Lung stereotactic body radiation therapy: personalized PTV margins according to tumor location and number of four-dimensional CT scans

OBJECTIVES: To characterise the motion of pulmonary tumours during stereotactic body radiation therapy (SBRT) and to evaluate different margins when creating the planning target volume (PTV) on a single 4D CT scan (4DCT). METHODS: We conducted a retrospective single-site analysis on 30 patients undergoing lung SBRT. Two 4DCTs (4DCT1 and 4DCT2) were performed on all patients. First, motion was recorded for each 4DCT in anterior–posterior (AP), superior-inferior (SI) and rightleft (RL) directions. Then, we used 3 different margins (3,4 and 5 mm) to create the PTV, from the internal target volume (ITV) of 4DCT1 only (PTV D1 + 3, PTV D1 + 4, PTV D1 + 5). We compared, using the Dice coefficient, the volumes of these 3 PTVs, to the PTV actually used for the treatment (PTV(ttt)). Finally, new treatment plans were calculated using only these 3 PTVs. We studied the ratio of the D2%, D50% and D98% between each new plan and the plan actually used for the treatment (D2% PTV(ttt), D50% PTV(ttt), D50% ITVttt D98% PTV(ttt)). RESULTS: 30 lesions were studied. The greatest motion was observed in the SI axis (8.8 ± 6.6 [0.4–25.8] mm). The Dice index was higher when comparing PTVttt to PTV D1 + 4 mm (0.89 ± 0.04 [0.82–0.98]). Large differences were observed when comparing plans relative to PTVttt and PTV D1 + 3 for D98% PTV(ttt) (0.85 ± 0.24 [0.19–1.00]). and also for D98% ITV(ttt) (0.93 ± 0.12 [0.4–1.0]).D98% PTV(ttt) (0.85 ± 0.24 [0.19–1.00], p value = 0.003) was statistically different when comparing plans relative to PTV(ttt) and PTV D1 + 3. No stastistically differences were observed when comparing plans relative to PTV(ttt) and PTV D1 + 4. A difference greater than 10% relative to D98% PTV(ttt) was found for only in one UL lesion, located under the carina. CONCLUSION: A single 4DCT appears feasible for upper lobe lesions located above the carina, using a 4-mm margin to generate the PTV. ADVANCE IN KNOWLEDGE: Propostion of a personalized SBRT treatment (number of 4DCT, margins) according to tumor location (above or under the carina).