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

摘要

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.