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Spectroscopy in Prostate Cancer: Hope or Hype?

Spectroscopy in Prostate Cancer: Hope or Hype?

No greater technologic advances have been seen anywhere in medicine than in
the fields of diagnostic imaging and radiation therapy. The two have gone hand
in hand in defining the tumor and providing better treatment planning and
delivery for prostate cancer patients. Technology in both areas has
simultaneously progressed, and the authors have beautifully illustrated the
potential for innovative interaction.

From the radiation oncologist’s standpoint, when distant disease has been
ruled out, treatment choices and applications hinge on defining the tumor
volume. Neither computed tomography (CT) nor pelvic magnetic resonance imaging (MRI)
has been very helpful in delineating the extent of prostate cancer within the
gland or in assessing the possibility of extracapsular extension and seminal
vesicle invasion. As the authors note, the addition of the endorectal coil
component and fast spin echo imaging has increased the accuracy of MRI in
detecting local extension outside the prostate to approximately 80%.[1] Although
the negative predictive value of MRI is 80% to 96% for seminal vesicle disease,
it is significantly less for extracapsular extension, ranging from 67% to
90%.[1-5] This is important for our urology colleagues, as they try to determine
preoperatively whether prostatectomy has a high probability of eradicating all
local disease.

As a procedure to outline tumor within the gland, MRI is not highly sensitive
(78%) and even less specific (55%), owing to the high false-positive rate.[1]
The advantage of combining the sensitivity of MRI with the specificity of
magnetic resonance spectroscopic imaging (MRSI) is a total specificity of 91% in
detecting cancer located in a certain area of the prostate gland if both are
positive. As the authors suggest, this may be very helpful in directing biopsies
that have been negative despite a rising prostate-specific antigen (PSA) level.
Another advantage for radiation oncologists is the ability to more accurately
visualize the dominant or bulky disease locations.

Clinical Advantages of Improved Imaging

The tremendous progress made in developing treatment equipment that can
preferentially direct more dose to the site of greatest tumor burden allows us
to take advantage of the described imaging techniques. Intensity-modulated
radiation therapy enables us to apply an accurate dose gradient differentially,
according to the various amounts of tumor within the prostate depicted by the
combined MRI/MRSI study. It appears to make sense that more dose to the sites
where radiation is most needed may lead to more local cures. The authors seem to
have been applying this technique consistently, although outcome studies have
not matured.

Data are accruing, however, on the effect of increasing radiation doses to
the prostate in general. A randomized study from M. D. Anderson Cancer Center
recently reported an advantage for patients receiving a 78-Gy dose vs a 70-Gy
dose.[6] Other institutions likewise have reported promising results with dose
escalation.[7,8] To date, it appears that intermediate-risk patients (T2,
Gleason score £ 7, PSA 10 to 20 ng/mL) may benefit
most; MRI/MRSI would likely delineate these prostatic lesions for treatment
planning and differential dose delivery.

Follow-up after irradiation can be problematic, since the first sign of
recurrence is usually an elevated PSA without the appearance of a local or
distant clinical site. PSA levels can fluctuate, producing a sawtooth curve
instead of a steady, gradual rise. There is also the PSA "bounce"
phenomenon—a one-time rise, followed by a subsequent decline to baseline. Any
adverse changes in PSA generally cause great alarm to patients, of course.

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