Abstract Book

S225

ESTRO 37

PV-0428 Adaptive radiotherapy in head and neck cancer to correct tumor underdose and parotid gland overdose J. Castelli 1 , A. Simon 2 , B. Rigaud 2 , E. Chajon 1 , K. Benezery 3 , E. Vauleon 4 , F. Jegoux 5 , O. Henry 1 , C. Lafond 1 , R. De Crevoisier 1 1 Centre Eugène Marquis, Radiotherapy, Rennes CEDEX, France 2 INSERM U 1099, LTSI, Rennes, France 3 Centre Antoine Lacassagne, Radiotherapy, Nice, France 4 Centre Eugène Marquis, Oncology, Rennes CEDEX, France 5 CHU Rennes, Head and Neck, Rennes, France Purpose or Objective In the context of locally advanced oropharyngeal cancer (LAOC) treated with definitive radiotherapy (RT) (combined with chemotherapy or cetuximab), the aims of this study were: 1) to estimate the dosimetric impact of anatomical variations on the tumor and the parotid gland; and 2) to estimate the benefit of adaptive radiotherapy (ART) for both decrease the mean parotid gland dose and increase tumor coverage. Material and Methods Thirty-seven patients with a LAOC treated with IMRT (70 Gy, 2 Gy per fraction) had weekly computed tomography scans during the seven weeks of IMRT. Weekly doses were calculated without and with replanning. The dose delivered to 98% of the CTV (D98_CTV), the near maximum dose to the CTV (D2_CTV) and the mean dose of CTV and parotid gland were calculated. The delivered dose was estimated as the mean of the weekly dose for each parameter (without or with replanning). The delivered dose without ART was compared to the planned dose, and the delivered dose with ART to the delivered dose without ART. Comparison was performed using nonparametric test (Wilcoxon test). Results The median D98_ CTV delivered without ART was 68 Gy, compared to 69.1 Gy at the planning (p<0.01) (figure1). Ninety-four percent the patients had either CTV underdosage, PG overdose or both (table 1). A decrease of the D98_CTV of more than 2 Gy combined with an increase of the mean PG dose of 2.4 Gy was shown for 30% of the patients. Compared to standard IMRT without replanning, ART allowed increasing the D98_ CTV (69.2 Gy compared to 68 Gy, p<0.01) (figure 1). At the same time, the D2_CTV with ART was decreased from 74 Gy to 72.85 Gy (p<0.01) and the mean CTV dose from 71.4 Gy to 70.9 Gy (p=0.01). The mean PG dose was decreased from 27.9 Gy to 25.9 Gy (p<0.01).

Figure 1

Figure 2

Conclusion We found a significant volume reduction of the parotid glands. With daily CBCT for setup and replanning in case of significant changes in dose distribution, no clinically relevant difference between planned and delivered dose to the parotid glands or PCMs was observed. There was no significant difference in the dose response relationships of parotid glands or PCMs, whether these were based on planned or delivered mean doses.

Figure 1: Population repartition of D98 CTV: at the planning, delivered without adaptive radiotherapy (ART) and delivered with ART The D98_CTV delivered without ART (red area) was decreased compared to the planned dose (blue area). A weekly ART allows correcting tumor underdose (green area). D98_CTV = Dose delivered to 98% of the CTV.