone modelling is observed: fracture callus, active Paget's disease, tumour metastasis.
The total radioactivity is extremely low (740 MBq) with no known danger to patients.
A local punctiform hyperfixation is rather significant of a bone metastasis, however it can also be observed in benign pathologies (such as arthritis, bone infection, post traumatic sequela).
In principle, only cancers bearing an osteoid reaction should fix the tracer, however almost all lytic bone destruction is accompanied by a minor reconstruction process hence provoking hyperfixation.
Main bone metastases detected by scintigraphy are breast cancer metastases, prostate cancer metastases, and thyroid cancer. Scintigraphy enables avoiding radiographies of all bone parts and offers the possibility to limit radiography to enhanced bones. Primary bone lesions are generally highly enhanced (for instance Ewing's sarcoma ).
Hyperscan is a particular aspect used for certain prostatic bone or mammary cancer metastases. The fixation is so high than generalised metabolic radiotherapy may be proposed treating all the bone metastases in one shot.
However, to locally treat a metastasis (by radiotherapy for instance) or refute the exeresis of a primary tumour (in relation to supposed distant metastases), further confirmation is required (such as bone radiography or RMI). Other pathologies provoke hyperfixation, particularly among aged people, and include arthrosis or metabolic bone disease such as Paget's disease).
Thyroid scintigraphies are very interesting for the delineation of 'cold' nodules and to eliminate other thyroid pathologies which induce increased volume of the gland.
Radioactive Iodine is the reference physiological tracer: it is a reliable tracer of hormonogenesis and permits a cartography of Iodine fixation. However the use of 131I is complicated by radioprotection issues related to its long half life (8 days) and its major ß emitting characteristics. 99Technetium or 99mTc is generally preferred and provides quality nodule distinction (cold or hot) and the surveillance of a non-toxic goiter.
With thyroid scintigraphy, we can distinguish the following pathologies:
123I enables the measurement of pretherapeutic fixation and follow-up of differentiated thyroid carcinoma treated with radioactive Iodine (cf. metabolic radiotherapy)
The isotopic tracer is used to study the function of an organ.
For instance, we can study the ventricular contraction fraction by studying the evolution of radioactivity of the precardiac area after intection of a radioactive tracer.
Pulmonary ventilation is studied by the absorption of xenon, enabling the seperation of each part of the lung: before deciding on pneumonectomy or lobectomy to treat lung cancer, knowledge of the capacity of the remaining lung is essential
Pulmonary perfusion study is an essential step for diagnosing pulmonary embolism, a frequent complication in cancer after lower limb or pelvis thrombophlebitis.
Renal scintigraphy is used toimprove definition of renal clearance.
These techniques are often used for the study of chemotherapy toxicity; however they often need to be compared with other classical functional methods.
This technique uses an antibody specific to a particular tissue or antigen which has been labelled with a radioactive tracer.
The antibody should be specific, and not absorbed by circulating antigens and, if possible, should not disturb further immunological assays of the antigen.
This technique has been used to detect micrometastases which can be treated by localised surgery: choriocarcinoma (β-HCG), digestive metastases (ACE), ovarian carcinoma (Ca-125), and so on.
With the use of a scintigraphy probe during operation, the surgeon may detect non-visible tumours and perform a complete exeresis (for instance, for local thyroid relapse, microscopic digestive relapse, satellite node study in breast carcinoma.
This latter technique avoids postoperative complications from unnecessary lymphadenectomies.
67Gallium is an anolog of iron which is linked to various plasma proteins (transferrin, lactoferrin, ferritin) for which lymphoma cells express surface receptors: study of 67Gallium enables the physiological evaluation of tumour cell viability.
After chemotherapy, when complete remission seem to be obtained, the X-Ray scanner can still show tumour masses which, however, do not fix 67Gallium. The absence of fixation defines complete remission. Sometimes (thorax or abdominal views), the interpretation may be more delicate due to the digestive elimination of Gallium.
High grade non-Hodgkin lymphoma and most Hodgkin's lymphoma have a very great affinity for 67Gallium, wheras low grade lymphoma has a lesser affinity. Another products used for these latter lymphoma are 201Thallium or MIBI.
Some isotopes with a very short half-life emit postrons, allowing precise metabolism studies of pathological tissues. Since they have strong but short radioactivity and since their location can be precisely determined by coincidence study, they can very precisely detect metabolically active tumours (increased glucose consumption).
This is the principle of PETScan (see linked chapter) which is more and more useful due to the increased availability of positron devices and the possibility of combining them with an X-Ray scanner (PET-CT)
The use of positron emitting isotopes is very promising for the detection of mediastinal or abdominal tumours and for the study of their limits.
However, since they require very expensive devices and costly isotopes, a precise evaluation of the practical value of PETScan (useful modifications of therapeutic schedules) should be carried out within the framework of international studies.
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