ESTRO 36 Abstract Book

S502

ESTRO 36 2017

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Czech Technical University in Prague, Institute of

Experimental and Applied Physics, Prague, Czech

Republic

German Cancer Research Center DKFZ, Department of

Medical Physics, Heidelberg, Germany

Purpose or Objective

(1) Purpose: In highly conformal radiotherapy, like ion

beam radiotherapy, inter- and intrafractional monitoring

of the target is desirable. Due to the steepness of the

rising part of the Bragg curve, ion beam radiography can

in principle provide high resolution of the traversed tissue

thickness. Ion beam radiography is furthermore attractive

due to its potential to measure the stopping power of the

tissue directly. However, currently there is no detection

system for clinical imaging of patients. Helium ions as the

imaging modality provide the advantages of decreased

multiple scattering in comparison to protons and lower

biological effectiveness than the carbon ions.

Material and Methods

(2) Methods: Plastic phantoms contain ning 1mm deep

step-like inhomogeneities were imaged with helium ion

beams at the Heidelberg Ion Beam Therapy facility in

Germany. To register the radiation, a system of 5 parallel

layers, based on the semiconductor pixelized detector

Timepix, which was developed by the Medipix

Collaboration at CERN, was used. Two layers in front of

the phantom enabled us to measure the position and

direction of incoming helium ions. Another pair of

detection layers, located behind the phantom, registered

the outgoing particles and an additional layer was used to

measure their energy loss and to identify the ion type.

Synchronization of all the five detector layers enabled us

to associate the outgoing particles to the incoming ones.

To build the image of the phantom, we used the measured

information about the transversal position of the incoming

and outgoing particle, their direction and type (He or H).

Results

(3) Results: With this system we imaged a 1 mm step in a

160 mm thick PMMA phantom. Spatial resolution below 2

mm was reached when the inhomogeneity was located in

the phantom, while resolution below 1 mm was achieved

in the cases where the step was located at the front or at

the end of the phantom. Hereby we have shown that the

information about flight direction of the incoming and

outgoing ion, together with the capability to identify them

and thus select solely helium ions, enables to improve the

spatial resolution by a factor of more than three.

Conclusion

(4) Conclusion: We have shown experimentally that helium

beam radiography reaches in simple phantoms spatial

resolution in the region which is attractive for highly

conformal radiotherapy. In the presentation the results

obtained with helium beams as the imaging modality will

be compared to proton-based imaging.

PO-0915 Performance study of a prototype straight-

through linac delivery system with an EPID assembly

R. Scheuermann

, C. Kennedy

, D. Mihailidis

, J. Metz

University of Pennsylvania, Radiation Oncology,

Philadelphia, USA

Purpose or Objective

To study and expand the use of the Machine Performance

Check (MPC) tool in monitoring the continuous operational

performance of a prototype delivery system composed of

a straight-through-linac and an in-line MV portal imaging

panel (Proof-of-Concept).

Material and Methods

The MPC, as implemented in TrueBeam (TB), is an

integrated self-check tool that assures that critical

machine performance are within specifications, e.g.

mechanical accuracy and radiation output. As adapted to

the prototype straight-through linac delivering 6X-FFF

(filter-free) beam, the automated tests are based on its

in-line MV EPID. The MPC acquires a series of MV images

of an IsoCal phantom, capturing beam properties and

mechanical data such as MLC and gantry accuracies. A

new MPC test monitors output stability in terms of percent

deviation from the baseline data of the actual measured

beam. All measured data are automatically processed,

analyzed, and displayed for evaluation, thus providing a

reliable and fast method for routine machine performance

assessment. Independent tests such as star-shots,

Winston-Lutz, MLC picket fence patterns and output

measurements on a daily basis were employed to

benchmark the MPC test results for the prototype system.

Results

MPC results were collected daily for six months on both

the prototype and a TB. The independent tests on the

prototype system were repeated weekly to validate the

MPC results. A sample comparison of the MPC results for

the prototype against independent tests are shown in

Table 1. The output stability of the prototype system, as

measured with the MPC and a DailyQA

3 device, is

comparable (Fig. 1), and within 0.5% of independent

output measurement for the period shown. All tests

performed were within the tolerances allowed by the MPC

and agreed in most cases with the result of the

independent tests. The prototype system performs as well

as the TB system. A summary of MPC test results and

comparisons with independent measurements will be

shown alongside with the TB MPC results.

Table 1

Figure 1

Conclusion

The performance of a straight-through linac d elivering 6X-

FFF (filter-free) beam with an EPID panel wa s investigated

with the MPC testing tool and that method w as validated

against independent tests for proof of con cept. MPC is a

complete, reliable and quick test suite that monitors the

performance of a treatment unit on routine basis.