Treating moving targets with scanned proton therapy: Is 5
mm initial tumour motion a safe threshold?
L.A. Den Otter1, R.M. Anakotta1, M.D. Weessies1, M. Dieters1, C.T. Muijs1, S. Both1, A.C. Knopf1
1: University of Groningen – University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands
Purpose/Objective
Pencil beam scanning (PBS) is a highly conformal technology to treat cancer. The time structure of PBS makes the treatment of moving tumours challenging due to the interplay effect. According to literature, PBS in combination with rescanning can be safely applied without motion mitigation strategies to lung tumours that move 5 mm or less. However, the question is whether the motion measured during treatment simulation will remain below 5 mm during the course of treatment. We investigated the inter-fractional lung tumour motion variation in a unique data set providing five repeated 4DCTs per patient, to evaluate if a 5 mm threshold is a reliable indicator for considering PBS treatments.
Material/Methods
For 40 NSCLC patients (24 male, 16 female, age: 47-89, stage: II-IV) weekly 4DCT imaging was performed during treatment simulation before and during the treatment course to monitor the anatomical changes and differences in motion. Gross tumour volumes (GTV) were delineated on all phases of the planning 4DCT and on the weekly repeat 4DCTs. GTV volume changes were acquired and the weekly inter-fraction motion variation was evaluated by measuring the GTV centroid translations in all three directions.
Proton radiotherapy is a very precise technology to treat cancer and at the same time spare healthy tissue. However, treating lung tumors that move due to breathing remains a
challenge. According to literature, tumors that move less than 5 mm can safely be treated without additional techniques to limit breathing motion. We investigated the potential use of this threshold in clinical practice, by measuring the variation in tumor motion during the five weeks of radiotherapy treatment for 40 lung cancer patients. We found that the initial motion of less than 5 mm remained under 5 mm for 66% of the patients. With one third of the patients showing larger variations in motion, motion monitoring remains crucial, even for seemingly minor moving lung tumors. This way a safe and highly precise treatment can be guaranteed.
