DIR based dose accumulation to validate reduced PTV and PRV margins in head-and-neck radiotherapy
PH-0123
Abstract
DIR based dose accumulation to validate reduced PTV and PRV margins in head-and-neck radiotherapy
Authors: Lowther|, Nicholas(1)*[nick.lowther@ccdhb.org.nz];Marsh|, Steven(2);Louwe|, Robert(1);
(1)Wellington Blood & Cancer Centre, Department of Radiation Oncology, Wellington, New Zealand;(2)University of Canterbury, School of Physical and Chemical Sciences, Christchurch, New Zealand;
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Purpose or Objective
Emerging literature has reported reduced treatment toxicity in head-and-neck radiotherapy (HNRT) with 3 instead of 5 mm planning target volume (PTV) margins [1–3]. However, the loco-regional (LR) control rate was not preserved in one study [3]. The current study investigates the occurrence, location and trends where clinical target volume (CTV) coverage and critical organ at risk (OAR) sparing may be at risk in the context of reduced PTV and planning risk volume (PRV) margins. It specifically focusses on locations where the high-dose target coverage is commonly tight during treatment planning.
Material and Methods
VMAT plans for 12 patients were optimised using either 3 or 5 mm PTV and PRV margins (prescribed dose, Dpresc 66 Gy in 30 fractions). The planning CT (pCT) was first registered to each daily CBCT using demons deformable image registration (DIR). Subsequently, the inverse registration was used to reconstruct and accumulate the delivered dose (accD) over all fractions. The 95% level of confidence of accD was determined using the structure- and voxel-specific uncertainty as described previously. The CTV and critical OAR doses were assessed using the dose-volume histogram (DVH) metrics D99% and D0.1cc of accD, respectively. In addition, the planned and accumulated minimum dose (planDmin and accDmin) were determined for local target volumes defined at the point of minimum distance to the brainstem (BS), spinal cord (SC) and skin.
Results
For each critical OAR and for both margin expansions, accD0.1cc was below the specific OAR tolerance dose. For the CTVs, accD99% was less than 95% Dpresc at the 95% level of confidence in two and one cases for 3 and 5 mm PTV/PRV margin plans, respectively, with the under dosage occurring near the skin. Fig. 1 shows that target volumes closer to the skin generally exhibited a lower planDmin and a larger drop in local coverage during treatment compared to less superficial CTVs, independent of the applied PTV/PRV margin. Analysis of the local target coverages near the BS showed that for one patient, an increased BS PRV dose could be avoided using 3 mm margins. For cases with more proximal CTVs, a higher planDmin to the local PTV was achievable with 3 mm plans. Nevertheless, the drop in coverage during treatment for the local CTV was always less than 2%, with accDmin > 95% Dpresc (Fig 2). Similar results were obtained for CTV coverage near the SC.
Conclusion
For CTVs proximal to BS and SC, 3 mm margin plans with improved PTV coverage and PRV sparing preserved CTV coverage during treatment. Consistent with results of Chen et al. [2] who reported that ~50% of marginal LR recurrences were observed in superficial areas, our study only showed a drop in CTV coverage below 95% Dpresc in skin regions. Careful monitoring of the delivered dose is therefore warranted both for 3 and 5 mm PTV/PRV margins.
[1] Navran et al. 2019. doi.org/10.1016/j.radonc.2018.06.032
[2] Chen et al. 2014. doi.org/10.1002/hed.23532
[3] Franzese et al. 2019. doi.org/10.1002/lary.28048