Stereotactic centralized ablative radiation therapy: A novel methodology in treating bulky tumor and its technical realization.

159 Background: Bulky tumors pose significant challenges to traditional treatment modalities such as surgery, chemotherapy, and conventional radiotherapy. This study introduces a novel therapeutic approach named as Stereotactic-Core-Ablative-Radiation-Therapy (SCART), designed to deliver an ablative dose to a substantial core of the bulky tumor while quickly decreasing to a lower dose at the tumor periphery. Methods: A SCART-Treatment-Volume (STV) at the core of the GTV was predefined to administer an ablative radiation dose, aiming to induce DNA damage in cancer cells and potentially trigger biological cancer-killing effects. Utilizing Linac-based VMAT or Cyberknife techniques with 6MV photon beams, multiple radiation fields intersect and optimize at STV, generating an ablative dose at the STV. The dose rapidly diminishes to a safe level at the edge of GTV sparing the surrounding tissue. In the phase-1 trial, nineteen patients with 21 bulky tumors enrolled, receiving SCART at five dose levels (15GyX1, 15GyX2, 15GyX3, 18GyX3, 21GyX3, and 24GyX3), while maintaining the GTV's peripheral dose at 5Gy each fraction. Results: All patients completed treatment with average beam-on time of 8.9min and average treatment time of 18.5min. Mean follow-up time is 15.4 month. No grade-III or higher toxicity was observed. 7/19 patients still survive, with the overall survival of 40% at 30 months. Mean tumor volume shrinks by 60% between initial 301cc and post-SCART volumes of 118cc. Long-term follow-up revealed that 14/21 tumors achieved Partial Response, 2/21 Complete-Response, 3/21 Stable-Disease, and 1/21 Progressive-Disease, leading to an encouraging local control of 95%. Conclusions: SCART emerges as a safe and effective strategy for treating bulky malignant tumors, demonstrating excellent local control and overall survival. Multiple treatment courses were feasible. The results from phase-1 study suggest that SCART could revolutionize the treatment landscape for bulky tumors, offering a promising avenue for further exploration and application in clinical practice.