ESTRO 36 Abstract Book

S157

ESTRO 36

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weeks, keeping the same positioning. The intra-fraction

reproducibility of the lung anatomy during breath hold was

investigated, by comparing the MRI of the first breath hold

with the three other MRIs of the same session. The inter-

fraction anatomical reproducibility was investigated by

comparing the first breath hold MRI of the first session

with the four MRIs during the second session. To avoid any

influence of setup variation, first a global rigid image

registration was performed. Then the lung volume was

semi-automatically segmented to define a region of

interest for the deformable image registration (DIR). DIR

was performed using Mirada RTx v1.2 (Mirada Medical,

Ltd.), with a DIR grid resolution of 3.5x2x3 mm

. The

deformation vector fields were analyzed using MATLAB

v2014b. Magnitudes of the deformation vectors were

calculated and combined for all five volunteers. The lung

volumes were divided into six segments, to analyze the

anatomical displacements on a local level. A boxplot

showing the intra- and inter-fraction displacements with a

schematic view of the six segments can be seen in figure

Results

The lung volumes for all breath holds varied by 2% within

and 7% between fractions. Looking at all five volunteers,

up to 2 mm median intra- and inter-fraction displacements

were found for all lung segments. The anatomical

reproducibility decreased towards the caudal regions.

Inter-fraction displacements were larger than intra-

fractional displacements. Maximum displacements (99.3%

of the magnitude vectors) reached 6 mm intra-fractionally

and did not exceed 8 mm inter-fractionally.

Conclusion

While the lung volume differences were insignificant,

relevant anatomical displacements were found. Moreover,

a trend of increased displacements over time could be

seen. ABC mitigates motion to some extent. Nevertheless,

the remaining reproducibility uncertainties need to be

considered during scanned proton therapy treatments. As

next step, we aim to include this knowledge in a model to

estimate their dosimetric influence for scanning proton

therapy.

OC-0304 Real-time gamma evaluations of motion

induced dose errors as QA of liver SBRT tumour

tracking

T. Ravkilde

, S. Skouboe

, R. Hansen

, E.S. Worm

, P.R.

Poulse n

Aarhus University Hospital, Department of Oncology,

Aarhus N, Denmark

Purpose or Objective

Organ motion during radiotherapy can lead to serious

deterioration of the intended dose distribution. As modern

radiotherapy shifts increasingly towards escalated doses,

steeper dose gradients and hypofractionation, the

demands on accurate delivery increase concurrently. A

large body of studies show that tumour tracking can be

applied to mitigate the effects of motion and restore dose

fidelity, yet clinical introduction seems reluctant. In this

study we report on a method for continuous evaluation of

the tracking dose delivery that conforms to common dose

analysis practice and can be acted upon in real time.

Material and Methods

Experiments were performed on a TrueBeam linear

accelerator (Varian Medical Systems) with target motion

being recorded by an electromagnetic transponder system

(Calypso, Varian Medical Systems). A HexaMotion motion

stage (Scandidos) reproduced the liver motion traces for

five different liver SBRT patients as previously measured

using intrafraction kV imaging. VMAT SBRT treatment

plans were delivered to the moving phantom with MLC

tracking, without tracking (simulating the actual delivery)

as well as to a static phantom for reference (planned

delivery). Temporally resolved dose distributions were

measured at 72 Hz using a Delta4 dosimeter (Scandidos).

Accelerator parameters (monitor units, gantry angle, MLC

leaf positions, etc.) were streamed at 21 Hz to prototype

software that performed continuous reconstruction of the

dose in real time by a simplified non-voxel based 4D pencil

beam convolution algorithm. Also in real time, but on a

separate thread, 3%/3mm gamma evaluations were

calculated continuously throughout beam delivery to

quantify the deviation from the planned intent. After

experiments, the time-resolved gamma tests were

compared with the same quantities from the measured

data.

Results

The motion induced gamma errors were well

reconstructed both spatially (Figure 1) and temporally

(Figure 2). In 95% of the time both actual and planned

doses were reconstructed within 100 ms. The median time

for reconstruction was 65 ms, which translates into a

typical frequency of about 15 Hz. Asynchronously, but also

continuously, 95% of gamma evaluations were performed

within 1.5 s with the median being at 1.2 s. Over all

experiments the root-mean-square difference between

reconstructed and measured gamma failure rates was

2.9%.