Prostate cancer -Follow-up of systemic therapy

Both bone scanning and CT are heavily used to assess response to systemic treatment for bone metastases, but they have substantial limitations. On bone scanning, uptake usually decreases after chemotherapy/hormone therapy or radiotherapy if a response is obtained. However, in prostate cancer patients, a "flare" phenomenon may be observed, where uptake initially increases after chemotherapy or hormone therapy, peaking at 6 weeks after treatment as bone turnover increases as part of the healing process [65, 66]. Thus care must be taken to avoid mistaking apparent new lesions for areas of new metastatic disease, when subtle changes were in fact present in prior studies. Computed tomography cannot be used to differentiate healing bone from progressive disease, but it can play a role in the assessment of more aggressive lytic lesions with potential for pathologic fracture. Bone marrow MRI or FDG-PET (or FDG-PET/CT) is better than bone scanning or CT for distinguishing between active bony metastasis and healing bone [67]. In addition, FDG-PET or FDF-PET/CT allows quantitative assessment of tumor response not only in terms of tumor volume and size, but also in terms of the degree of metabolic activity.

¹⁸ F-Fluorodihydrotestosterone (FDHT) PET may have potential for predicting and assessing the impact of androgen receptor blocking therapies in patients with metastatic prostate cancer. Recent studies have found a mismatch between FDG-PET and FDHT-PET findings in patients with metastatic disease that appeared to suggest variations in the androgen dependence of different disease sites [68, 69].

In patients with androgen-independent prostate cancer, calculation of the bone scan index (BSI) allows quantitative assessment of bone involvement [70, 71]. The BSI estimates the fraction of the skeleton that is involved by tumor and the regional distribution of bone metastases [71]. It may be useful for stratifying patients entering treatment protocols and for assessing the response to treatment, particularly in clinical trials of agents for which PSA changes do not accurately reflect clinical outcomes [70, 71].

It appears that FDG-PET may be a promising modality for the assessment of response to chemotherapy. A preliminary study [72] found that in patients undergoing antimicrotubule chemotherapy for castrate metastatic prostate cancer, the change in the average maximum standardized uptake (SUVmaxavg) value on FDG-PET scans indicated treatment effects usually described by a combination of PSA, bone scan and soft-tissue imaging. FDG-PET optimally distinguished between progressors and non-progressors when progression was defined as a more than 33% increase in SUVmaxavg or the appearance of a new lesion [72].

Prostate cancer is a heterogeneous disease with numerous treatment options and widely varying outcomes. While imaging cannot provide a definitive answer to every question that arises in the management of the disease, it has an essential role to play in reducing uncertainty and allowing more effective and evidence-based care. Magnetic resonance imaging and PET may be the most powerful weapons in the imaging arsenal. Not only does conventional MRI offer detailed anatomic assessment of the prostate and surrounding structures, but MRSI and PET allow assessment of disease biology. As more radiologists become familiar with MRI techniques and as research with new PET tracers continues, imaging can be expected to permit increasingly precise discrimination between high- and low-risk disease, more appropriate treatment selection, and more accurate assessment of treatment response.