Effect on Tumor Regrowth Delay in a Murine Tumor of
Scheduling Split Dose Radiation
Based on Direct pO2 Measurements by EPR Oximetry
1Julia O'Hara, 1,2Fuminori Goda, 1Harold M. Swartz
EPR Center for the Study of Viable Biological Systems, Department of
Diagnostic Radiology,
Dartmouth Medical School, Hanover, NH 03755, USA
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
tumor systems,
typically an increase in hypoxia followed by reoxygenation. The occurrence of
reoxygenation and its
fundamental importance in the success of fractionated radiation therapy is
widely accepted, but the
means to assess the extent and timing of reoxygenation have not been available
until recently. EPR
oximetry has the potential to be very useful for the repeated and sensitive
measurements that are
necessary to obtain such data, since tumor pO2 can be monitored at the same
place in a tumor over
the time of response to treatment. Using the new capabilities developed with
in vivo electron
paramagnetic resonance (EPR) oximetry, previously we assessed tumor pO2
changes after single
doses of radiation and reported a tumor dependent biphasic time course: an
increase in hypoxia
followed by reoxygenation. In murine fibrosarcoma tumors (RIF-1), the nadir of
pO2 occurred 24 hr
after a priming dose (10-40 Gy) and reoxygenation was maximal 72 post
irradiation. We now have
extended this work to determine experimentally if tumor pO2 measurements by
EPR could be
exploited to enhance the effectiveness of radiation treatment by measuring the
effect on tumor
volume of two different intervals between 10 Gy doses, with the interval
matched to the most
hypoxic (24 hr) time or after maximum reoxygenation had occurred (72 hr
interval).
METHODS: Subcutaneous RIF-1 tumors were established in C3H mice and
propagated by alternating
in vitro and in vivo passes. Mice were randomly assigned at the time of tumor
implantation to one of
four treatment plans: Control (Sham-Irradiated), Single (20 Gy on Day 0), Oxic
(10 Gy on Day 0
and 10 Gy 72 hr later) or Hypoxic (10 Gy on Day 0 and 10 Gy 24 hr later). Two
experiments were
completed which were the same except that the tumor volume at the time of
irradiation was 114 ± 53
mm3 in the first and 231 ± 73 mm3 in the second experiment. One day prior to
irradiation, 20 µl of
India ink (this material has an EPR spectrum which reflects the pO2) was
injected directly into the
tumor. Tumor pO2 was measured daily in each tumor by EPR oximetry. The mouse,
restrained but
not anesthetized, was placed in the EPR spectrometer operating at 1.2 GHz with
a loop resonator
positioned over the tumor and India ink spectra were recorded using modulation
less than 1/3 of the
LW. Only one injection of ink 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 compare treatments.
RESULTS: Initial pO2 in the first experiment was 7.2 ± 0.8 torr (mean ±
SD, n=20) and 6.5 ± 0.5 in the
second, reflecting a significant dependence of tumor pO2 on tumor volume.
After radiation,
characteristic pO2 changes were observed in all irradiated groups: an increase
in hypoxia 24 hr after
radiation, followed by slower reoxygenation. A 24 hr interval between 10 Gy
doses resulted in a
delay of reoxygenation, while a 72 hr interval resulted in a second cycle of
hypoxia/reoxygenation in
the tumors. In both experiments, tumor regrowth delay was significantly longer
in the "oxic"
compared to the "hypoxic" group, indicating that the measured pO2 changes
reflected changes in
tumor radiosensitivity. Individual variation in degree and timing of
reoxygenation was observed but
the choice for intervals between radiation doses was made based on mean values
for the treatment
groups.
DISCUSSION: These results provide the first direct evidence that
observed post-irradiation changes in
pO2 reflect changes in tumor radiosensitivity. The results also demonstrate
the power of in vivo EPR
oximetry to provide repeated measurements of pO2 in an individual tumor.
Future studies will focus
on extending our observations to lower, more clinically relevant radiation
doses and to investigate
systematically the effects of altered dose fraction size, fraction number, and
interval between
fractions on tumor pO2 and tumor control. A major long term goal of the
project is to develop EPR
oximetry for clinical use to allow the principles of the relationship between
local control and pO2 to
be applied to improve treatment of individual human tumors.
REFERENCE:
1. Department of Radiology, Dartmouth Medical School, Hanover, NH 03755, and
2Current Address:
First Department of Surgery, Kagawa Medical School, 1750-1 Ikenobe Miki,
Kagawa, 761-07
Japan.