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S504

ESTRO 36 2017

_______________________________________________________________________________________________

For Elekta accelerators, all the calculation results show a

deviation from the reference values lower than 3%. For

Siemens and Varian accelerators, the resulting

calculations for fields larger than 2×2 cm

2

differ less than

4%. For 2×2 cm

2

large fields formed by Siemens and Varian

MLC, the differences between the calculated and

measured output factors often exceed 5%, but still are

below 10%.

Conclusion

The RPC measured values provide a consistent dataset for

small field output factors that can be used as a redundant

QA check of a treatment planning system dosimetry data

for small-field treatments. The results of the audit are

very useful for the participants who should carefully

investigate any detected discrepancies between the

standard dataset and calculated values, with attention to

the specific beam model.

PO-0918 Radiotherapy and Her2 targeting agents:

synergism and antagonism in clonogenic and confluence

assays

N. Suchowerska

1

, J. Toohey

2

, S. Carroll

2

, L. Rogers

2

, G.

Lyons

3

, J. Beith

4

, A. Dos Santos Esteves

2

, D.R. McKenzie

5

1

Chris O'Brien Lifehouse, Radiation Oncology,

Camperdown- Sydney, Australia

2

Chris O'Brien Lifehouse, Radiation Oncology,

Camperdown Sydney, Australia

3

Royal Prince Alfred Hospital, Dermatology Cancer,

Sydney, Australia

4

Chris O'Brien Lifehouse, Medical Oncology, Camperdown

Sydney, Australia

5

University of Sydney, School of Physics, Sydney,

Australia

Purpose or Objective

Her2 amplified cancers, comprising 15-20% of patients

presenting with breast cancer, are now routinely

prescribed Trastuzumab (Herceptin), a monoclonal

antibody targeting Her2 receptors, leading to a significant

improvement in outcomes in this previously high risk

breast cancer subtype. Such targeting agents are rapidly

being introduced into the clinic, based on trials showing a

survival advantage. Now combination therapies with drug

conjugates have emerged. The biological interactions of

combined targeting agents, when given concurrently with

radiation, are not well described. Our aim is to identify

whether there is a synergistic or antagonistic interaction

between targeting agents and ionising radiation for two

distinct Her2+ subtypes.

Material and Methods

Two molecular subtypes of HER2+ breast cancer cell lines

were used: HCC-1954, which is ER and PR hormone

negative and BT-474, a luminal B which is ER negative and

PR positive. Both cell lines were treated to Her2 targeting

agents (Trastuzumab and T-DM1) and radiation (6MV

photons, 0 to 4Gy), individually and in combination to

identify whether the response was synergistic, additive or

antagonistic. The alpha/beta ratio was experimentally

determined for each cell line. Synergy (

S

) is defined as the

fractional difference between the observed (

S

o

) and the

predicted survival for each treatment given alone (

S

1

x

S

2

):

The observed response was determined using two assays:

the clonogenic assay and the confluence assay.

Results

The alpha/beta ratio for HCC-1954 (ER-/PR-/Her2+) and

BT474 (Luminal B ER-/PR+/Her2+) are found to be 35 Gy

and 5 Gy respectively, highlighting a heterogeneous

treatment response. The survival of HCC-1954 was not

affected by Trastuzumab alone, but when combined with

radiation, a synergistic interaction was observed. BT-474

showed a 20% decrease in survival when exposed to

Trastuzumab alone, but a combined treatment with

radiation did not yield the expected decrease in survival,

indicating an antagonistic interaction.

Conclusion

Our results show that before starting clinical trials, the

combination of radiation therapy and combined targeting

agents needs to be closely examined for each sub-type

under consideration. The assumption that a combination

of treatments will result in a synergistic response is clearly

not always true.

Acknowledgements

We acknowledge funding from the Sydney Breast Cancer

Foundation

PO-0919 Stereotactic radiotherapy for brain

metastases : Cyberknife versus VersaHD / ExacTrac

M. Perdrieux

1

, M. Celeste

1

, I. Lecouillard

1

, E. Nouhaud

1

,

C. Blay

1

, F. Jouyaux

1

, N. Delaby

1

, J. Bellec

1

, C. Lafond

1

1

Centre Eugène Marquis, Radiotherapy, Rennes CEDEX,

France

Purpose or Objective

The aim of this study was to compare dosimetric and

geometric performances of the CyberKnife (Accuray) and

VersaHD (Elekta) with the ExacTrac system (BrainLab) in

stereotactic radiotherapy for brain metastases.

Material and Methods

This study was conducted on 10 patients for Cyberknife M6

v10.6 with Iris collimator and VersaHD equipped with

ExacTrac v6.1 and the Frameless system (BrainLab). The

prescribed dose was 27 Gy in 3 fractions with 1mm margin

between CTV and PTV for both modalities. The dosimetric

study was also conducted with 2 mm margin for VersaHD

plans in accordance to our clinical practices.

Plans have been computed for CyberKnife with non-

isocentric non-coplanar beams generated by inverse

optimization on Multiplan v5.3 (Accuray) with the

RayTracing dose calculation algorithm. For VersaHD, 4

non-coplanar arcs (VMAT) have been generated b y inverse

optimization on Pinnacle v9.10 (Philips ) with the

Adaptative Convolution algorithm. For each case, plans

were normalized to obtain the same PTV co verage at +/-

0.2 %.

The comparison was based on the brain volume outside

PTV receiving 23.1 Gy. The volume of isodoses 6 Gy, 2.7

Gy and 1 Gy have been reported as well as the Paddick’s

Gradient Index to characterize the dose gradient around

PTV and the spread of low doses.

Quality controls have been performed with Gafchromic

EBT3 films (Ashland) and with an ionization chamber

(Pinpoint 31014 /PTW) in an anthropomorphic phantom

(STEEV/CIRS). The measured dose with film has been

compared to the calculated dose according to the gamma

index method with a 3% (local) / 2 mm criteria (analytical

threshold : 30% of the maximum dose). The geometric shift

between the measured and calculated dose distribution

has been also reported.

Results

Table 1 shows that dosimetric criteria for plan validation

were reached for both modalities and both margins.

Compared to VersaHD, dose gradients obtained with

Cyberknife were greater and lower volumes of healthy

tissue received doses below 6 Gy.

Ionization chamber measurements showed mean

differences with the calculated dose of 2.53% and 0.03%

for Cyberknife and VersaHD respectively. The mean value

of the gamma index was 0.42 for the Cyberknife and 0.38

for the VersaHD. The mean geometric shifts between the

measured and calculated dose distributions were 0.87 mm

and 0.84 mm for Cyberknife and VersaHD respectively.