CNS relapse despite prophylactic cranial irradiation (PCI) in small cell lung cancer (SCLC).

CNS relapse after PCI may reflect either suboptimal radiation dose schedules or reseeding from other sites of active disease. A retrospective study has been undertaken to investigate these alternative mechanisms of treatment failure. Between August 1981 and December 1983, 30 patients with SCLC treated by induction chemotherapy, followed by high-dose cyclophosphamide (7 Gm/m2), were selected for PCI on the basis of clinical, radiological, and bronchoscopic CR. Pretreatment CT brain scans were normal in all patients, and 20 Gy mid-plane dose in 5 daily fractions were delivered by lateral fields to whole brain using megavoltage X rays and localizing check films. Progressive focal neurological signs of cranial metastases developed in 7/30 (23%) patients 3-11 months after PCI, confirmed on CT scanning in 4 patients. Relapse at other sites, predominantly the thorax, occurred in all seven patients at intervals of 1-6 months prior to CNS relapse. Published clinical data of tumor volume doubling times in SCLC predict longer CNS relapse-free intervals after PCI than those observed in our patients if reseeding was responsible for relapse. This suggests that incomplete eradication of intracranial disease is the main cause of CNS relapse after PCI. Higher equivalent radiation doses may improve the results of PCI.