Introduction: In the context of radiotherapy, control of breakthrough cancer pain (BTPc) is particularly challenging. BTPc has been defined by the Spanish Society of Pain (SED), the Spanish Society of Medical Oncology (SEOM) and the Spanish Society for Palliative Care (SECPAL) as a sudden and transient exacerbation of pain of great intensity (VAS > 7) and short (less than 20-30 minutes), which appears on the basis of a stable persistent pain when it is reduced to a tolerable level (VAS < 5) by using major opioids. Objectives: The main objective of this study was to assess the intensity of BTPc induced by cancer treatments that included radiotherapy (RT), both exclusive and associated with che-motherapy (RT/CT). Secondly, the efficacy of treatment was evaluated with fentanyl sublingual scheduled for BTPc control. Material and methods: Retrospective, observational study in 110 patients recruited in 19 Spanish Radiotherapy Services. Patients must have BTPc induced by RT or RT/CT, with or without medication prescribed and with an intensity outside a VAS > 6 in the last 24-48 h. Controls were established at baseline and at 3, 7, 15 and 30 days. Results: There was a decrease in mean values on the VAS scale as the study progressed (VAS = 6 in the control 0 to VAS = 3 in the control 3) and the differences were significant (p < 0.0001). Treatment satisfaction was rated as good or excellent by 85.3% of patients and 92.7% of researches. Conclusions: The results of this study demonstrate the efficacy of BTPc treatment with sublingual fentanyl in the context of the radiotherapy cancer treatment, with a significant decrease in VAS from baseline values. The high satisfaction among physicians and patients with this treatment reflects the efficacy and convenience of sublingual fentanyl in controlling BTPc.