['To assess the efficacy of multiple treatment of phosphatidylinositol-3-kinase (PI3K) inhibitor on autochthonous tumours in phosphatase and tensin homologue (Pten)-deficient genetically engineered mouse cancer models using a longitudinal magnetic resonance imaging (MRI) protocol.', 'Using 3D MRI, B-cell follicular lymphoma growth was quantified in a Pten(+/-)Lkb1(+/hypo) mouse line, before, during and after repeated treatments with a PI3K inhibitor GDC-0941 (75 mg/kg).', "Mean pre-treatment linear tumour growth rate was 16.5±12.8 mm(3)/week. Repeated 28-day GDC-0941 administration, with 21 days 'off-treatment', induced average tumour regression of 41±7%. Upon cessation of the second treatment (which was not permanently cytocidal), tumours re-grew with an average linear growth rate of 40.1±15.5 mm(3)/week. There was no evidence of chemoresistance."] Does quantitative MRI establish the efficacy of PI3K inhibitor ( GDC-0941 ) multi-treatments in PTEN-deficient mice lymphoma?