Direct pO2 Measurements in Experimental Tumors During
Fractionated Radiation
Therapy Measured by EPR Oximetry
Julia O'Hara, Carmen Wilmot, Harold M. Swartz
EPR Center for the Study of Viable Biological Systems, Department of
Diagnostic Radiology,
Dartmouth Medical School, HB7785, Hanover, NH, USA, 03755
INTRODUCTION: Oxygen is an important determinant of the response of
tumors to radiation
therapy. Changes in tumor pO2 after radiation have been reported in most
experimental and some
human tumors, typically an increase in hypoxia followed by reoxygenation.
Thus radiation
therapy may be made more effective if serial monitoring of tumor pO2 were
used to determine
the extent and timing of post-irradiation reoxygenation in individual
tumors. Treatment planning
could then be modified to exploit improved oxygenation or to direct patients
to therapy
appropriate for hypoxic tumors. Clinical studies with microelectrodes have
shown that
information on tumor pO2 can be useful even if limited to one time point.
EPR oximetry can
provide more information since, once the oxygen sensitive material has been
placed within the
tumor tissue, the pO2 can be monitored repeatedly without further invasion.
Using the new capabilities developed with in vivo electron paramagnetic
resonance
(EPR) oximetry, previously we have shown that changes in tumor pO2 after
split dose irradiation
reflected changes in tumor radiosensitivity1. Presently, we are extending
the work to lower doses
per fraction, similar to those used in the clinic, and measuring tumor pO2
before and during
courses of radiation therapy that simulate clinical regimens (e.g.2Gy/day x
20days) in
experimental tumors. The goal is to identify principles and methods that
allow for possible
modification of radiation therapy plans to exploit improved post-irradiation
reoxygenation for
improved tumor control.
METHODS: Subcutaneous tumors were established in mice and grown to a
size (about 5mm
diameter) that could be implanted with solid oxygen-sensitive paramagnetic
material (usually
lithium phthalocyanine). Two days after implantation of LiPc, pretreatment
tumor pO2 is
measured in each tumor by in vivo EPR oximetry. Radiation began on Day 0
when tumors were
about 150 mm3 in volume. Tumor pO2 was then assessed 2-3 times per week
during the course of
radiation therapy and for up to a week post-therapy. Only one injection of
LiPc was necessary to
monitor pO2 in a tumor over the whole course of the experiment. Tumor
regrowth delay, defined
as the time required for the irradiated tumors to grow to double the volume
at the time of
treatment, was used to assess the effectiveness of treatments. Animals were
anesthetized with
isoflurane with spontaneous breathing of during irradiation and pO2
measurements.
RESULTS: We have completed several experiments with two tumor lines
and two different
treatment plans. There was heterogeneity of response among the tumors of a
given type given the
same doses of radiation on the same schedule. Approximately one third of the
tumors responded
with radiobiologically significant improvements in pO2 (ie, a mean above 10
mm Hg). One third
responded with increasing hypoxia or no improvement at all, and the other
third showed modest
improvement but not to a mean above the arbitrary level of 10 mm Hg.
Results from one experiment with RIF-1 (radiation-induced fibrosarcoma)
tumors in
C3H/HeJ mice are described here. The radiation treatment plan was 2Gy/day
for 10 days without
interruption starting on Day 0. The experiment included 23 mice, divided
into treated and control
groups, with these divided into three groups that started irradiation on
three consecutive days.
There was heterogeneity of tumor pO2 in mice before and during treatment.
The mean (±SE)
pretreatment pO2 was 7.4 ± 1.3 mm Hg (n=26 measures, 23 mice). The range of
pO2 was 14.0-
1.7mm Hg. Tumor volume (± SD) at the time of treatment was 140 ± 32 mm3. The
effect of
irradiation on tumor growth was to increase the time to reach 3x the volume
at treatment by 4
days.
These results indicate that reoxygenation does occur during fractionated
radiation but
investigations into the time course between doses will need to be carried
out to determine the
detailed time course of the cumulative effect of the continuing radiation
treatments on
oxygenation.
RIF-1 Tumor pO2 during
Fractionated Radiation (10x2Gy/Day) Beginning Day
0.
Reoxygenation between doses was observed
when pO2 was measured 19 hr after the 5th -2Gy dose.
No reoxygenation was seen after doses 1-4. The Interval between
irradiation and pO2 measurement
varied and was longest (19 hr) after dose 4.
DISCUSSION: Our previous results provided direct evidence that
observed post-irradiation changes
in pO2 after large doses of radiation reflect changes in tumor
radiosensitivity. The results also
demonstrated the power of in vivo EPR oximetry to provide repeated
measurements of pO2 in an
individual tumor. Our current studies show that potentially
radiobiologically significant pO2
changes occur after lower, more clinically relevant doses per fraction. Due
to the high degree of
heterogeneity however we plan to proceed with utilizing the newly developing
capabilities for
multi-site and high resolution multisite measurements to investigate
systematically the effects of
intra-tumor heterogeneity on our assessments of post-irradiation
reoxygenation. The goal is to
develop EPR oximetry for clinical use to allow the principles of the
relationship between local
control and tumor pO2 to be applied to improve treatment of individual human
tumors.
REFERENCE:
1) J. O'Hara, F. Goda, E. Demidenko and H. Swartz, "Effect on regrowth delay
in a tumor of scheduling split dose
radiation based on direct pO2 measurements by EPR oximetry", Radiat Res,
150, 549-556,1998.