Recurrent Spinal Cord Cystic Astrocytomas: Treatment with Rhenium-186 Intracavitary Radiation
Endocavitary irradiation or brachytherapy is frequently used for the treatment of intracranial cystic tumors. This treatment can also be used to treat low-grade astrocytoma of the spinal cord in the very specific case of symptomatic recurrent cysts, despite repetition of classical methods such as surgery or CT scan guided taping. We use 186 Rhenium isotope, which is able to penetrate the cyst wall from 1 to 2 mm. The isotope is injected using a CT scan, which checks the position of the top of the needle during the taping of the cyst and after its opacification with iodine contrast. Follow-up consists of spinal cord MR every 3–6 months. A second injection may be performed if the cyst keeps growing.
KeywordsSpinal Cord Cyst Wall Cystic Tumor Pilocytic Astrocytoma Conventional Radiotherapy
Primitive spinal cord tumors are rare, representing only 4–8% of tumors in the central nervous system (Schellinger et al. 2008). Their rate of occurrence is between 0.74 and 2.5 on 100,000 (Elia-Pasquet et al. 2004). Ependymoma and astrocytoma are the most common histological types (Abul-Kasim et al. 2008; Epstein et al. 1992; Schellinger et al. 2008). Surgical treatment is standard practice notably for low-grade tumors whose prognostic correlates to the quality of surgical resection. (Abdel-Wahab et al. 2006; Epstein et al. 1992; Nakamura et al. 2006). The efficiency of conventional radiotherapy is subject to controversies (Abdel-Wahab et al. 2006; Kim et al. 2001; Minehan et al. 2009). Low grade astrocytoma can present a cystic component which is particularly difficult to treat due to frequent recurrences, despite repeated surgery. These cysts can jeopardize the good prognostic usually observed with this type of tumor. Brachytherapy or endocavitary irradiation is widely used for treating intracranial tumors such as craniopharyngioma (Derrey et al. 2008; Hasegawa et al. 2004; Marchal et al. 2005; Szikla et al. 1984) or pilocytic astrocytoma (Hood et al. 1987; Narayana et al. 2005; Herrera et al. 2007; Proust et al. 1998). Its main advantage is to be an efficient way to treat the cystic parts of a tumor. We shall describe the application of this treatment for patients presenting with cystic tumors of the spinal cord.
Endocavitary irradiation aims to treat the cystic component of benign tumors such as astrocytomas, insofar as the cyst, via a mechanical compression effect, is responsible for neurological symptoms, thus compromising the functional prognosis.
When facing a recurrent cyst following one or multiple surgeries, percutaneous cyst tapping should be proposed before considering brachytherapy. In this case CT scan guided tapping is performed under local anaesthetic. Thus, one can check the neurological condition of the patient and avoid side effects that might occur during the procedure.
However, a very short neuroleptanalgesia may be proposed to prevent the patient from feeling the electric charge that sometimes accompanies the passage across the posterior cords of the spinal cord. The cyst’s volume is calculated prior to this procedure using T2 weighted images; the optimal volume of liquid can then be ascertained.
The patient is installed in lateral position in the scanner. The needle is introduced (whilst checking with the CT scan) onto the midline at the level of the cyst where the wall is the thinnest, in order to avoid wounding the spinal cord. Physician should not attempt to empty the cyst completely: this is usually impossible due to the septa that could separate it. Furthermore, removing too much liquid can modify the volume of the tumor (the tip of the needle must remain inside the cyst). The septa in the cyst are likely to prevent the diffusion of rhenium. This can be assessed by adding iodine contrast and carrying out CT scans 3 and 6 h after tapping, with the patient positioned upright to facilitate caudal diffusion. For this reason, we advised to tap the top of the cyst.
Injection of Rhenium
The first step before injecting is to tap the cyst following the procedure described above. This tapping must not completely drain the cyst so as to allow injection of the isotope with maximum visibility and control.
Pilocystic astrocytomas of the spinal cord are rare tumors, and their prognosis depends on their histological type (Innocenzi et al. 1997; Kim et al. 2001). The first line of therapy is surgery, which is associated with high survival levels (Epstein et al. 1992; Guidetti et al. 1981; Nakamura et al. 2006; Przybylski et al. 1997).
Some of these tumors may have a recurring cystic component, which is particularly difficult to treat. The cyst’s growth leads to mechanical compression of the spinal cord, thus affecting the functional prognostic for patients whose vital prognostic is otherwise preserved. The cyst walls are a component of a cystic tumor. That is why the cyst can recur. Only the destruction (because the removal is dangerous) of this thin pathological lining of tumoral cells is able to make the cyst disappear. This wall is usually left intact during the surgery so as to avoid impairing the neurological conditions. Subsequent surgical operations and CT guided taping allow transient diminution of the cystic component; however we consider the cyst recurrences are the rules. Because of the neurological consequences linked to multiple surgical procedures and taps, we favour brachytherapy as a long-term treatment. Brachytherapy is widely used to treat intracranial cystic tumors as craniopharyngiomas (Hasegawa et al. 2004; Marchal et al. 2005; Szikla et al. 1984). Some authors have suggested the use of this technique for treatment of pilocytic astrocytoma. Szikla et al. (1984) treated 21 patients presenting with low-grade intracranial cystic astrocytoma via endocavitary irradiation using 186 Rhenium. Stabilization and reduction of the volume of the cyst volume was observed in 15 patients, and disappearance in 6 patients. Three of the patients whose cyst recurred after injection of isotopes were given a second injection that was tolerated without difficulty by all patients. Narayana et al. (2005) used the 32 Phosphorous to deal with a cyst that was recurred after taping, conventional radiotherapy and intracavitary injection of bleomycin. Similarly, Proust et al. (1998) used brachytherapy alongside conventional radiotherapy to treat a child presenting with a parieto-occipital cystic astrocytoma. Different varieties of isotopes can be used for endocavitary irradiation: 32 Phosphorous (Hasegawa et al.. 2004; Hood et al. 1987; Narayana et al. 2005), 125 Iodine (Herrera et al. 2007), or 186 Rhenium (Marchal et al. 2005; Proust et al. 1998; Szikla et al. 1984). 186 Rhenium diffuses both a beta and a gamma radiation.
The beta radiation is a low energy type, which limits penetration of radioactivity at the cyst wall to 1 or 2 mm. This reduces potential side effects on the adjacent intra-medullary functional structures. Gamma radiation may be detected using a gamma camera, to have a clear view of the diffusion of the isotope and, possibly, a leakage out of the cyst.
Determining the necessary amount of radioactivity to be delivered requires a compromise between therapeutic efficiency and any potential side effects. Indeed, cerebral oedema has been observed following brachytherapy for intracranial tumors. This may constitute an early complication (Szikla et al. 1984) or one that may be delayed by several months (Herrera et al. 2007). The standard dose is between 200 and 400 Gys. In our experience, the doses administered at the level of the cyst wall are higher (Colnat-Coulbois et al. 2010) for medullary cystic tumors because the isotope is left in the cavity so as to avoid further medullary taping. Despite the low penetration of the isotope, there is a theoretical risk of post radiation myelitis. Long-term follow-up of patients enables assessment of the patient’s tolerance of this treatment.
Other endocavitary treatments have been proposed for recurring cystic intracranial tumors. To this end Disabato et al. (1999) used bleomycin in an intracavitary injection to treat a pilocytic astrocytoma. This treatment is, however, associated with a high morbidity rate (Belen et al. 2007; Savas et al. 2000).
Endocavitary irradiation with injection of 186 Rhenium enables a long-term disappearance of the cystic component of spinal tumors. The side effects are limited. Long-term follow-up should confirm the tolerance of the spinal cord for this treatment.
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