ESTRO 2024 - Abstract Book

S3508

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

OARs, dose-volume constraints are seldom used, and then only for the most parallel OARs. Therefore, more prospective studies with rigorous dose reporting are pressingly needed. We urge that not only cumulative EQD2 data but also alpha/beta and recovery factors be reported. This would simplify the conversion to different fractionations and aid in the development of strategies to convert first-course dose constraints to reRT constraints.

Keywords: dose constraints, re-irradiation, multi-center

841

Mini-Oral

The effect of dosimetric and radio-sensitivity variation on dose constraints

Joep Stroom, Sandra Vieira, Carlo Greco

Champalimaud Foundation, Radiation Oncology, Lisbon, Portugal

Purpose/Objective:

In previous work, we showed that published dose constraints (D p ) depend on the delivered dose variation SD(D p ) in the patient cohort during the dose-effect studies. Assuming that patients receiving the x% highest doses cause the (accepted) x% NTCP, the true critical doses are higher than the published values given by the following recipe:

-1 (1-NTCP)×SD(D

D true = D p + Φ

p ), (1)

with Φ -1 the inverse cumulative normal distribution. The difference between D p and D true can be considered a safety margin for OARs. This can then be used to e.g. estimate true dose constraints or to adjust planning dose constraints in different situations. However, in Eq.1 it was assumed that inter-patient radio-sensitivity variations will cancel out. Here, the combined effect of dose and radio-sensitivity variations on dose constraints is derived and validated with monte-carlo simulations.

Material/Methods:

In this case, we assume that patients with the x% severest radiation effect E will yield the x% NTCP. Similar to Eq.1 we can write for the true threshold effect E true

-1 (1-NTCP) × SD(E

E true = E p + Φ

p ). (2)

To calculate E we chose the LQ model:

2 /(α/β)), (3)

E = n f × α (d + d

with radio-sensitivity given by α and β . We can now obtain d true (=D true /n f ) by replacing E in Eq.2 with Eq.3, applying the ABC-formula to obtain d true , and reordering the terms: