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S269

ESTRO 36 2017

_______________________________________________________________________________________________

treated with PBS PT between 1999 and 2015. Median age

at diagnosis was 3.3 years (range, 0.3-17) and the

male/female ratio was 1.44. The median delivered dose

was 54 Gy(RBE) (range, 40–74.1). Post PT brain alterations

(white matter lesions [WML] and RN) were defined as a

new area of abnormal signal intensity on T2-weighted

images or increased signal intensity on T2-weighted

images and contrast enhancement on T1 occurring in the

brain parenchyma included in the radiation treatment

field, which did not demonstrate any abnormality before

PT. RN was graded according to the National Cancer

Institute Common Terminology Criteria for Adverse Events

v4.03 and EORTC grading systems. The median follow-up

period for the surviving patients was 49.8 months (range,

5.9-194.7).

Results

Twenty-nine (17%) patients developed RN with a median

time of 5 months (range, 1-26), the majority of them

(

n

=17; 59%) being asymptomatic (grade 1). Grade 2, 4 and

5 toxicities were observed in 8, 2 and 2 patients,

respectively. The observed toxicity reversed in a majority

of cases (

n

=18; 62%). The observed WML rate was 11%

(n=18 patients). The 5-year RN-free survival was 71%. In

univariate analysis, neo-adjuvant chemotherapy (

p

=0.025)

and hydrocephalus before PT (

p

=0.035) were significant

predictors of RN.

Conclusion

Children treated with PT demonstrated a low prevalence

(7%) of symptomatic RN. Exposure to chemotherapy before

PT and hydrocephalus as an initial symptom were

significant risk factors associated with RN in these

children/AYAs.

OC-0516 Brainstem linear energy transfer in intensity-

modulated proton therapy of paediatric brain tumours

L.F. Fjaera

1

, Z. Li

2

, K.S. Ytre-Hauge

1

, L.P. Muren

3

, D.

Indelicato

2

, Y. Lassen-Ramshad

4

, G.M. Engeseth

5

, M.

Brydøy

5

, S. Flampouri

2

, O. Dahl

5

, C.H. Stokkevåg

5

1

University of Bergen, Department of Physics and

Technology, Bergen, Norway

2

University of Florida, Department of Radiation

Oncology, Jacksonville, USA

3

Aarhus University Hospital, Department of Medical

Physics, Aarhus, Denmark

4

Aarhus University Hospital, Department of Oncology,

Aarhus, Denmark

5

Haukeland University Hospital, Department of Oncology

and Medical Physics, Bergen, Norway

Purpose or Objective

The enhanced linear energy transfer (LET) and relative

biological effectiveness (RBE) at the end of a proton track

is usually only accounted for qualitatively during

treatment planning. Intensity-modulated proton therapy

(IMPT) plans are currently optimised using dose/volume

constraints for the brainstem, but there is growing

concern about the clinical consequences of the

elevated LET surrounding a tumour volume and potentially

within nearby organs at risk. For posterior fossa tumours

invading or in close proximity to the brainstem, the

brainstem may receive an unintended increased biological

dose. The aim of this study was to investigate how various

posterior fossa tumour locations impact the LET and

biological dose distributions in the brainstem.

Material and Methods

The brainstem was contoured on CT/MRI images on a five

year old male patient with a posterior fossa tumour

treated with protons. Multiple IMPT treatment plans were

generated (in Eclipse, Varian) for different simulated

tumour locations relative to the brainstem: full overlap

between tumour and brainstem (A), half overlap (B),

juxtaposed posteriorly (C) and 1 cm posterior of the

brainstem (D). Two lateral and one posterior non-coplanar

fields were applied for all plans. The dose prescription was

54 Gy(RBE) and brainstem constraints were applied based

on published metrics to keep the risk of brainstem necrosis

<5%. All plans were optimized using a fixed RBE value of

1.1. The dose-averaged LET (LET

d

) as well as the dose-

weighted LET

d

(LET

d

x dose) were subsequently calculated

using the FLUKA Monte Carlo code. The dose-weighted