Adaptive radiotherapy for head and neck cancer - Dosimetric results from a prospective clinical trial

Purpose: To conduct a clinical trial evaluating adaptive head and neck radiotherapy (ART). Methods: Patients with locally advanced oropharyngeal cancer were prospectively enrolled. Daily CT-guided setup and deformable image registration permitted mapping of dose to avoidance structures and CTVs. We compared four planning scenarios: (1) original IMRT plan aligned daily to marked isocenter (BB); (2) original plan aligned daily to bone (IGRT); (3) IGRT with one adaptive replan (ART1); and (4) actual treatment received by each study patient (IGRT with one or two adaptive replans, ART2). Results: All 22 study patients underwent one replan (ART1); eight patients had two replans (ART2). ART1 reduced mean dose to contralateral parotid by 0.6 Gy or 2.8% (paired t-test; p = 0.003) and ipsilateral parotid by 1.3 Gy (3.9%) (p = 0.002) over the IGRT alone. ART2 further reduced the mean contralateral parotid dose by 0.8 Gy or 3.8% (p = 0.026) and ipsilateral parotid by 4.1 Gy or 9% (p = 0.001). ART significantly reduced integral body dose. Conclusions: This pilot trial suggests that head and neck ART dosimetrically outperforms IMRT. IGRT that leverages conventional PTV margins does not improve dosimetry. One properly timed replan delivers the majority of achievable dosimetric improvement. The clinical impact of ART must be confirmed by future trials.