Evaluation of dosimetric and radiobiological parameters (TCP/NTCP) based on daily CBCT in lung cancer patients

Radiotherapy for non-small-cell lung cancer (NSCLC) requires precise tumour targeting and monitoring of anatomical changes to optimize outcomes. Daily kilovoltage cone-beam computed tomography (kV-CBCT) enables real-time tracking of tumour volume and electron density changes, which may affect dose distribution and radiobiological outcomes, such as Tumour Control Probability (TCP) and Normal Tissue Complication Probability (NTCP). This study evaluated inter-fractional changes in gross tumour volume (GTV) and electron density using kV-CBCT and their impact on dosimetric and radiobiological endpoints to assess the role of CBCT in adaptive radiotherapy. Twenty inoperable NSCLC patients receiving intensity-modulated radiotherapy (IMRT) with a prescribed dose of 60 Gy in 30 fractions were included in this retrospective study. Daily kV-CBCT images were acquired on the 1st, 15th, and 30th fraction to re-contour GTV and measure electron density changes in the GTV and ipsilateral lung (V30 volume, defined as lung volume receiving ≥ 30 Gy excluding GTV). CBCT images were fused with planning CT for dose recalculation using a Hounsfield unit-to-density calibration. Changes in GTV volume, electron density, maximum dose, and volume receiving ≥ 2.1 Gy/fraction were analyzed. TCP and NTCP were calculated using the Equivalent Uniform Dose (EUD) and Lyman-Kutcher-Burman (LKB) models, respectively. Statistical significance was assessed using ANOVA (p < 0.05). GTV volume decreased significantly by 3.4-59.7% by the 30th fraction (p < 0.05). Mean electron density in the GTV reduced by 3.04% at 30 Gy and 5.76% at 60 Gy, while the V30 volume showed reductions of 4.61% and 17.17% at the 15th and 30th fractions, respectively (p < 0.05). These changes resulted in a 1.39% increase in maximum GTV dose and a 7.48% increase in the volume receiving ≥ 2.1 Gy/fraction by the 30th fraction. TCP increased modestly from 81.91 ± 10.25% to 83.4 ± 11.32%, while NTCP rose significantly from 6.64 ± 2.31% to 10.54 ± 1.94% (p < 0.05). Thus, daily kV-CBCT demonstrated significant GTV volume and electron density reductions during NSCLC radiotherapy, leading to dose heterogeneity and increased NTCP. These findings underscore the potential of CBCT as a tool for adaptive radiotherapy to enhance treatment precision and minimize radiation toxicity. Prospective studies with larger cohorts and clinical outcome data are needed to establish thresholds for adaptive replanning.