UCLA Journal of Radiation Oncology APRIL 2023 - Flipbook - Page 13
UCLA RADIATION ONCOLOGY JOURNAL
M R I - G U I D E D RA D I OT H E RA PY F O R P RO S TAT E CA N C E R
L E S S O N S F RO M T H E M I RAG E T R I A L
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potentially leading to reduced toxicity.
he National Cancer Institute defines
precision medicine in cancer as a strategy
that “uses specific information about
a person’s tumor to help make a diagnosis,
plan treatment, find out how well treatment is
working, or make a prognosis.” 1
This concept is extremely relevant for patients
with prostate cancer specifically. Approximately
288,000 American men will be diagnosed with
prostate cancer in 2023.2 Thankfully, the vast
majority will be diagnosed with clinically
localized disease, and many men can be cured
with either surgery or radiotherapy. Given
the high cure rate and long life-expectancy
following treatment for prostate cancer, quality
of life (QOL) often remains paramount in
decision making.3 4
Photo Credit: Kevin Shanmugam
A quick perusal of the table of contents from the
latest issues of any major oncology journal or a
survey of ongoing cooperative group trials will
underscore that a large proportion of clinical
and basic investigation is focused on how
oncologists might leverage precision medicine
to improve treatment outcomes for patients.
When treating the prostate with radiotherapy,
the major potential QOL declines are in the
Less often recognized, but critically important
genitourinary (GU), gastrointestinal (GI), and
nonetheless, is the concept of physical
sexual domains.5 With the exception of the
precision. This is particularly relevant in
prostatic urethra, the organs-at-risk with respect
radiation oncology, where the balance between
to GU, GI, and sexual toxicity after radiotherapy
delivering adequate tumoricidal doses of
are presumed to be structures adjacent to
radiation must be balanced with the risk of
the prostate (i.e., the bladder, rectum, and
damaging adjacent normal structures.
neurovascular structures). The PTV margin
needed to ensure adequate dosing of the
Built into this calculus is the inherent physical
prostate will necessitate that portions of these
uncertainty with radiation therapy, particularly
external beam radiotherapy. That is, the location organs-at-risk are included in the target volume.
and shape of the target, as well as the nearby
Arguably, the importance of precision and
normal tissues, can vary not only from day to
accuracy becomes greater as the dose of
day, but also during the delivery of a single dose
radiation delivered per treatment session
of radiation. Because of these uncertainties,
increases. Importantly, prostate cancer appears
radiation oncologists often prescribe radiation
to be more sensitive than most cancers to the
not just to the intended clinical target, but
dose of radiation delivered per day, suggesting
rather to that target plus a margin around it.
that condensing the radiation into a few high
This total volume constitutes the “planning
dose sessions might yield results that are, at
target volume” (PTV), and often overlaps with
minimum, equivalent to longer courses of
nearby organs. Within the context of radiation
radiation.6
oncology, enhanced physical precision might
lead to reducing the overall PTV, therefore
Emerging clinical trial data have cemented
stereotactic body radiotherapy (SBRT), a form of
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