©TheCanadian Journal ofUrology™: International Supplement, April 2014
Metabolicimagingwithfluorine-18fludeoxyglucose
(FDG) has been studied in prostate cancer, and has
been demonstrated to targetmetastases, particularly
in castration resistant prostate cancer (CRPC).
12
Moreover, at least one study has demonstrated that
improvementonFDGPET/CTbeingconcordantwith
PSAdecreases.
13
FDG-avidcancersareprobablymore
likely to be castration resistant, and thus FDGmay
be useful both for the identification of a castration
resistant phenotype as well as a pharmacodynamic
biomarker, Figure 2.
PET/CTwith radiolabeledcholinehasbeen found
tobeextremelyuseful in the identificationofprostate
cancer,
14-16
in the treatment-naïve as well as the
castrationresistantpatient,withnoevidencecurrently
ofdifferentialphenotype-specificmetabolism. Choline
is essential to theproductionof phosphotidyl choline
necessary for cell membrane integrity; cancer cell
membranes have elevated phosphatidyl choline
levels, resulting in increased amounts of exogenous
(andperhapsendogenous,detectedbyMRI)amounts
of trapped choline in tumor cell membranes.
14
Initial studies were carried out with carbon-11
labeled choline. An Italian study
15
found that while
radiolabeled choline was useful in identification of
bonemetastases, conventional bone scintigraphyhad
higheroverallaccuracy;positron-labeledcholinePET/
CT therefore is no substitute for bone scintigraphy at
this time. The U.S. Food and Drug Administration
(FDA) recently approved a New Drug Application
filed by theMayo Clinic for the production and use
of 11C-choline for PET imaging.
16
It is expected that
the agentwill have high accuracy in identification of
recurrent disease after primarydefinitive therapy.
The 20-minute half-life of carbon-11 precludes
centralized production and distribution of the
radiopharmaceutical. Fluorine-18 isapositron-emitting
nuclideused forPET, primarilybecauseof its favorable
imaging characteristics and its nearly 2-hour half-life.
Fluorocholinehas thereforebeen studiedbynumerous
groupsandhasbeenshowntohaveutilityinthedetection
of recurrent/metastaticprostatecancer.
17
Fluorocholine
hasbeen shown tohavebetter accuracy thanNaFbone
PET in identificationofbonemetastases inCRPC.
18
Another metabolic agent that has been studied in
prostatecancerhasbeenradiolabeledacetate,afattyacid.
Most studieshavereported theuseofcarbon-11 labeled
acetate,
19,20
andalso shown that [11C]-acetatemayhave
better accuracyboth indetectionaswell as in response
determinationofprostatecancermetastases.
21-23
Arecentreview
24
providesacomprehensiveoverview
of theutilizationof these tracers inprostate cancer, and
highlights their characteristics.
Imaging of cell surface receptors
Most prostate cancers are abundant in androgen
receptors (AR) at the outset. These receptors may
thereforebeimagedusingapositron-labeledandrogen.
25
These promising early results by Katzenellenbogen
et al led to the clinical exploration of [18F]-labeled
dihydroxytestosterone, or FDHT, in the assessment of
AR expression in CRPC.
26,27
These studies have not
been developed systematically to assess the utility of
this novel hormone receptor imaging agent in CRPC,
theyhaveservedtoillustratethecontinuumbetweenAR
expressionandloss,anditsrelationshiptothe“castration
resistant” state, in theprogressionof thisdisease.
Anotherreceptorthat isbeing increasinglystudied in
prostatecanceristheprostatespecificmembraneantigen
(PSMA). Thistransmembranereceptorwasfirst imaged
with a single photon emitter, indium-111 linked via a
chelate (pendetide) to amurinemonoclonal antibody,
capromab.Indium-111 labeledcapromabpendetidewas
approvedby theFDAfor the identificationof recurrent
prostate cancer after primary definitive therapy.
28
However, its relatively low accuracy has restricted
its use to those instances where MR is equivocal for
prostatebedrecurrence,and imagingwith thisagent is
fraughtwithtechnicalchallenges; it isconsequentlynot
utilized inmost centers.
29
It is generallybelieved that
its lowaccuracy isduepartly to theantibody targeting
an intracellulardomainof thePSMAmolecule.
30
Figure 2.
FDGPET and bone scans in a patientwith
CRPCreceivingchemotherapy. Upperpanel,baseline
images. Lowerpanel, after4cyclesof taxane therapy.
Note that the lesions seen on FDG PET at baseline
have largely disappeared, while conventional bone
scintigraphy appearsunchanged.
44
Imaging approacheswith advancedprostate cancer: techniques and timing
