Dose accumulation to assess the validity of treatment plans with reduced margins in radiotherapy of head and neck cancer

Abstract

Background and purpose: Literature has reported reduced treatment toxicity in head-and-neck radiotherapy (HNRT) when reducing the planning target volume (PTV) margin from 5 to 3 mm but loco-regional control was not always preserved. This study used deformable image registration (DIR)-facilitated dose accumulation to assess clinical target volume (CTV) coverage in the presence of anatomical changes.

Materials and methods: VMAT plans for 12 patients were optimized using 3 or 5 mm PTV and planning risk volume (PRV) margins. The planning computed tomography (pCT) scan was registered to each daily cone beam CT (CBCT) using DIR. The inverse registration was used to reconstruct and accumulate dose ( $D^{\mathrm{acc}}$ ). CTV coverage was assessed using the dose-volume histogram (DVH) metric $D_{99\%}^{\mathrm{acc}}$ and by individual voxel analysis. Both approaches included an uncertainty estimate using the 95% level of confidence.

Results: $D_{99\%}^{\mathrm{acc}}$ was less than 95% of the prescribed dose $D^{\mathrm{presc}}$ for three cases including only one case where this was at the 95% level of confidence. However for many patients, the accumulated dose included a substantial volume of voxels receiving less than 95% $D^{\mathrm{presc}}$ independent of margin expansion, which predominantly occurred in the subdermal region. Loss in target coverage was very patient specific but tightness of target volume coverage at planning was a common factor leading to underdosage.

Conclusion: This study agrees with previous literature that PTV/PRV margin reduction did not significantly reduce CTV coverage during treatment, but also highlighted that tight coverage of target volumes at planning increases the risk of clinically unacceptable dose delivery. Patient-specific verification of dose delivery to assess the dose delivered to each voxel is recommended.

Keywords: Anatomical changes; DIR dose accumulation; Head-and-neck radiotherapy; PTV margin reduction; Plan robustness.

Fig. 1

Dose-volume histograms (DVHs) of $D^{\mathit{plan}}$and $D^{\mathit{acc}}$ for the high-dose clinical target volume (CTV) of patient 8 (a) and patient 10 (b). In contrast to (a), a CTV underdosage is observed for case (b) when utilizing 3 and 5 mm planning target volume (PTV) margin plans at the 95% level of confidence (b). $D^{\mathit{plan}}$ = planned dose. $D^{\mathit{acc}}$ = accumulated dose. D_99% = dose to 99% of volume. $D^{\mathit{presc}}$ = prescribed dose.

Fig. 2

Plots of dose-volume histograms (DVH) metrics of interest for the high-dose (HD) clinical target volume (CTV) and planning target volume (PTV) when utilizing 3 or 5 mm planning target volume (PTV) and planning risk volume (PRV) margin plans. ${\mathrm{D}}^{\mathrm{p}\mathrm{l}\mathrm{a}\mathrm{n}}$ = planned dose. ${\mathrm{D}}^{\mathrm{a}\mathrm{c}\mathrm{c}}$ = accumulated dose. D99% = dose to 99% of volume. D98% = dose to 98% of volume. ${\mathrm{D}}^{\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{s}\mathrm{c}}$ = prescribed dose.

Fig. 3

Plot of individual voxels within the high-dose clinical target volumes (CTVs) of all patients when utilizing 3 or 5 mm planning target volume (PTV) and planning risk volume (PRV) margin plans. $D^{\mathit{plan}}$ = planned dose. $D^{\mathit{acc}}$ = accumulated dose. $D^{\mathit{presc}}$ = prescribed dose.

Fig. 4

Progression of the minimum planned dose $D_{\mathit{min}}^{\mathit{plan}}$ to the minimum accumulated dose $D_{\mathit{min}}^{\mathit{acc}}$ for local clinical target volumes (CTVs) near the skin for 3 and 5 mm planning target volume (PTV)/planning risk volume (PRV) margin plans. $D_{\mathit{min}}^{\mathit{plan}}$ is plotted as a function of the minimum distance between the CTV and skin. $D_{\mathit{min}}^{\mathit{acc}}$is plotted as a function of the minimum distance between the CTV and skin, and the local shift of the skin at the end of treatment to indicate observed anatomical changes.