ESTRO 36 Abstract Book

S765

ESTRO 36

_______________________________________________________________________________________________

the neutrons are produced when the electron beam hits

the tungsten target and then the primary collimation

occurs. Both the target and the primary collimator are

located at the top of the gantry head. The maximum

thermal

neutron

flux

obtained

3x10

neutrons/cm

.second which is higher than a standard

americium-beryllium

(Am-Be)

neutron

source.

At the isocenter plane (SSD=100 cm), the fluxes were

5.4x10

at the center, 1.5x10

at 2.5 m away and 9.9x10

n/cm

.s at the room wall which is 3.8 m away from

isocenter. The flux at the maze entrance was measured

nearly six in a ten thousand less (81 n/cm

.s).

Conclusion

The neutron flux distribution within the bunker was

measured with detail using 91 points. Neutron flux

distribution within the bunker found and the graph was

plotted. Thus neutron flux can found any desired point in

the room by iterations. The flux decreases as we move

away the isocenter which is compatible with the

literature. The magnitude of the neutron fluxes shows that

there is a significant amount of neutron dose within the

room. The corresponding neutron dose to the patient

however is only 0.1-0.3 % of the total dose. However,

neutrons have a high RBE and this unwanted dose is not

calculated with the TPS. The future work would be to

compare the results with the Monte Carlo simulations.

EP-1434 Comparison of small-field output factor

measurements

C. Oliver

, V. Takau

, D. Butler

, I. Williams

ARPANSA, Radiotherapy, Yallambie, Australia

Purpose or Objective

The Australian Radiation Protection and Nuclear Safety

Agency (ARPANSA) held a comparison in April 2016

whereby participants came to ARPANSA and measured the

output factor of a 5 mm cone . The goal of the comparison

was to compare the consistency of the small-field output

factor measured by independent medical physicists with

their own apparatus.

Material and Methods

The participants measured the output factor of the 5 mm

cone using a 6 MV photon beam at a source to surface

distance of 95 cm and depth in water of 5 cm. ARPANSA

provided a 3D scanning water tank for detector positioning

but all detectors were brought by participants. The

participant was asked to measure the output factor as

accurately as possible. All post measurement analysis,

correction factor determination and uncertainty

calculations were supplied by the participant.

Results

Fifteen groups travelled to ARPANSA and a total of thirty

independent measurements of the output factor were

made. The most popular method of measurement was with

film but measurements were also made with ionisation

chambers, semiconductor detectors, diamond detectors

and a scintillation detector. A large volume ionisation

chamber measuring dose area product was also used in the

comparison. The standard deviation of all the

measurements was 5.6 % with the maximum variation

between two results being 42 %.

Conclusion

This exercise gave an indication of the consistency of the

small-field dosimetry being performed in Australia at the

present time. There is no currently accepted protocol for

these measurements and a wide range of detectors are

being used with correction factors being applied from a

variety of sources. The dissemination of the small-field

methods and techniques currently being used will aid the

consistency of these measurements.

EP-1435 Evaluation of single material and multi-

material patient-specific, 3D-printed radiotherapy

phantoms

D. Craft

, E. Burgett

, R. Howell

The University of Texas MD Anderson Cancer Center,

Radiation Physics, Houston, USA

Idaho State Univeresity, Department of Nuclear

Engineering, Pocatello Idaho, USA

Purpose or Objective

Anthropomorphic phantoms are used in a variety of ways

in radiation therapy for both research and quality

assurance purposes. Most anthropomorphic phantoms are

of generalized patients, but 3D printing technology can be

used to fabricate patient-realistic phantoms for special QA

and verification procedures. Most 3D printers, however,

can only print in one or two materials at a time, so true

patient heterogeneity is limited. In this study, we

examined two different patient specific, 3D printed

phantoms created based on the same patient to determine

the accuracy of single and multi-material phantoms.

Material and Methods

The phantoms used in this study were designed from the

clinical CT data for a post-mastectomy patient treated at

our institution. The CT data was trimmed to remove the

patient’s head and arms to preserve anonymity and

simplify printing. Phantom 1 was designed by converting

the trimmed CT data into a 3D model with a CT threshold

of >-500 Hounsfield units (HU). This model was sliced into

2.5-cm-thick sagittal slices and printed one slice at a time.

All slices were printed with polylactic acid (PLA)

representing all body tissues, but with air cavities and

lower density regions like the lungs left open. Sagittal

slices were chosen for their superior fit with each other,

and minimal material warping relative to axial slices.

Phantom 2 was designed by converting the CT data into

three separate 3D models with a CT threshold of <-147 HU

for air cavities, -147 to 320 HU for soft tissue, and >320

HU for bone. The models were sliced into 1-cm-thick axial

slices, and printed. The slices were printed from the soft

tissue model using a custom formulated high impact

polystyrene (HIPS) with the air and bone models left open.

After printing, the open bone model sections were filled

with a liquid resin polymer with an equivalent density to

bone.

The phantoms were evaluated for their materials and

overall accuracy to the original patient CT. Blocks of PLA,

HIPS, and the bone resin material were all imaged to

determine their average HU. The phantoms were also each

imaged and registered with each other and the original

patient CT to determine the consistency and accuracy of

each phantom.

Results

The materials used and their properties are summarized

in Table 1. Phantom 1 was fabricated from PLA, which

isn’t particularly tissue equivalent, but did print relatively

consistently. The bone resin and HIPS of phantom 2 more

accurately reflect tissue heterogeneity, but have more

variations in their printed consistency.