ESTRO 2022 - Abstract Book

S430

Abstract book

ESTRO 2022

evolving from malignant tissue without the evidence of remaining tumor tissue. Patient and treatment related risk factors were analyzed with uni- and multivariable regression analysis. In a subgroup of 25 patients imaging at time of PRMU was available. The PRMU in these patients were delineated on the original planning CT scan and location with respect to isodose lines was determined. Results A total of 64 patients developed PRMU grade ≥ 2, with a two year cumulative incidence of 19%. For conventional fractionation, all PRMU occurred within 9 months (N=18). For the hypofractionation boost group (N = 46), 31 PRMU developed within 9 months, and an additional 15 PRMU developed after 9 to 22 months post-RT (see figure). All PRMU were successfully managed (50% with HBOT), with none developing into grade 3 PRMU. 34% of PRMU persisted after more than 6 months. The cumulative incidence of PRMU after the hypofractionation boost was significantly higher compared to conventional fractionation (26% vs 12% at two yrs; p = 0.003). In the hypofractionation boost group, tonsil subsite (SHR = 2.70; p = 0.003), female (SHR = 2.07; p = 0.016), and acute tube feeding (SHR = 3.10; p < 0.001) were associated with increased PRMU risk in multivariable regression. In the conventional fractionation group, no statistically significant risk factors were identified. 17% of PRMU (N = 11) developed directly after tumor regression and the remaining 83% were late PRMU (see table). All 25 delineated PRMU were situated within the 95% isodose lines.

Conclusion The increased risk of PRMU should be considered when introducing hypofractionation in radiotherapy of oropharyngeal cancer. In this group, requiring a feeding tube (surrogate for severe acute mucositis) and being female was associated with increased risk.

MO-0480 Risk of acute mucositis following an increase in high dose CTV1 in HNSCC patients

R. Zukauskaite 1,2 , J. Grau Eriksen 3,4 , E. Andersen 5 , J. Johansen 1 , E. Samsøe 5,6 , S. Long Krogh 7 , J. Overgaard 4 , C. Grau 3,8 , C. Rønn Hansen 8,9,10 1 Odense University Hospital, Department of Oncology, Odense, Denmark; 2 University of Southern Denmark, Odense, Department of Clinical Research, Odense , Denmark; 3 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark; 4 Aarhus University Hospital, Department of Experimental Clinical Oncology, Aarhus, Denmark; 5 Copenhagen University Hospital Herlev, Department of Oncology, Copenhagen, Denmark; 6 Zealand University Hospital, Department of Oncology, Naestved, Denmark; 7 Odense University Hospital, Laboratory of Radiation Physics, Odense , Denmark; 8 Aarhus University Hospital, Danish Centre for Particle Therapy, Aarhus, Denmark; 9 University of Southern Denmark, Department of Clinical Research, Odense , Denmark; 10 Laboratory of Radiation Physics, Odense University Hospital, Odense , Denmark Purpose or Objective Radiotherapy (RT) in the head and neck region bears a risk of adverse effects due to often relatively large treatment volumes in a narrow anatomical region. According to the latest DAHANCA RT guidelines, there are at least 24 organs at