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I have a query for list members.
Our institution has recently had a patient with an
Ommaya reservoir through which he received an injection
of methotrexate (MTX).
The oncologist treating this patient ordered a dose of
12mg. We use a preservative-free stock solution of MTX
with a concentration of 10mg/mL. The oncologist wanted
the syringe diluted with an equivalent volume of NS, ie,
1.2mL of MTX solution + 1.2mL of NS. But she also phoned
the pharmacy (a phone call which I caught) about the
constitution of the solution. She wanted to know if her
order had been appropriate.
Surprisingly, our department has *no* information about
making up dosages in syringes for instillation through an
Ommaya. Empirically, I could only tell her that the
volume should be no greater than that given intrathecally.
Here is what I want to know:
(1) What concentration of MTX can be administered via an
rationale: avoid the risk of chemically-induced
(2) What volume, etc?
rationale: dose is being instilled ultimately through a
catheter lying in a ventricle; I am concerned about a
mass effect if the volume (and rate of administration)
is too great
I have tried to find some definitive answers through
MEDLINE and have not been successful. What I've found
have been references to a variety of treatment protocols
which have investigated the clinical effectiveness of
giving MTX via a reservoir in order to maintain a certain
CSF concentration of drug over a certain period of time.
This is not exactly what I'm interested in (although it
*is* something that I intend to pass along to the oncologist).
I have gotten very valuable information from the British
Columbia Cancer Agency and St. Jude's in Memphis about how
they make up dosages for instillation through an Ommaya,
the former in adults and the latter in peds patients. I
would like, however, to conduct a wider canvas of practice.
While I am interested in empirical practice, I am also
interested in what evidence there is to support the
manufacture of MTX dosages for instillation through an
Ommaya, particularly (1) drug concentration, (2) total
volume, and (3) rate of instillation.
I have gotten an excellent description of a nursing
procedure from the (US) National Institutes of Health
describing the steps for the procedure itself. But this
only suggests (by implication) that the total volume
is 3-4mL. In contrast, the BCCA makes up syringes (for
adults) to a total volume of 6mL (which would work out
to a dose of 12mg at a concentration of 2mg/mL). This
is the same concentration used at St. Jude's.
I would appreciate any information that list members
have about their practice in manufacturing MTX (or
other drugs) for instillation through an Ommaya. I
have to say that I find it curious that the information
I'm looking for seems so hard to come by. These devices
have been around for about 30 years now and one would
think that there would be enough experience with them
to have prompted the development of some standards with
respect to preparation of dosages.
The experience of having looked into this has given me
the idea that an article about this would be useful. If
list members would like, I will put together the info I
get and repost to the list.
Randy Trinkle, BScPharm, BA
Cross Cancer Institute
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