ESTRO 36 Abstract Book

S404

ESTRO 36 2017

_______________________________________________________________________________________________

Campus Pius Hospital Carl von Ossietzky University,

Oldenburg, Germany

Prof em. Medical Physics and Biophysics, Georg August

University, Göttingen, Germany

Purpose or Objective

The development of therapeutic devices combining

clinical linear accelerators and MRI scanners for MR guided

radiotherapy leads to new challenges in the clinical

dosimetry since the trajectories of the secondary

electrons are influenced by the Lorentz force. In this

study, the lateral dose response functions of a clinical air-

filled ionization chamber in the presence of a magnetic

field were examined depending on beam quality and

magnetic field following the approach of a convolution

model (Looe

et al

2015, Harder

et al

2014).

Material and Methods

In the convolution model, the 1D lateral dose response

function

(

x-ξ

) is defined as the convolution kernel

transforming the true dose profile

(

) into the disturbed

signal profile

(

) measured with a detector. For an air-

filled ionization chamber, type T31021 (PTW Freiburg,

Germany), the lateral dose response functions were

determined by Monte-Carlo simulation using 0.25 mm wide

Co and 6 MV slit beams. The chamber was modelled

according to manufacturer’s detailed specification and

placed at 5 cm water depth in three different

orientations, i.e. axial, lateral and longitudinal. For each

chamber orientation, a magnetic field oriented

perpendicular to the beam axis was applied. Simulations

were performed for magnetic fields of 0, 0.5, 1 and 1.5 T

using the EGSnrc package and the

egs_chamber

code.

To verify the simulation results, the lateral dose response

functions without magnetic field were compared against

measurements with a 5 mm wide collimated 6 MV photon

slit beam using tertiary lead blocks following the approach

of Poppinga

et al

2015.

Results

Fig. 1 shows good agreement between the simulated and

measured dose response functions

(

x-ξ

) of the

investigated ionization chamber in the three investigated

orientations. The structures of the measured functions are

not as evident as those of the simulated functions possibly

due different scanning step widths used in the experiment

and the calculation.

Fig. 2 shows the lateral dose response function

(

x-ξ

) with

and without magnetic field obtained exemplary for the

detector in lateral orientation. The distortion of the dose

response function

(

x-ξ

) corresponds to the change in the

trajectory lengths of the secondary electrons in the air of

the ionization chamber due to the Lorentz force, as

compared to the trajectories in a small sample of water.

Fig. 1. Area-normalized simulated and measured dose

response functions

(

)

Fig. 2. Area-normalized dose response functions

(

) for

the T31021 in lateral orientation for magnetic fields of 0,

0.5,1 and 1.5 T

Conclusion

The distortions of the lateral dose response function

(

) will alter the measured signal profile

(

) of a detector

in magnetic field, as demonstrated in this study. The

variety of the possible combinations of detector

orientation and magnetic field direction and the strong

dependence of the distortion on the magnetic field

strength require careful consideration whenever a non-

water equivalent detector is used in magnetic field.

PO-0772 Patient-specific realtime error detection for

VMAT based on transmission detector measurements

M. Pasler

, K. Michel

, L. Marrazzo

, M. Obenland

, S.

Pallotta

, H. Wirtz

, J. Lutterbach

Lake Constance Radiation Oncology Center, Department

for Medical Physics, Friedrichshafen, Germany

Lake Constance Radiation Oncology Center- Martin-

Luther-Universität Halle-Wittenberg, Department for

Medical Physics- Naturwissenschaftliche Fakultät II,

Friedrichshafen, Germany

AOU Careggi, Medical Physics Unit, Florence, Italy

Lake Constance Radiation Oncology Center, Department

for Medical Physics, Singen, Germany

University of Florence- AOU Careggi, Medical Physics

Unit- Department of Biomedical- Experimental and

Clinical Sciences, Florence, Italy

Lake Constance Radiation Oncology Center,

Radiooncology, Singen, Germany

Purpose or Objective

To investigate a new transmission detector for online dose

verification. Error detection ability was examined and the

correlation between the changes in detector output signal

with γ passing rate and DVH variations was evaluated.

Material and Methods

The integral quality monitor detector (IQM, iRT Systems

GmbH, Germany) consists of a single large area ionization

chamber which is positioned between the treatment head

and the patient. The ionization chamber has a gradient

along the direction of MLC motion and is thus spatially

sensitive. The detector provides an output for each single

control point (segment-by-segment) and a cumulative

output which is compared with a calculated value.

Signal stability and error detection sensitivity were

investigated. Ten types of errors were induced in clinical

VMAT plans for three treatment sites: head and neck (HN),

prostate (PC) and breast cancer (MC). Treatment plans

were generated with Pinnacle (V.14) for an Elekta synergy

linac (MLCi2). Geometric errors included shifts of one or

both leaf banks for all control points toward (i) and away

(ii) from the central axis of the beam and unidirectional

shifts of both leaf banks (iii) by 1 and 2mm, respectively.

Dosimetric errors were induced by increasing the number

of MUs by 2% and 5%.

Deviations in dose distributions between the original and

error-induced plans were compared in terms of IQM signal

deviation, 2D γ passing rate (2%/2mm and3%/3mm) and

DVH metrics (D

mean

, D

and D

98%

for PTV and OARs).

Results

For segment-by-segment evaluation, calculated and

measured IQM signal differed by 4.7%±5.5%, -2.6%±4.6%

and 4.19%±6.56% for MC, PC und HN plans, respectively.

Concerning the cumulative evaluation, the deviation was

-1.4±0.25%, -6.0±0.3% und -1.47%±0.97%, respectively.

Signal stability for ten successive measurements was

within 0.5% and 2% for the cumulative and the segment-

by-segment analysis.

The IQM system is highly sensitive in detecting geometric

errors down to 1mm MLC bank displacement and

dosimetric errors of 2% if a measured signal is used as

reference. Table 1 reports IQM signal deviations for a

range of introduced errors.