2173 - Investigating Target Coverage and Organ-at-Risk Sparing for Adaptive Radiotherapy of Head and Neck Cancer with Direct Dose Calculation on Cone Beam CT

Purpose/Objective(s): This work evaluates the potential benefit of adaptive radiotherapy planning for head and neck (H&N) cancer using direct dose calculation on a high quality cone beam CT (CBCT) platform, which has demonstrated accurate dose calculation relative to conventional fan beam CT.¹ Our hypothesis was that at fraction 21, adaptive planning would allow for improved planning target volume (PTV) coverage and organ-at-risk (OAR) sparing relative to the original plan.

Materials/Methods: Thirty H&N cancer patients were enrolled in the HyperSight ARCHER trial.² Subjects were imaged on the high quality CBCT platform on the same day as planning CT acquisition (Day 0), and again on fraction 21 of radiotherapy treatment (Day 21). To date, structures have been contoured on nine out of thirty subjects, with target contouring performed by radiation oncologists. For these nine subjects (oropharynx site; prescribed 70 Gy/35 fractions), clinically acceptable plans were created using the CBCTs acquired at Day 0 and Day 21 (denoted Day 21-Adapted) with the same optimization directives. The Day 0 and Day 21 CBCTs were rigidly registered, and the Day 0 plans were forward-calculated on the Day 21 images (denoted Day 21-Original). The Day 21-Adapted and Day 21-Original plans were compared for PTV coverage and OAR sparing. While this analysis was performed on nine subjects with all contouring completed to date, analysis for the full thirty subject dataset will be prepared prior to the annual meeting.

Results: Each metric in Table 1 demonstrated a statistically significant difference between the Day 21-Adapted and Day 21-Original plans (p<0.05, Wilcoxon signed rank), indicating an increase in target dose and decrease in parotid dose for the adapted plans. Furthermore, clinically unacceptable PTV coverage was observed in 7/9 of the Day 21-Original plans (i.e. V100% < 93%, D99% < 61.6 Gy or D95% < 66.5 Gy for PTV high risk; D99% < 50 Gy or D95% < 53.2 Gy for PTV low risk). No significant differences were observed between the Day 0 and Day 21-Adapted plans.

Conclusion: Adaptive planning using this CBCT platform allowed for improved target coverage and sparing of the parotid glands at fraction 21 of treatment. These results highlight the dosimetric benefits of adaptive radiotherapy planning for H&N cancer patients, made efficient through direct dose calculation on CBCT.

Abstract 2173 - Table 1: DVH data for day 0, day 21-adapted and day 21-original plans
DVH data (median ± IQR)	Day 0	Day 21-Adapted	Day 21-Original
PTV High V100%	95.23 ± 0.27 %	95.20 ± 0.50 %	91.71 ± 4.34 %
PTV High D99%	67.17 ± 0.74 Gy	67.37 ± 0.51 Gy	63.64 ± 1.80 Gy
PTV High D95%	70.06 ± 0.07 Gy	70.07 ± 0.12 Gy	68.55 ± 2.19 Gy
PTV Low D99%	52.51 ± 0.75 Gy	52.67 ± 1.07 Gy	49.39 ± 4.30 Gy
PTV Low D95%	55.06 ± 1.11 Gy	55.09 ± 0.78 Gy	53.87 ± 1.65 Gy
Parotids Dmean	28.84 ± 5.93 Gy	27.70 ± 7.48 Gy	32.64 ± 8.85 Gy