Initial Results Of 2 Different Dynamic Tracking Techniques For Stereotactic Body Radiation Therapy For Solitary Lung Tumors

Recently, stereotactic body radiation therapy (SBRT) using dynamic tumor tracking (DTT) techniques has been increasingly used. DTT technique has been successful in reducing the size of the PTV and doses to the normal tissue. We have been performing 2 different DTT techniques, namely, robot arm-based DTT (R-DTT) and gimbal-based DTT (G-DTT). We investigated the initial results of SBRT for solitary lung tumors (SLTs) treated with these DTT techniques. Eligibility criteria are as follows; primary or metastatic SLT, 10mm or more movement on planning CT, high risk operable, inoperable or operation refusal, size not exceeding 5 cm. Between March 2013 and December 2015, 28 cases received DTT SBRT in our hospital. Among them 10 received with R-DTT and the other 18 received with G-DTT. There were 15 primary and 13 metastatic lung tumors. Twenty-seven tumors were located in the lower lobe. The median age was 73 (range: 40-88). The gender ratio was 20 to 8. The T-stages for primary lung cancers were T1a in 6, T1b in 6 and T2a in 3. There were 6 adenocarcinomas, 4 squamous, 2 NSCLC, and 3 histologically unknown. As for the primary site of metastatic lung cancers, there were 4 H & N, 3 esophagus, 2 colon, etc. Regarding the reasons for declining surgery, there were metastatic tumors in 13, poor respiratory function in 8, old age in 6, refusal of surgery in 3, etc. Average CTV was 12.9 +/- 11.7 ml (range: 1.2 ∼ 32.5 ml). Average respiratory tumor movement was 16.9 +/- 6.4 mm (range: 6.7 ∼ 31.2 mm). Fractionation regimen was 50 Gy / 4 fr / 1 wk with the prescription point of D95 of PTV. Eighty to 100 beams were used for R-DTT whereas 7 to 8 beams were used for G-DTT. Median follow-up period was 12.2 months (range: 2.5 ∼ 32.4 months). With these DTT techniques, all cases successfully completed the SBRT treatment without any respiratory modeling and/or fiducial marker problem. At the time of this analysis, only one metastatic tumor has recurred. Overall local control rate at 1 year (LCR1) was 96%. LCR1 was 100% for primary lung cancers, and 91% for metastatic lung cancers. Regarding overall survival rate at 1 year, it was 100% for primary lung cancer and 78% for metastatic lung cancer. As for toxicities, there were only 2 cases (7%) whose lung V20 exceeded 15%, and both of them developed grade 3 radiation pneumonitis. One of them was primary lung cancer and the other was metastatic. No cases whose lung V20 was smaller than 15% developed severe radiation pneumonitis (P<0.001). There was no difference in the safety and efficacy between the two DTT techniques. Although our clinical results are preliminary, DTT SBRT for moving SLTs might be promising with high LCR and acceptable toxicity. However, higher lung V20 value (> 15%) might cause severe radiation pneumonitis even if this technique has been reported to decrease V20 values significantly compared with conventional static SBRT. Further studies are necessary to see the safety and efficacy of these DTT techniques in the treatment of solitary lung tumors.