525 / 1096
S510
ESTRO 36
_______________________________________________________________________________________________
beams (0.4 ± 3.2%). Comparing C4 to C2, log file accuracy
is analyzed for MC. LF resulted in lower σ values for 20/22
arcs (-5.4 ± 3.4%) and improved pass rates for 14/18 arcs
(1.1 ± 1.4%). Comparing C5 to C2, LF and AC QA
techniques are compared. The LF technique yielded
decreased σ values for 22/22 arcs (-51 ± 7%) and improved
pass rates for 18/18 fields (9.9 ± 3.8%). The LF technique
also eliminated systematic AC errors; mean dose errors
decreased from 3.2% to 0.1%. For 1/2/3/4° LF-CS control
point spacing, 1%/1mm pass rates were 80.0 ± 5.0%, 78.0
± 4.2%, 74.0 ± 5.1%, and 68.8 ± 5.3%. Plan-CS pass rates
were 80.2 ± 4.0%. Figure 2 plots difference in pass rates
[(LF-CS vs. AC) minus (Plan-CS vs. AC)] as a function of
control point spacing for each arc. Calculation times for
CS and MC were 12s per control point and 3 minutes per
VMAT arc respectively.
Conclusion
MC doses proved more accurate than CS when compared
to AC measurement. LF-MC plans yielded superior
accuracy and shorter calculation times than LF-CS plans.
By cutting out the phantom and comparing LF dose to that
of the original plan, systematic error was eliminated and
random error greatly reduced.
PO-0921 Dose considerations of IGRT using MV
projection and MV CBCT on a prototype linear
accelerator
P. Balter
1
, T. Netherton
1
, Y. Li
1
, P. Nitsch
1
, S. Gao
1
, M.
Muruganandham
1
, S. Shaitelman-
1
, S. Frank
1
, S. Hahn
1
, A.
Klopp
1
, L. Court
1
1
UT MD Anderson Cancer Center Radiation Physics,
Radiation Physics, Houston- TX, USA
Purpose or Objective
The use of the mega-voltage treatment beam for image-
guided patient setup has some potential advantages over
kV imaging, especially reduced equipment and QA
requirements. One of the challenges that MV imaging
introduces is the increase in daily imaging dose. Here we
investigate (1) whether the MV imaging dose can be
correctly calculated and incorporated into the treatment
plan, and (2) the impact of MV imaging dose on the dose
to normal tissues such as the lung and heart.
Material and Methods
MV imaging dose to the lung, heart and other soft tissue
was measured using an ion chamber in anthropomorphic
thorax phantom (CIRS), and compared with dose
calculated in the TPS (Eclipse) for orthogonal MV-MV
imaging fields and MV CBCT images using a prototype
linear accelerator, each with a low-dose and high-quality
mode (total 4 modes). The impact of the imaging
technique (orthogonal vs. CBCT and high vs. low quality)
on the doses to normal tissue was evaluated using Eclipse,
where the imaging doses were used as based plans in the
treatment planning process. For breast plans, doses to the
heart and lung were evaluated. For head/neck plans,
doses to all the normal tissues were compared.
Figure 1. Anthropomorphic phantom (CIRS) with dose
measurement points identified.
Results
Average imaging dose was measured as 1.3, 2.5, 3.7, and
7.6cGy for daily low dose MV pairs, high quality MV pairs,
low dose CBCT and high quality CBCT, respectively. Over
a 30 fraction treatment with daily IGRT, this equates to 38
- 227cGy. The average agreement between measured and
calculated tissue doses due to imaging was 0.4±0.4cGy.
The largest difference was 1.3cGy, found in the lung for
high quality CBCT imaging (~39cGy over a 30 fraction
treatment).
With imaging dose incorporated into the treatment
planning process, it was possible to create clinically
acceptable treatment plans for a range of treatment sites,
including breast, head and neck and prostate. The imaging
technique did, however, increase the heart and lung dose
for breast plans. For an example left breast treatment,
the mean heart dose in our original, clinically delivered
plan was 60cGy. With daily MV imaging included, this
increased to 140, 150, 190 and 260cGy for daily low dose
MV pairs, high quality MV pairs, low dose CBCT and high
quality CBCT, respectively. The corresponding values for
mean lung dose were 360cGy (original clinical) and 470,
490, 510 and 570cGy.
Table 1: Tissue doses(cGy/fraction) at different points in
the anthropomorphic phantom. M: Measured. C:
Calculated
Conclusion