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Electron ARC therapy dose calculations
??Biology ??Physics
Radiation Oncology
182
October 1984, Volume 10, Sup. 2
1072 PHOTON
ENERGY
AND ANGULAR
Radhe Mohan, Memorial
Ph.
DISTRIBUTIONS
D., Chen-Shou
Sloan Kettering
OF LINEAR ACCELERATORS
Chui, M.
Cancer
Center,
S. New York, N.
Y.
10021.
For accurate three-dimensional treatment planning, new models of dose calculations are being developed that require knowledge of the energy spectrum of photons incident on the surface of the patient. Knowledge of spectra may also be useful in other applications including routine clinical quality assurance and design of filters and beam modifying devices etc. We have used Monte Carlo code (EGS) to compute photon spectra for a number of different linear acclerators. Both the target and the flattening filter have been accurately modelled. We found the mean photon energy to have a value lower than the generally perceived value of one-third the peak energy. As expected, the spectra become softer as the distance from the central axis increases. For 15 MV photons from a Clinac-20, for example, we found the mean energy to be 3.5 MeV on the central part (between 0 and 10 cm) of the beam and 2.87 MeV between 15 and 20 cm from the central axis. The impact of softening of spectra on dose computations is being investigated. Verification of the spectra was performed by computing dose distributions in water using the calculated spectra and comparing the results with measured data. We also examined the angular distributions of photons incident on the surface of the phantom. In all models of dose computations, it is assumed that the angular distribution of photons with respect to fan lines emanating from the source is negligible. The computed angular spreads were found to support this assumption.
1073 ELECTRON ARC THERAPY
DOSE CALCULATIONS
Finn
Thomadsen**
Laursen?:,
Radiofysisk,
Bruce
Kdbenhavns
Generally it is detail the influence not be necessarv for
Amts
and
Bhudatt
Sygehusi
Paliwal+”
Herlev”,
agreed that electron of surface curvatures electron arc planning,
Div.
of
Radiation
Oncology,
Univ.
of
Wisconsin-Madison*+:
dose planning systems need complicated and internal inhomogeneities on dose since all fine details are “blurred
algorithms to describe in distributions. This might out” during the rotation. This proqram is based in electron arc therapy. The cal cuwater or water equivalent phantom. results from arc treatments of a body shaped
A simplified proqram has been developed for dose calculation only upon beam data measured for stationary beams in a flat lated dose values of the program are compared with measured phantom of tissue equivalent material. in a patient to a ooint P, as shown in the fiqure, The dose,
due
to
electron
arc
could
be
given
by:
-2
D,
=
where
SF(s) SF(s)
01,
6 -1 is
02,
d(O)
is
the
slit
is
factor
and
e are
the
the
depth
of
i s the
fsd (0) r(0)
F[d(O)*fsd(O).r(O)*p(4,
the
virtual
patient
0,
in start,
Gy/mu
for
slit
stop
and
instantaneous
P along
the
source
to
curvature
s)ldO
ray skin
at
size
angle
distance
s angle
0 dependence
dependence
for the slit widths as a function of 0, s) is the dose profile I beam size angle 0. For oractical reasons the function F has to be simplified. A traditional approach is seoarate the variables as F[d(O)*fsd(0)*r(O).p(Q, 0, s)] = D(o)*FSD(O)*R(O)*P(@, S) where D(O) = central axis depth dose curves FSD(O) = inverse square law using the virtual source to skin distance P(Q, s) = stationary beam orofile for a flat phantom at the depth of dose maximum R(O) = function describing the chanqes in the dose distribution caused by the curved P(Q,
to
try
to
surface.
02 The
dose
equation
can
then
be
simplified
This poster considers the problems speed calculations when the patient Using this approach with rotations measured doses agree to within ?r 3%. to
of can of
R(Q) P(Q, s)dO D(0) FSD(0) / 01 findinq a solution for any patient contour, and simp 1 ications be considered a cylinder. and 8, 12, and 18 MeV electrons, agreement between calcu lated to:
D,
= SF(s)
Radiation Oncology
182
October 1984, Volume 10, Sup. 2
1072 PHOTON
ENERGY
AND ANGULAR
Radhe Mohan, Memorial
Ph.
DISTRIBUTIONS
D., Chen-Shou
Sloan Kettering
OF LINEAR ACCELERATORS
Chui, M.
Cancer
Center,
S. New York, N.
Y.
10021.
For accurate three-dimensional treatment planning, new models of dose calculations are being developed that require knowledge of the energy spectrum of photons incident on the surface of the patient. Knowledge of spectra may also be useful in other applications including routine clinical quality assurance and design of filters and beam modifying devices etc. We have used Monte Carlo code (EGS) to compute photon spectra for a number of different linear acclerators. Both the target and the flattening filter have been accurately modelled. We found the mean photon energy to have a value lower than the generally perceived value of one-third the peak energy. As expected, the spectra become softer as the distance from the central axis increases. For 15 MV photons from a Clinac-20, for example, we found the mean energy to be 3.5 MeV on the central part (between 0 and 10 cm) of the beam and 2.87 MeV between 15 and 20 cm from the central axis. The impact of softening of spectra on dose computations is being investigated. Verification of the spectra was performed by computing dose distributions in water using the calculated spectra and comparing the results with measured data. We also examined the angular distributions of photons incident on the surface of the phantom. In all models of dose computations, it is assumed that the angular distribution of photons with respect to fan lines emanating from the source is negligible. The computed angular spreads were found to support this assumption.
1073 ELECTRON ARC THERAPY
DOSE CALCULATIONS
Finn
Thomadsen**
Laursen?:,
Radiofysisk,
Bruce
Kdbenhavns
Generally it is detail the influence not be necessarv for
Amts
and
Bhudatt
Sygehusi
Paliwal+”
Herlev”,
agreed that electron of surface curvatures electron arc planning,
Div.
of
Radiation
Oncology,
Univ.
of
Wisconsin-Madison*+:
dose planning systems need complicated and internal inhomogeneities on dose since all fine details are “blurred
algorithms to describe in distributions. This might out” during the rotation. This proqram is based in electron arc therapy. The cal cuwater or water equivalent phantom. results from arc treatments of a body shaped
A simplified proqram has been developed for dose calculation only upon beam data measured for stationary beams in a flat lated dose values of the program are compared with measured phantom of tissue equivalent material. in a patient to a ooint P, as shown in the fiqure, The dose,
due
to
electron
arc
could
be
given
by:
-2
D,
=
where
SF(s) SF(s)
01,
6 -1 is
02,
d(O)
is
the
slit
is
factor
and
e are
the
the
depth
of
i s the
fsd (0) r(0)
F[d(O)*fsd(O).r(O)*p(4,
the
virtual
patient
0,
in start,
Gy/mu
for
slit
stop
and
instantaneous
P along
the
source
to
curvature
s)ldO
ray skin
at
size
angle
distance
s angle
0 dependence
dependence
for the slit widths as a function of 0, s) is the dose profile I beam size angle 0. For oractical reasons the function F has to be simplified. A traditional approach is seoarate the variables as F[d(O)*fsd(0)*r(O).p(Q, 0, s)] = D(o)*FSD(O)*R(O)*P(@, S) where D(O) = central axis depth dose curves FSD(O) = inverse square law using the virtual source to skin distance P(Q, s) = stationary beam orofile for a flat phantom at the depth of dose maximum R(O) = function describing the chanqes in the dose distribution caused by the curved P(Q,
to
try
to
surface.
02 The
dose
equation
can
then
be
simplified
This poster considers the problems speed calculations when the patient Using this approach with rotations measured doses agree to within ?r 3%. to
of can of
R(Q) P(Q, s)dO D(0) FSD(0) / 01 findinq a solution for any patient contour, and simp 1 ications be considered a cylinder. and 8, 12, and 18 MeV electrons, agreement between calcu lated to:
D,
= SF(s)