ESTRO 2024 - Abstract Book

S4624

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

evaluated variable RBE weighted dose distributions in the brainstem and spinal cord with the Unkelbach model [3]. This model was chosen since it assumes an RBE of 1.1 in the midpoint of a spread out Bragg peak; models based on in vitro data (e.g. McNamara) might predict higher RBE than the Unkelbach model [4], given the low α/β of the brainstem injury endpoint and dose per fraction but may have little validity for this endpoint.

Results:

For the ATRT treatment plans, the mean LETd in parts of the brainstem receiving more than 50 Gy(RBE) varied highly as seen in Table 1 and Figure 2, and was higher than 3 keV/µm in all but one of the treatment plans. That plan, in turn, had the highest LETd in the spinal cord, due to the use of fields with parts of the distal edge in the spinal cord, while all other centers used fields with parts of the distal edges overlapping with the brainstem. The D2% to the brainstem and spinal cord was increased across all plans when applying the Unkelbach RBE (Table 1). For the ependymoma treatment plans, the mean LETd in the brainstem volume receiving more than 50 Gy(RBE) was lower and varied much less across plans due to the fields typically ranging out beyond the brainstem and not in the brainstem as in the ATRT case. The same tendency was seen for the spinal cord (Table 1). Table 1 : Median and range of mean LETd within the 50 Gy(RBE) isodose overlap with the brainstem, spinal cord or CTV, and the range of the increment in the D2% from applying the Unkelbach model rather than RBE=1.1 across the ten treatment plans.

ATRT

Ependymoma

Brainstem

Spinal cord C1 CTV

Brainstem

Spinal cord C1

CTV

LETd range (keV/µm) [2.8;3.6]

[2.7;3.8]

[2.7;3.0]

[2.5;2.8]

[2.5;3.0]

[2.5;2.7]

LETd

Median

3.3

3.0

2.9

2.7

2.8

2.7

(keV/µm)

Unkelbach

D2%

increment

range

[0.7;3.3]

[0.4;2.0]

[0.1;1.2]

[0;1.0]

(Gy(RBE))