Oral Presentation 37th TROG Cancer Research Annual Scientific Meeting 2025

Optimisation of Contralateral and Midline Organs at Risk (OARs) in Tonsil Patients treated with Unilateral Radiotherapy (URT): a Re-optimisation Planning Study from TROG 12.01 (#50)

Glen Osbourne 1 , Nadia Walsh 1 , Aimee Devlin 2 , Nick Hardcastle 1 , Alisha Moore 3 , Danny Rischin 4 5 , Tsien Fua 1 , Lizbeth Kenny 6 7 , Andrew McCann 8 , Sandro Porceddu 1 3 , Christopher Wratten 9 , June Corry 10 11 , Lachlan McDowell 2 7
  1. Department of Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia
  2. Department of Radiation Oncology, Princess Alexandra Hospital, Brisbane, Queensland, Australia
  3. Department Radiation Quality Assurance, Trans-Tasman Radiation Oncology Group, Newcastle, New South Wales, Australia
  4. Department of Medical Oncology, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia
  5. Sir Peter MacCallum Department of Oncology, University of Melbourne, Melbourne, Victoria, Australia
  6. Department of Radiation Oncology, Royal Brisbane & Women's Hospital, Brisbane, Queensland, Australia
  7. Faculty of Medicine, University of Queensland, Brisbane, Queensland, Australia
  8. Department of Radiation Oncology, Auckland City Hospital and University of Auckland, Auckland, New Zealand
  9. Department of Radiation Oncology, Calvary Mater Hospital & University of Newcastle, Newcastle, New South Wales, Australia
  10. Radiation Oncology, Genesiscare St Vincent's Hospital, Melbourne, Victoria, Australia
  11. Department of Medicine, University of Melbourne, Melbourne, Victoria, Australia

1.Background

URT is an effective treatment strategy in selected patients with lateralized tonsil cancer. However, there are no established guidelines for URT planning, potentially resulting in suboptimal optimization of contralateral and midline OARs.

2.Aims

To establish OAR goals in URT planning.

3.Methods

Treatment plan data for 22/26 available patients treated with URT on TROG 12.01 were imported into Eclipse V16.01. All reported structures were recontoured according to consensus guidelines.  For each case, a volumetric modulated arc therapy plan was produced and peer-reviewed by three experienced radiation therapists and subsequently reviewed for clinical acceptability by two experienced radiation oncologists. Optimisation objectives prioritised protocol-stipulated TROG 12.01 target volume coverage (gross tumour volumes, GTV; planning target volumes, PTV) but also included contralateral and midline OARs aiming for maximal OAR sparing without compromising target coverage while also not exceeding clinically acceptable doses to the mandible, brainstem or spinal cord. Metrics for all targets and OARs were compared with the initially delivered plan.

4.Results

Target volume metrics on re-optimized plans were protocol-compliant albeit with a small reduction in the D95% of the PTV7000 (delivered vs replanned, 68.0 vs 66.7Gy, p<0.0001) and PTV6300 (62.5 vs 61.3, p=0.03), but not the PTV5600, or the D100% of the primary or nodal GTVs.

Compared with the delivered plan, the re-optimized plans demonstrated significant reductions (all p<0.001) in mean doses to the pharyngeal constrictors (48.5 vs 37.4Gy), contralateral hemi-pharyngeal constrictor (38.1 vs 23.1Gy), contralateral submandibular gland (14.9 vs 6.7Gy), contralateral parotid gland (9.2 vs 4.9Gy), oral cavity (39.5 vs 35.9) and glottis (21.5 vs 9.8).  The majority of differences were considered clinically relevant.

5.Conclusions

Significant midline and contralateral OAR sparing can be achieved whilst maintaining target coverage, reducing the risk of acute and late toxicities. These results provide the first available guidance on achievable OAR constraints for URT planning.