Speaker
Description
Objective: To quantify inter-fractional tumor variation in upright lung cancer radiotherapy and compare fraction-level and accumulated target coverage and normal-tissue sparing across five ITV-based planning strategies.
Methods: Eight patients with lung cancer underwent upright planning CT and serial daily in-room CT. Following rigid alignment, target contours were propagated to daily CTs by deformable image registration. Inter-fractional variation was quantified using Dice similarity coefficient (DSC), 95th-percentile Hausdorff distance (HD95), relative volume change (ΔV), and center-of-mass (COM) displacement. Five CIRT plans were generated for each patient: conventional optimization using 3-, 5-, or 7-mm ITV expansions (M3, M5, and M7), and ITV-based robust optimization with 3- or 5-mm setup uncertainty and 3.5% range uncertainty (RO3 and RO5). Each plan was recalculated on every available daily CT, yielding 650 plan–fraction evaluations. Fractional RBE-weighted doses were deformably mapped to the planning CT and accumulated over available fractions. Target coverage was evaluated relative to the prescription dose corresponding to the accumulated fractions.
Results: Across patients, the mean inter-fractional GTV COM displacements ranged from 0.81 to 2.52 mm, DSC from 0.749 to 0.967, HD95 from 1.50 to 4.04 mm, and ΔV from −14.17% to 9.06%. Fraction-level pass rates for meeting both ITV D95 and V95 ≥ 95% were 84.6%, 94.6%, 96.2%, 93.1%, and 95.4% for M3, M5, M7, RO3, and RO5, respectively. M7 achieved the highest median accumulated ITV D95 (98.85%) and V95 (99.84%). Only M3 for patient P6 failed both accumulated coverage criteria (D95, 94.87%; V95, 94.92%); all other strategies met both criteria in all patients. RO3 achieved the highest median conformity index (CI, 0.614) and lowest median homogeneity index (HI, 0.051). Compared with M7, RO3 reduced ipsilateral-lung V20Gy in all patients, with a median paired relative reduction of 25.2%, and reduced esophageal D0.1cc in four of five patients with evaluable DVH data.
Outlook & Discussion: Larger ITV expansions improved target coverage but was associated with higher OAR exposure and less favorable conformity and homogeneity. In this exploratory cohort, RO3 maintained accumulated target coverage in all patients while providing favorable conformity, homogeneity, and OAR sparing compared with larger ITV expansion. These findings highlight the trade-off between target robustness and normal-tissue exposure and support further investigation of patient-specific robust and adaptive planning strategies for upright lung CIRT.