Compared with RO, BM usually means a case far from no evidence of disease (NED) and is associated poor survival. Thus, identifying BM and RO correctly is vital for advanced CRC patients. Pathology remains the gold-standard for diagnosing BM, however, it is invasive. Imaging is the most widely used method for diagnosing BM ( O'Sullivan et al.2015). MRI is acknowledged for diagnosing BM, especially for early BM. BM often character as discrete foci of low T1-signals because of the replacement of normal fatty marrow by malignant cells; on a T2-weighted sequence, BM usually present T2 hyperintensity due to the elevated water content and gadolinium enhancement due to increased vascularity associated therewith ( O'Sullivan et al.2015). Biochemical-markers contribute to diagnosis of BM usually include tumor biomarkers, ALP, N- and C-terminal propeptide of procollagen type I (PINP and PICP), tartrate-resistant acid phosphatase isoform, bone sialoprotein, etc. [(Huang, Q.et al, 2012).
A history of pelvic irradiation is important for diagnosing RO. Besides, RO is featured by mottled areas of bone, with osteopenia, coarse trabeculation, and areas of focally increased bone density. The changes usually start at the iliac side of the sacroiliac joint and process to involve the entire joint (Bluemke et al., 2019). Blastic, or mixed blastic and lytic, lesions in the field of radiotherapy without associated masses usually suggest RO in CT (Yousem et al., 1989) . Yoshioka et al. found MRI signal patterns in RO are of low signal intensity (in T1-weighted images), mixed signal intensity around sacroiliac joints, and high signal intensity in the peripheral areas (in T2-weighted images), indicating edematous changes, and gadolinium enhancement may be seen due to fibrotic changes (Yoshioka et al., 2000). Meixel et al. defined a RISC (Radiation-Induced Sacral Change) classification according to MRI, in which RO showed iso-/hypointense responses in T1wi, hyperintense responses in T2wi, hyperintense (punctuated or confluent) responses in T1wi +contrast, and hyperintense response in turbo-inversion recovery magnitude (Meixel et al., 2018).
These case series provide experience for differentiating RO from BM correctly in advanced CRC patients. There’s no study with a focus on this condition at the time of writing. In our cases, we found that the age of both patients was around 60 years, and the time-to-onset of RO was about one year after radiotherapy with the dose exceeding 50 Gy. The main regions of bone changes were the sacrum and ilium. These were consistent with previous studies which focused on RO ( Meixel et al., 2018, Yoshioka et al., 2014) . Increased CEA was not sufficiently valuable to indicate changes in bone because of the solitary metastasis of other part, and we found the ALP was normal during treatment. Radiology, especially MRI, was considered as the best technique to diagnose RO. In Case 1, PET/CT showed the abnormal uptake of sacrum, which usually implies bone metastasis. Although manifestations in MRI/CT implied that the bone changes were unilateral with cortical destruction therein, but there were no significant changes after anti-tumor therapy and anti-osteoporosis. We finally diagnosed the patient as solitary metastasis of peritoneal cavity with RO. In Case 2, during the anti-tumor treatment, PET/CT exhibited the abnormal uptake of sacrum. This was confused when trying to confirm the bone change was BM or some other cause. However, the bone changes in MRI/CT showed bilateral and complete cortical bone. The patient was considered as solitary metastasis of the left accessory area with RO and underwent a surgery to remove the solitary metastasis in October 2022.
The strength of the case series is such that we provide complete and novel imaging related to RO, which is vital for the radiologist to understand the imaging characteristics, location, and morphology of RO better. The limitation of the present research is that the lesions were not biopsied because of the age of the patient and risk of complications. However, MDT discussion was undertaken, which was acknowledged as being important for patients in that it avoided unnecessary biopsy.
In conclusion, our cases indicated that isolated bone change in advanced CRC patients may be RO rather than BM, so the patients could receive different treatments. However, more specific standards for differential diagnosis are warranted.