Dose–volume histogram analysis and clinical evaluation of knowledge-based plans with manual objective constraints for pharyngeal cancer

The present study aimed to evaluate whether knowledge-based plans (KBP) from a single optimization could be used clinically, and to compare dose–volume histogram (DVH) parameters and plan quality between KBP with (KBP(CONST)) and without (KBP(ORIG)) manual objective constraints and clinical manual optimized (CMO) plans for pharyngeal cancer. KBPs were produced from a system trained on clinical plans from 55 patients with pharyngeal cancer who had undergone intensity-modulated radiation therapy or volumetric-modulated arc therapy (VMAT). For another 15 patients, DVH parameters of KBP(CONST) and KBP(ORIG) from a single optimization were compared with CMO plans with respect to the planning target volume (D(98%), D(50%), D(2%)), brainstem maximum dose (D(max)), spinal cord D(max), parotid gland median and mean dose (D(med) and D(mean)), monitor units and modulation complexity score for VMAT. The D(max) of spinal cord and brainstem and the D(med) and D(mean) of ipsilateral parotid glands were unacceptably high for KBP(ORIG), although the KBP(CONST) DVH parameters met our goal for most patients. KBP(CONST) and CMO plans produced comparable DVH parameters. The monitor units of KBP(CONST) were significantly lower than those of the CMO plans (P < 0.001). Dose distribution of the KBP(CONST) was better than or comparable to that of the CMO plans for 13 (87%) of the 15 patients. In conclusion, KBP(ORIG) was found to be clinically unacceptable, while KBP(CONST) from a single optimization was comparable or superior to CMO plans for most patients with head and neck cancer.