Peter Magnus Trock Lægdsmand

PhD student The Danish Centre for Particle Therapy, Aarhus University Hospital

Optimal Proton Therapy for Pediatric Posterior Fossa Tumors - PhD project

Central nervous system tumors are the most common solid tumors occurring in children. Radiotherapy (in particular, proton therapy, PT) is an essential part of the multi-modality treatment regimes for these tumors. However, these tumors often occur in the posterior fossa, which makes treatment difficult because sparing the brainstem is a critical priority. Treatment planning of PT is, therefore, a delicate balance between maximizing target coverage and minimizing dose to the brainstem.

Unfortunately, there are many uncertainties regarding the biological effect of proton therapy compared to conventional photon therapy. In particular, the relative biological effectiveness (RBE) varies with the pattern of energy deposition - so-called linear energy transfer (LET). To mitigate this uncertainty, treatment planners are wary of minimizing high LET, found near the end of range of the proton beams. Current strategies are based on rules of thumb obtained from older treatment techniques and may vary significantly between planners. In this project, we aim to harmonize the strategies for handling these uncertainties through inter-center collaborations and developing clinical guidelines. By calculating LET distributions in treatment plans, treatment plans with significant uncertainties in the relative biological effect can be identified. Analyzing these distributions allows a more precise balance between sparing the brainstem and target coverage.

An already scheduled treatment planning workshop surveying the clinical practice across European PT centers will form the basis of the project. The study will assess the techniques used by different centers to mitigate uncertainties in biological effects in the brainstem and the priority of different treatment goals in the planning process. The workshop will provide insight into how different practices affect LET distributions and how the RBE uncertainty in the brainstem can be controlled without high costs regarding target coverage and normal tissue sparing. Based on the study, clinical guidelines will be proposed to harmonize the treatment planning process. The uptake of these guidelines among PT centers will be assessed through a follow-up survey.

A separate study will examine the capability of using LET optimization in the treatment planning process. LET optimization will be used on a cohort of posterior fossa patients treated at the Danish Center for Particle Therapy. The resulting treatment plans will be compared to those produced using a conventional planning technique. This comparison will assess the benefits of using LET optimization clinically.

The project is ultimately expected to lead to more homogenous and effective treatments of pediatric patients with posterior fossa tumors and a reduction in the risk of normal tissue effects after radiotherapy.

Duration: 2022-2025
Ludvig Muren, Professor and medical physicist, Aarhus University, DCPT
Laura Toussaint, Postdoc at Aarhus University, DCPT
Anne Vestergaard, Medical Physicist, DCPT
Yasmin Lassen-Ramshad, Oncologist, DCPT