Primary study on target reproducibility of peripheral non-small cell lung cancer in cone-beam CT based image-guided radiotherapy

Abstract

7596 Background: To evaluate impact of the change of position, volume and shape of the targets of peripheral non-small cell lung cancer (NSCLC) on the target reproducibility, analyze PTV margins and guide the modification of the treatment planning. Methods: Ten patients with peripheral NSCLC treated with 3D-CRT were scanned by KV CBCT per week until treatment planning changed and 3∼5 sets of CBCT image for per patient were got. Set-up errors were corrected on-line by registration of the CBCT images and planning CT images and the CBCT image sets were transferred to the TPS. In the TPS, CBCT images and CT images were registered off-line, and the volumes of the primary tumor were delineated on the CBCT images registered to CT images, which were denominated as CTCT-IGTV consisted of GTV plus IM to analyze the change of position, volume and shape of the targets. A group margin was got by means of the formula (M=2.0Σ+0.7σ) which was proposed by Stroom. Through DVH, the volume percent of CTCT-IGTVs irradiated with 95% prescribed dose were compared with that of CT-PTV, so as to judge whether part of CTCT-IGTVs not be irradiated in some weeks. Characterized PTV margin was got by increasing the margin to CT-GTV inch by inch until the volume percent of CTCT-IGTVs irradiated with 95% prescribed dose less than that of characterized PTV. Results: The group margin due to targets displacement in different fraction of radiotherapy was 0.80cm, 0.96cm, and 0.87cm on X,Y,Z axis respectively. Volume of the targets significantly decreased at the third week (39.7±29.1 cm3) compared to that at the first (51.1±33.9cm3) (P=0.006) and second week (48.6±34.0 cm3) (P=0.013) during radiotherapy. When PTV margins were prescribed as 8∼10mm, three targets (3/41 weeks) were not irradiated perfectly. Characterized PTV margins were 4∼11mm which were not significantly different from that 8∼10mm prescribed for treatment planning (P>0.05), but the variations for individual patient were greater. Conclusions: Target displacement and volume enlargement is the major reason to result in leakage of irradiation to part of the target, image-guided radiotherapy can monitor and evaluate target repetitiveness during radiotherapy to guide modification of the treatment planning of NSCLC. Author Disclosure Employment or Leadership Consultant or Advisory Role Stock Ownership Honoraria Research Expert Testimony Other Remuneration Li Jianbin