Impact of the type of imaging modality on target volumes delineation and dose distribution in pharyngo-laryngeal squamous cell carcinoma: comparison between pre- and per-treatment studies

Background and purpose It has been shown that the use of pre-treatment FDG-PET impacted on the GTV delineation of pharyngo-laryngeal tumors. The goals of this study were to evaluate (1) the impact of FDG-PET GTV on dose distribution, and (2) the impact of per-treatment re-imaging on target volume delineation and dose distribution. Materials and methods Eighteen patients with squamous cell carcinoma of the oropharynx or larynx/hypopharynx were treated with curative intent by forward planning IMRT. Prior to treatment and on average after a dose of 46 Gy, all patients underwent contrast-enhanced CT, MRI and FDG-PET. After coregistration, GTVs were delineated manually on CT and MRI and automatically on FDG-PET. From these volumes, CTVs and PTVs were derived using consistent guidelines. Planning was performed using conformal radiotherapy. Results GTVs, CTVs and PTVs based on pre-treatment FDG-PET were significantly smaller than those based on pre-treatment CT. Such difference in target volumes (TV) translated into a significant reduction in the irradiated volumes (reduction of 13 and 18% of the V50 and V95, respectively), Dmean to ipsilateral parotids (30.7 and 38.6% for FDG-PET and CT based plans, respectively) and to controlateral parotids (11.2 and 14.4% for FDG-PET and CT based plans, respectively). TVs based on per-treatment CT or MRI were also significantly smaller compared to those delineated from pre-treatment CT. Volumes delineated with MRI were significantly smaller than those delineated with CT. Due to radiotherapy-induced peri-tumoral inflammation, automatic delineation of FDG-PET GTV could not be performed. Such reductions in TVs translated into a reduction of the irradiated volumes compared to pre-treatment CT planning (reduction for V50 of 19 and 32%, and for V95 of 22 and 40%, for CT and MRI, respectively); Dmean to the ipsilateral parotids were also reduced (ipsilateral parotid Dmean of 20.4% for CT and of 20.1% for MRI compared to 24.7% for pre-treatment CT). Conclusions : The use of pre-treatment FDG-PET and per-treatment CT or MRI significantly impacts on the delineation of TVs in pharyngo-laryngeal SCC, translating into more normal tissue sparing after conformal radiotherapy planning.