Assessing Response to Radiation Therapy Treatment of Bone Metastases: Short-Term Followup of Radiation Therapy Treatment of Bone Metastases with Diffusion-Weighted Magnetic Resonance Imaging

This study examined the usefulness of diffusion-weighted (DW) Magnetic Resonance Imaging (MRI) in monitoring bone metastases response to radiation therapy in 15 oligometastatic patients. For each metastasis, both mean apparent diffusion coefficient (ADC) changes and high b-value DW metastasis/muscle signal intensity ratio (SIR) variations were evaluated at 30 ± 5 days and 60 ± 7 days after the end of treatment. On baseline DW-MRI, all bone metastases were hyperintense and had signal intensities higher than normal bone marrow on calculated ADC maps. At follow-up evaluations, 4 patterns of response were identified: (I) decreased high b-value DW SIR associated with increased mean ADC (83.3% of cases); (II) increased mean ADC with no change of high b-value DW SIR (10% of cases); (III) decreased both high b-value DW SIR and mean ADC (3.3% of cases); (IV) a reduction in mean ADC associated with an increase in high b-value DW SIR compared to pretreatment values (3.3% of cases). Patterns (I) and (II) suggested a good response to therapy; pattern (III) was classified as indeterminate, while pattern (IV) was suggestive of disease progression. This pattern approach may represent a useful tool in the differentiation between treatment-induced necrosis and highly cellular residual tumor. 1. Introduction Bone metastases occur from 30% to 70% of all patients suffering from cancer and commonly involve the axial skeleton [1, 2]. Breast, prostate, and lung cancers represent the main sources of bone metastases, with prostate and lung cancers being most common in males and breast cancer being most common in females [2]. Once bony metastases occur, cancer cure becomes impossible and in these cases radiation therapy, associated or not with systemic chemotherapy, may be performed for palliative intent [1, 3]. Therapy goals are to delay progression, palliate symptoms, improve quality of life, and achieve any possible survival benefit [3]. Currently, there are no universally accepted methods for evaluating the response to treatment, making it difficult to give patient the optimal management to minimize radiation dose and prevent recurrences [1, 3]. Bone scintigraphy (99mTc-methylene diphosphonate-MDP-bone scans) with plain radiographs or cross-sectional imaging, such as Computed Tomography (CT) or Magnetic Resonance Imaging (MRI), and sometimes 18F-fluoride Positron Emission Tomography (PET)/CT remain the commonest imaging methods, complementing one another in order to characterize and follow up bone marrow metastases [1, 3, 4]. Although conventionally used, none of these