Toward optimal organ at risk sparing in complex volumetric modulated arc therapy: An exponential trade-off with target volume dose homogeneity

Purpose: Conventional radiotherapy typically aims for homogenous dose in the planning target volume (PTV) while sparing organs at risk (OAR). The authors quantified and characterized the trade-off between PTV dose inhomogeneity (IH) and OAR sparing in complex head and neck volumetric modulated arc therapy plans. Methods: Thirteen simultaneous integrated boost plans were created per patient, for ten patients. PTV boost(B)/elective(E) optimization priorities were systematically increased. IHB and IHE, defined as (100% -V95%) + V107%, were evaluated against the average of the mean dose to the combined composite swallowing and combined salivary organs (D-OARcomp). To investigate the influence of OAR size and position with respect to PTVB/E, OAR dose was evaluated against a modified Euclidean distance (DMB/DME) between OAR and PTV. Results: Although the achievable D-OARcomp for a given level of PTV IH differed between patients, excellent logarithmic fits described the D-OARcomp/IHB and IHE relationship in all patients (mean R2 of 0.98 and 0.97, respectively). Allowing an increase in average IHB and IHE over a clinically acceptable range, e. g., from 0.4% +/- 0.5% to 2.0% +/- 2.0% and 6.9% +/- 2.8% to 14.8% +/- 2.7%, respectively, corresponded to a decrease in average dose to the composite salivary and swallowing structures from 30.3 +/- 6.5 to 23.6 +/- 4.7 Gy and 32.5 +/- 8.3 to 26.8 +/- 9.3 Gy. The increase in PTVE IH was mainly accounted for by an increase in V107, by on average 5.9%, rather than a reduction in V95, which was on average only 2%. A linear correlation was found between the OAR dose to composite swallowing structures and contralateral parotid and submandibular gland, with DME (R2 = 0.83, 0.88, 0.95). Only mean ipsilateral parotid dose correlated with DMB (R2 = 0.87). Conclusions: OAR sparing is highly dependent on the permitted PTVB/E IH. PTVE IH substantially influences OAR doses. These results are relevant for clinical practice and for future automated treatment-planning strategies. c 2014 American Association of Physicists in Medicine.