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Background  The advent of IMRT and image-guided radiotherapy (IGRT) in combination with involved-field radiotherapy (IF-RT) in inoperable non-small-cell lung cancer results in a decreased incidental dose deposition in elective nodal stations. While incidental nodal irradiation is considered a relevant by-product of 3D-CRT to control microscopic disease this planning study analyzed the impact of IMRT on dosimetric parameters and tumor control probabilities (TCP) in elective nodal stations in direct comparison with 3D-CRT.    Methods and materials  The retrospective planning study was performed on 41 patients with NSCLC (stages II-III). The CTV was defined as the primary tumor (GTV + 3 mm) and all FDG-PET-positive lymph node stations. As to the PTV (CTV + 7 mm), both an IMRT plan and a 3D-CRT plan were established. Plans were escalated until the pre-defined dose-constraints of normal tissues (spinal cord, lung, esophagus and heart) were reached. Additionally, IMRT plans were normalized to the total dose of the corresponding 3D-CRT. For two groups of out-of-field mediastinal node stations (all lymph node stations not included in the CTV (LN all_el ) and those directly adjacent to the CTV (LN adj_el )) the equivalent uniform dose (EUD) and the TCP (for microscopic disease a D50 of 36.5 Gy was assumed) for the treatment with IMRT vs 3D-CRT were calculated.    Results  In comparison, a significantly higher total dose for the PTV could be achieved with the IMRT planning as opposed to conventional 3D-CRT planning (74.3 Gy vs 70.1 Gy;  p  = 0.03). In identical total reference doses, the EUD of LN adj_el  is significantly lower with IMRT than with 3D-CRT (40.4 Gy vs. 44.2 Gy.  P  = 0.05) and a significant reduction of TCP with IMRT vs 3D-CRT was demonstrated for LN all_el  and LN adj_el  (12.6 % vs. 14.8 %; and 23.6 % vs 27.3 %, respectively).    Conclusions  In comparison with 3D-CRT, IMRT comes along with a decreased EUD in out-of-field lymph node stations. This translates into a statistically significant decrease in TCP-values. Yet, the combination of IF-RT and IMRT leads to a significantly better sparing of normal tissues and higher total doses whereas the potential therapeutic drawback of decreased incidental irradiation of elective lymph nodes is moderate.

radiation oncology

Dose distribution and tumor control probability in out-of-field lymph node stations in intensity modulated radiotherapy (IMRT) vs 3D-conformal radiotherapy (3D-CRT) of non-small-cell lung cancer: an in silico analysis