2439 - Application of Monitoring Internal Tumor Motion with Surface Guided System for Lung Stereotactic Body Radiotherapy in Deep Inspiration Breath-Hold

J. Wang¹, B. Yang¹, T. Dong¹, Z. Wang¹, and X. Meng²; ¹Department of Radiation Oncology, Peking Union Medical College Hospital, Chinese Academy of Medical Sciences & Peking Union Medical College, Beijing, China, ²Peking Union Medical College Hospital, Chinese Academy of Medical Sciences and Peking Union Medical College, Beijing, China

The deep inspiration breath-hold (DIBH) technology used in lung cancer stereotactic body radiotherapy (SBRT) significantly controls the motion of target which brings dosimetric benefits to the organ at risk (OAR). The surface guided radiotherapy (SGRT) system is widely used for providing DIBH reproducibility, but as for the lung target, if the surface motion monitored by SGRT could represent the actual internal target motion still needs to be studied. Purpose/Objective(s): This study implemented lung SBRT in DIBH with SGRT. To study the consistency between surface motion monitored by SGRT and the actual internal target motion. To study the patients’ compliance in such new workflow of lung SBRT in DIBH with SGRT. Materials/

Methods: This study included five patients with either primary lung cancer or oligometastatic disease treated with SBRT in DIBH, only patients held breath over 30s were deemed eligible. A DIBH-CT scan and a free breathing four dimensions (FB-4D-CT) scan were acquired, the reference surface in DIBH was also acquired by SGRT, named as Ref1. Ref1 was used for patient’s set-up in DIBH, tolerance was set as ±3 mm translation and ±3? rotation. A fast cone-beam computed tomography (CBCT) was acquired in DIBH before each intrafraction, named as CBCT1. At the same time, SGRT recorded the surface differences compared to Ref1, named as ?Ref1. CBCT1 was registered to DIBH-CT, couch shifts (?shift1) were applied when the patient stayed in DIBH status, then a new surface (Ref2) was acquired which used for monitoring the surface motion in this intrafraction. Two radiotherapists monitored the surface motions compared with Ref2 in the whole delivery, beam would be interrupted manually if the difference exceeded tolerance. CBCT2 was acquired in the middle of treatment to verify if the tumor stayed in PTV. In the meanwhile, SGRT recorded the surface differences compared to Ref2, named as ?Ref2. CBCT2 was registered to the DIBH-CT, couch shifts (?shift2) were applied, then Ref3 was acquired for monitoring the surface motion in remaining delivery. The relationships between ?Ref1 and ?shift1, ?Ref2 and ?shift2 were analyzed with statistical software, results were considered significant when two-tailed p-value less than 0.05. Besides, treatment plannings were performed for both DIBH-CT and FB-4D-CT, dose to OARs in DIBH were compared to FB-4D. Both treatment plannings should meet the dose limit, and the latter was seemed as alternate treatment plan if the patient couldn’t repeat his/her DIBH status during beam delivery.
Results: The relationship between ?Ref1 and ?shift1, ?Ref2 and ?shift2 showed significant correlation with p=0.022, 0.038, separately. The tumors were inside PTV region according to CBCT2 as for each intrafraction of the five patients. DIBH significantly reduced the mean dose, V20 and V5 for ipsilateral lung and double lungs.
Conclusion: This study demonstrated the feasibility of monitoring the internal tumor motion with SGRT for lung SBRT in DIBH. DIBH technology achieved significant dose benefit for lung SBRT.