TLS is a potentially lethal oncological emergency in which massive tumor cell destruction causes severe electrolyte and metabolite abnormalities secondary to the release of intracellular components into the bloodstream, resulting in hyperuricemia, hyperkalemia, hyperphosphatemia, and secondary hypocalcemia. Hyperuricemia and hyperphosphatemia induce acute renal injury owing to uric acid precipitation and calcium phosphate deposition in the renal tubules. Hypocalcemia and hyperkalemia can also cause electrocardiographic abnormalities, arrhythmias, neuromuscular symptoms, and seizures. Following the introduction of the Cairo–Bishop definition, proposed in 2004, which provides TLS diagnostic criteria, TLS can now be diagnosed clinically, using laboratory values [5, 6]. The present patient met the Cairo–Bishop definitions of laboratory and grade II clinical TLS. After admission, the patient was treated with a high volume of rehydration fluid, glucose insulin therapy for hyperkalemia, and rasburicase for hyperuricemia. On the second day after admission, the patient developed progressive metabolic acidosis, and hemodialysis was initiated. On the third day after admission, the patient’s condition worsened. At this point, we considered the presence of the tumor to be related to the worsening condition; thus, we performed an emergency mastectomy under local anesthesia in the ICU. Unfortunately, the patient did not respond to these treatments and eventually died. At this point, we considered that the presence of the tumor was associated with worsening of the condition. Therefore, although not standard of care, we performed an emergency mastectomy under local anesthesia in the ICU. Unfortunately, the patient did not respond to these treatments and ultimately died.

There were two reasons why we decided to perform surgery. The first reason was that the patient’s general condition had deteriorated to the point where she developed consciousness disorders, and there was no other systemic treatment available. The second reason was that tumor removal might have been significant, considering the mechanism of tumor lysis syndrome. However, there have been no reports on the effectiveness of surgical resection of tumors in the treatment of tumor lysis syndrome, so it remains unclear whether this procedure was appropriate.

In the present case, although the patient had a solid tumor and was at a low risk for TLS, a prophylactic uric acid-lowering drug may have been indicated because of the large tumor volume and slightly elevated uric acid level (8.4 mg/dl prior to treatment). Therefore, control of elevated uric acid levels is important to prevent renal dysfunction. Furthermore, recognizing the high risk of TLS and assessing the risk factors prior to treatment are of utmost importance. In the present case, the high efficacy of letrozole was likely the trigger for TLS although the possibility of renal failure due to Mohs paste cannot be ruled out. In particular, clinical TLS reportedly increases mortality rates (83 vs. 24%; p < 0.001) [7]. The development of acute kidney injury associated with TLS is a strong predictor of mortality [8]. Regardless of cancer type, the mortality rate increases by 20–50% in cases of undiagnosed or delayed TLS diagnosis in solid tumors [9]. The best TLS management is prevention. Omori et al. [10] previously reported that prophylactic infusion and lowering uric acid levels prevented TLS in patients with breast cancer with high tumor volumes and hyperuricemia.

TLS has also been commonly reported in hematological malignancies but is becoming more frequently noted in solid tumors as treatments become more efficient [11]. Table 2 (modified from Watkinson and Hari Dass [3]) summarizes all reported TLS cases caused by breast cancer treatment. A total of 22 TLS cases associated with breast cancer have been reported, including three with hormone therapy only (one with tamoxifen and two with letrozole), three with hormone therapy plus a cyclin-dependent kinase 4/6 inhibitor or PIK3CA inhibitor, two with anti-HER2 therapy, nine with chemotherapy, two with radiation therapy, and three without therapy. Overall, chemotherapy, hormone therapy, molecular-targeted drug therapy, radiation therapy, and no therapy can all cause TLS.