ESTRO 2024 - Abstract Book

S4606

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

ESTR0 2024

received 54 GyRBE and/or above (V _54 ), subvolume receiving between 42-53 GyRBE (V _42 ) and subvolume receiving lower dose - between 32-41 GyRBE (V _32 ). Subsequently, their near max LETd values (L 2% ) were extracted.

Results:

Table 1. exemplifies the extracted values of L 2% for whole OARs, their subvolumes and their corresponding absolute volumes. Average L 2% values for the whole organ where between 6,0 keV/µm for chiasm and 8.7 keV/µm for the brainstem, with certain maximum values of 20.6 and 23.4 keV/µm for chiasm and optic nerve, respectively. For one of the patients max L 2% value in cochlea was reaching 52.1 keV/µm and 19.5 keV/µm in brainstem, however all those increased values of LETd were found in the very low dose regions (<5 GyRBE). Once the dose driven subvolumes were considered, that have received a meaningful dose, reported LETd value where much smaller. Average L 2% for OAR subvolume receiving 54 GyRBE and above was between 2.7 keV/µm (cochlea) and 3.4 keV/µm (brainstem) with the max value for brainstem of 5.2 keV/µm. It is worth to mention that absolute volumes of the subvolumes receiving 54 GyRBE were quite small (below 0.1 cc for optic nerve, bulbus and cochlea, while for chiasm it was below 0.3 cc and brainstem below 1 cc). Similar L 2% values were found for 2 other lower dose subvolumes (V _42 and V _32 ). Average for V _42 was between 3.1 keV/µm for cochlea and 4.2 keV/µm for brainstem, while for V _32 it was between 3.0 keV/µm (cochlea) and 5 keV/µm (brainstem).

Conclusion:

We have previously looked at the dose and LETd distributions of those patients in context of present RICE incidences. The ultimate goal of this evaluation is to report the LETd values for all the patients within the study, expand it for other OARs and correlate the received doses of the specific OAR with the LETd values. To predict what is the acceptable LETd value per organ, results will be compared with the reported toxicities. Our initial results show that our planning strategy is not only consistent in terms of the expected LETd values for evaluated OARs, but also safe from the OARs constraint dose limit perspective. What is the LETd constraint for the OARs remains to be answer with wider evaluation.

Keywords: Proton treatment planning, LETd of OARs,