Head and neck margin reduction with ART: robustness of treatment plans against anatomy changes

Publication date: Available online 21 July 2016
Source:International Journal of Radiation Oncology*Biology*Physics
Author(s): S. van Kranen, O. Hamming-Vrieze, A.L. Wolf, E. Damen, M. van Herk, J.-J. Sonke
PurposeTo investigate loss of target coverage from anatomy changes in head-and-neck cancer(HNC) patients as function of applied safety margins and to verify a CBCT based adaptive strategy with an average patient anatomy to overcome possible target underdosage.Methods& Materials: For 19 oropharyngeal cancer patients, VMAT treatments plans(2 arcs, SIB, 70/54.25Gy, 35 fractions) were automatically optimized with uniform CTV to PTV margins of 5, 3 and 0mm. B-spline CBCT-to-CT deformable registration was applied to allow recalculating the dose on modified CTs(planning CT deformed to daily CBCT following online positioning) and dose accumulation in the planning CT. Patients with deviations in primary or elective CTV-coverage >2Gy(ΔD99%) were identified as candidates for adaptive replanning. For these patients, a single adaptive intervention was simulated with an average anatomy from the first 10 fractions.ResultsMargin reduction from 5→3→0mm generally led to OAR sparing of ∼1Gy/mm(Dmean). CTV shrinkage was mainly seen in the elective volumes(up to 10%), likely related to weight-loss. Despite online repositioning, substantial systematic errors were present(>3mm) in lymph-node CTV, the parotid glands and the larynx. Nevertheless, average increase in OAR dose was small: max 1.2Gy(parotid glands, Dmean) for all applied margins. Loss of CTV-coverage >2Gy was found in 1, 3 and 7 of 73 CTVs respectively. Adaptive intervention in 0mm plans substantially improved coverage: in 5 of 7 CTVs(in 6 patients) to less than 2Gy of initially planned.ConclusionVMAT HNC treatment plans with 5mm margins are robust for anatomy changes and show modest increase in OAR dose. Margin reduction improves OAR sparing with ∼1Gy/mm at the expense of target coverage in a subgroup of patients. Patients at risk of CTV-underdosage>2Gy in 0mm plans may be identified early in treatment using dose accumulation. A single intervention with an average anatomy derived from CBCT effectively mitigates discrepancies.



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