Partial Stereotactic Ablative Boost Radiation Therapy in Bulky Tumors - A Retrospective, Single Center’s Experience

Abstract

Bulky tumors are difficult to achieve effective local control by conventional radiotherapy. This study evaluates the toxicity and effectiveness of Partial Stereotactic Ablative Boost Radiation Therapy (P-SABR) in treatment of bulky tumors. From Sep 2012 through Jul 2018, 108 patients with bulky tumors treated with P-SABR technique were analyzed. Median tumor size was 10.1 cm (5.03–22.73 cm). P-SABR consisted of one partial SABR plan (5 to 12 Gy per fraction in 2 to 6 fractions) to gross tumor boost (GTVb) followed by one conventional plan to the planning target volume (PTV). GTVb was the max volume receiving SABR to guarantee the PTV margin dose falloff to about 3 Gy per fraction, and the total dose of PTV margin must reach 60Gy. Forty-six treatments were delivered to lung, 22 to urothelial cancer, 13 to sarcoma and 22 to other sites in the body. With a median follow-up of 19 months (range, 2–81 months), 1 -year and 2-year overall survival were 63.8% and 37.0%, respectively. One patient (0.9%) had complete response (CR), 36 patients (33.6%) had partial response (PR), 28 patients (26.2%) had stable disease (SD) and 42 patients (39.3%) had progressive disease (PD). However, only 5 patients (4.7%) were presented with a RT-in-field- progression, the others all had local response but systemic progression. One-year local control rate was as high as 93.1% and 2-year was 86.5%. Furthermore, one-year and 2- year overall survival was 57% and 29.7%, respectively and 2-year local control rate was 89.8% even in patients with advanced bulky tumors (tumor size >8cm). In addition, B90 (the ratio of BED>90GyE to the in-field tumor) >80% was significantly associated with higher local control rate (HR = 0.436; 95% CI, 0.195–0.973). No severe acute side effects(i.e.>CTCAE grade III) was observed. For late side effects, only 1 patient developed grade IV esophagitis, 1 developed grade IV myelosuppression and 1 with grade IV radiation pneumonitis. P-SABR is feasible and well tolerated in bulky tumors. Local control rate is encouraging, especially for B90> 80% group.