A longitudinal evaluation of improvements in radiotherapy treatment plan quality for head and neck cancer patients

Purpose To investigate changes in head-and-neck cancer (HNC) plan quality following the introduction of new technologies and planning techniques in the last decade. Methods and materials Thirty plans were selected from each of four successive periods (P). P1: 7-field static intensity-modulated radiotherapy (IMRT) with parotid gland sparing; P2: dual-arc volumetric-modulated arc therapy (VMAT, similar to P3-P4), including submandibular gland sparing; P3: inclusion of individual swallowing muscles and attempts to further reduce parotid and oral cavity doses through manual interactive optimization; P4: containing the same organs-at-risk (OARs) as P3, but automatically interactively optimized. Plan benchmarking included mean salivary gland/swallowing muscle/oral cavity (D_sal/D_swal/D_oc) doses. Differences in mean doses between the periods were analyzed by an ANCOVA, taking geometric differences across periods into account. Results Compared to P1, P2 plans improved D_sal by 3.4 Gy on average. P3 improved D_sal/D_swal/D_oc by 6.9/11.5/7.2 Gy over P2, showing that D_swal and D_sal could be improved simultaneously. In P4, D_oc/D_swal slightly improved over P3 by 1.7/3.8 Gy. Improved OAR sparing in P3/P4 did not come at the cost of increased dose deposition elsewhere and planning target volume (PTV) dose homogeneity was similar. Conclusions New technologies and planning techniques were successfully implemented into routine clinical care and resulting in improved HNC plan quality.