Prostate cancer has a high risk for biochemical recurrence, with incidence ranging from 27–53% [25]. Some studies have demonstrated that the mean interval between biochemical recurrence (PSA re-elevation over 0.2 ng/ml twice after radical prostatectomy) and clinical recurrence (macroscopic appearance of recurrent lesion that can be identified by imaging or histological examination) is approximately 8 years, suggesting that long-term observation is required for patients with prostate cancer [26]. For this patient, we should have suspected the possibility of a clinical recurrence when CT revealed a lung nodule because biochemical recurrence after radical prostatectomy was already observed in the recent three years. However, it was not suspected because of the following reasons. The CT findings of this case included a solitary lesion with spiculation, which is more typical of primary lung cancer, rather than a metastatic lung tumor, and the serum PSA level was still within the normal range. Additionally, solitary lung metastasis from prostate cancer is extremely rare. Radiologic evidence of pulmonary metastases after surgery, chemotherapy, or radiotherapy for prostate cancer was detected in 48 out of 1290 patients (3.6%) while a solitary pulmonary nodule was detected in only 11 out of 1290 patients (0.85%) [2]. For these reasons, clinical recurrence of prostate cancer was not strongly suspected preoperatively.
To date, 35 patients with solitary lung metastasis from prostate cancer, including our case, have been reported. We reviewed 23 cases described in detail (Table 1). The mean interval from biochemical recurrence to clinical recurrence is 3.6 years (range 0 ~ 8 years), respectively. This result is shorter than the reported interval [26]. This might be influenced by risk factors for metastasis, such as advanced stage (pT3 or more) and high-grade cancer (Gleason Score > 8), because 57% (13/23) patients have either or both factors. The mean interval from initial treatment for prostate cancer to clinical recurrence is 6.5 years (range 0.9 ~ 15 years), respectively. This trend of recurrence seems completely different from other malignancies such as colon cancer. Based on these findings, the possibility of a solitary lung metastasis from prostate cancer should be considered even when initial treatment had been performed more than 5 years prior and moreover, once biochemical recurrence is detected.
Previous studies have shown that metastases from prostate cancer with normal serum PSA levels suggest high-grade cancer, small cell carcinoma, neuroendocrine tumor, or neuroendocrine differentiation [27]. Among the prior 23 cases, high-grade cancer or neuroendocrine differentiation was found in 9 patients, with most (8/9) of their serum PSA levels in the normal range. However, even among patients with low-grade cancer, normal serum PSA levels were found in more than half (6/10). In all cases, 74% (17/23) had isolated lung metastasis with normal PSA levels regardless of the histology. Therefore, it is necessary to consider the possibility of clinical recurrence from serum PSA fluctuations even in those with normal values like in our case.
The standard procedure for metastatic lung tumors is partial resection aimed at the preservation of lung parenchyma. Among 35 patients with solitary lung metastasis, 22 patients had undergone lung resection, with 14 of them reporting satisfactory outcomes. Lobectomy was performed in 9 patients, including our case. The reasons of this procedure in some cases are highly suggestive. In four cases, intraoperative frozen section examinations were unable to determine whether the tumor was metastasis or primary lung adenocarcinoma [5, 14, 17, 24]. Considering these previous case studies, there is a high likelihood of misdiagnosis of prostatic adenocarcinoma with lung metastasis as a primary pulmonary neoplasm in clinical practice. Copeland et al. reported the presence of a tubule-papillary or a carcinoid-like histologic pattern in a pulmonary tumor does not readily associate it with the histologic features of prostatic carcinoma [28]. Thus, in clinical practice, it is prudent to consider solitary lung metastasis from prostate cancer as a differential diagnosis for patients previously treated for prostate cancer.
Pulmonary metastatic lesions which underwent metastasectomies have been closely related to survival in various cancers, including colon cancer and uterine cervical cancer [29, 30]. The number of metastases from prostate cancer is considered an independent prognostic factor affecting the 5-year cancer-specific survival rate, of which characteristic is same as other malignancies previously mentioned. The 5-year cancer-specific survival rate for patients with one metastasis is 90%, compared to 32% for those with two or more. [31]. Surgical resection, especially for solitary pulmonary lesions, is associated with favorable outcomes in prostate cancer patients.