MLS analysis of INSIDE in-beam PET images for the detection of morphological changes in patients treated with protontherapy

Anatomical changes in proton therapy patients can affect the dose distribution and contribute to range uncertainty. In-beam Positron-Emission-Tomography (IB-PET) is a non-invasive technique that can be used to obtain information about morphological changes in the irradiated tissue. At the National Center of Oncological Hadrontherapy (CNAO) the INSIDE IB-PET scanner has acquired IB-PET data of eight proton treated patients enrolled for the clinical trial ID: NCT03662373. Our purpose is to assess the sensitivity of the INSIDE IB-PET system in detecting morphological changes by means of the Most-Likely-Shift (MLS) method.The MLS method allows to quantify range variations by comparing the profiles along the beam direction in two different PET images. It was tested on six patients without and two with morphological changes.The sensitivity of the INSIDE in-beam-PET scanner was evaluated as the standard deviation of the range difference distributions averaged over six unchanged patients, that was found to be 2.5 mm. For the patient where small morphological changes were identified in the control CT, the standard deviation was found to be larger and anomalously large range variations were localized in the same zones observed with the control CT.