Accelerated Hypofractionated Radiation Therapy For The Central And Ultracentral Tumors Of The Lung - Analysis Of Doses To The Organs At Risk

Accelerated hypofractionated radiation therapy (AHRT) has been considered effective in treating central tumors of lung in decreasing severe complications regarding serial organs at risk (OARs). We have been treating AHRT using 75 Gy/25 fr/5 wks regimen. To support the safety of AHRT, we investigated the doses to the OARs. Central tumor is defined as those located within 2cm from proximal bronchial tree (PBT) and/or located very close to mediastinal organs. Among central tumors ultracentral tumor is defined as those which is faced with lobar bronchus, esophagus, and/or major arteries, and/or whose planning target volume (PTV) overlaps trachea and/or main bronchus. Internal target volume (ITV) was created using 4-DCT. Then 5mm PTV margin was added to create PTV. D50% of PTV volume was set at the prescribed dose. AHRT was delivered with 7-8 fixed non-coplanar converging beams using 6MV X-rays. Usually, IMRT was used to decrease the dose to the OARs. No elective nodal irradiation was performed. Regarding OARs, average D1ccs of pulmonary artery (PA), aorta, superior vena cava (SVC), and proximal bronchial tree (PBT) were calculated. Also, average BED3Gys of D1cc in each OAR were calculated to evaluate late toxicities of late responding tissues. Sixty-six cases with 70 tumors treated between January 2013 and February 2019 were analyzed. Fifty-one tumors were primary, and 19 tumors were metastatic. Thirty-eight tumors were ultracentral and 32 tumors were central. Total dose ranged from 51 Gy to 75 Gy with the median of 75 Gy. Total dose was decreased depending on the high dose to OARs. Median follow-up period was 28.3 months. For ultracentral tumors, average D1cc of PA (15 cases), aorta (11 cases), SVC (2 cases), and PBT (17 cases) were 69.56 Gy±9.29 Gy, 71.72 Gy±5.19 Gy, 74.56 Gy±1.48 Gy, 59.60 Gy±13.64 Gy, respectively. Corresponding BED3Gys of D1cc were 139.53 Gy±25.16 Gy, 142.87 Gy±14.18 Gy, 150.20 Gy±2.31 Gy, 113.65 Gy±34.95 Gy, respectively. Corresponding figures for central tumors were relatively low compared with ultracentral tumors. No severe serial organ toxicity has been encountered. Overall local control rate at 2 years was 79.4%. Overall survival rate of overall cases at 2 years was 81.9%. AHRT was relatively safe treatment method in treating central and ultracentral tumors of lung without severe serial organ toxicities and the doses to OARs were restricted at the reasonably low level compared with ordinal SBRT, the BED3Gy of which would reach 200Gy.