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Dose to medium in head and neck radiotherapy: Clinical implications for target volume metrics
Nicholas Hardcastle1,2, Atousa Montaseri1, Jenny Lydon1
1Physical Sciences, Peter MacCallum Cancer Centre, 305 Grattan St, Melbourne, Victoria 3000, Australia.
Background And Purpose:
In radiotherapy dose calculation, advanced type-B dose calculation algorithms can calculate dose to medium (D ), as opposed to Type-B algorithms which compute dose to varying densities of water (D ). We investigate the impact of D on calculated dose and target coverage metrics in head and neck cancer patients.
Methods And Materials:
We reviewed 27 successfully treated (disease free at two-years post-(chemo)radiotherapy) human papillomavirus-associated (HPV) oropharyngeal cancer (ONC) patients treated with IMRT. Doses were calculated with Type-B and Linear Boltzman Transport Equation (LBTE) algorithms in a commercial treatment planning system, with the treated multi-leaf collimator patterns and monitor units. Coverage for primary Gross Tumour Volume (GTVp), high dose Planning Target Volume (PTV) (PTV_High), mandible within PTV_High (Mand ∩ PTV) and PTV_High excluding bone (PTV-bone) were compared between the algorithms.
Results:
Dose to 95% of PTV_High with LBTE was on average 1.1 Gy/1.7% lower than with Type-B (95%CI 1.5-1.9%, p < 0.0001). This magnitude was inversely linearly correlated with the relative volume of the PTV_High containing bone (pearson r = -0.81). Dose to 98% of the GTVp was 0.9 Gy/1.3% lower with LBTE compared with Type-B (95%CI 1.1-1.5%, p < 0.05). Dose to 98% of Mand ∩ PTV was on average 3.4 Gy/5.0% lower with LBTE than with Type-B (95%CI 4.6-5.4%, p < 0.0001).
Conclusion:
In OPC treated with IMRT, D results in significant reductions in dose to bone in high dose PTVs. Reported GTVp dose was reduced, but by a lower magnitude. Reduced coverage metrics should be expected for OPC patients treated with IMRT, with dose reductions limited to regions of bone.
