3452 - Dosimetric Assessment of iGTV-Guided Adaptive IMRT In Unresectable NSCLC

Purpose/Objective(s):

IMRT based SIB technique enables delivery of a higher dose to tumor while minimizing irradiation of OARs. Adaptive RT (ART), applied after tumor shrinkage, helps reducing geometric uncertainties and prevents unnecessary high dose to OARs. Additionally, the IMRT-SIB approach lowers the risk of tumor recurrence by effectively targeting potential microscopic disease.

Materials/Methods:

60 patients with stage III NSCLC treated with ART using SIB-IMRT were included. Definitive RT was planned with 66 Gy to the iGTV (PTV 66= iGTV) and 59.4 Gy to PTV 59.4 over 33 fractions. A 3 phase CT was done before treatment (PRE-TREATMENT SCAN) and repeated after 44 Gy for mid-treatment adaptation (ADAPTIVE SCAN).

CTV included iGTV with 6-8 mm margin in all directions. PTV 66 was defined as iGTV only and PTV 59.4 was defined as 5 mm margin from CTV. In adaptive scan, PTV66 was generated by delineating the shrunken tumor only. But in this study, the CTV and PTV 59.4 delineation in the adaptive/mid treatment scan were kept same as the pre-treatment scan to trade-off between high chance of LRF & better OAR sparing as numerous literatures have shown higher intrathoracic in-field recurrences. Adaptive plan was developed on the mid treatment scan using same prescription and optimization objectives after 22nd fraction.

Rigid registration was done to transfer the original 66 Gy plan to the adaptive CT, which was considered the NON-ADAPTIVE RT (SIMULATED PLAN). Whereas the COMBINED ART PLAN was generated by combining 22 fractions of the initial plan with 11 fractions of the adaptive replanned treatment. Dosimetric differences between the combined ART and non-ART plans were quantitatively assessed using DVH analysis.

Results:

The median iGTV before initiation of treatment was 247 cubic cm (range:79.3-512.7) and 176 cubic cm (range:69.3-251) post adaptation. Absolute values of (I/L lung–PTV) V20, V30 and mean lung dose (MLD) demonstrated average reductions of 1.4%, 2.7%, and 73 cGy respectively. Corresponding C/L lung parameters declined by 2.61%, 1.89%, and 129.7 cGy. Significant differences were also observed in mean esophageal and cardiac doses between NON-ART and COMBINED ART plans overall

Conclusion:

Adaptive replanning to the shrunk tumor and anatomical changes can have significant dosimetric benefits to OARs. This is more prominent in subgroups with higher initial dose to lung and significant tumor shrinkage. We have demonstrated that the routine adaptation can significantly reduce the dose to OARs, which has potential of lowering the associated pneumonitis and esophagitis as well. A limitation of this study is that fusion and matching were performed using rigid co-registration, which did not account for volumetric deformation during phase 2. Additionally, significant volume changes were observed in several cases even before the 22nd fraction, suggesting the potential need for earlier and multiple adaptive replanning. A multivariate analysis by site, laterality and IMRT/ARC technique is also warranted.