Using max standardized uptake value from positron emission tomography to assess tumor responses after lung stereotactic body radiotherapy for different prescriptions

PURPOSE: To retrospectively investigate tumor responses of lung SBRT patients for different prescriptions. To analyze the relation between optimal biologically equivalent dose (BED) and tumor responses. METHODS AND MATERIALS: Tumor responses after lung SBRT were compared by examining 48 treatments used four prescriptions. This study used simplified tumor response criteria: (a) Complete Response (CR) — post max SUV (SUV (post)) after SBRT in the treated tumor region was almost the same as the SUVs in the surrounding regions; (b) Partial Response (PR) — SUV (post) was smaller than previous max SUV (SUV (pre)), but was greater than the SUVs in the surrounding regions; (c) No Response (NR) — SUV (post) was the same as or greater than SUV (pre). Some SUV (post) reported as mild or favorable responses were classified as CR/PR. BED calculated using α/β of 10 Gy were analyzed with assessments of tumor responses for SBRT prescriptions. RESULTS: For the prescriptions (9 Gy × 5, 10 Gy × 5, 11 Gy × 5, and 12 Gy × 4) historically recommended by RTOG, we observed that higher BED (10) and lower tumor volume would achieve a higher complete response rate. The highest complete response rate was observed for smallest tumor volume (PTV (ave) = 6.8 cc) with higher BED (10) (105.6) of 12 Gy × 4 prescription. For 11 Gy × 5 prescription, the BED (10) (115.5) was the highest, but its complete response rate (58%) was lower than 79% of 12 Gy × 4 prescription. We observed the PTV (ave) of 11 Gy × 5 prescription was more than double of the PTV (ave) of 12 Gy × 4 prescription. For the same lung SBRT prescription (BED (10) > 100) earlier staging tumor had more favorable local control. CONCLUSION: We demonstrated post max SUV read from PET/CT could efficiently and accurately assess tumor response after lung SBRT. Although SBRT with prescriptions resulting in a BED (10) > 100 experienced favorable tumor responses for early staging cancer, escalation of BED (10) to higher levels would be beneficial for lung cancer patients with later staging and larger volume tumors.