ESTRO 2024 - Abstract Book

S137

Invited Speaker

ESTRO 2024

1. Moore, C., et al., Personalized Ultrafractionated Stereotactic Adaptive Radiotherapy (PULSAR) in Preclinical Models Enhances Single-Agent Immune Checkpoint Blockade. Int J Radiat Oncol Biol Phys, 2021. 110 (5): p. 1306-1316. 2. Tubin, S., et al., Shifting the Immune-Suppressive to Predominant Immune-Stimulatory Radiation Effects by SBRT PArtial Tumor Irradiation Targeting HYpoxic Segment (SBRT-PATHY). Cancers (Basel), 2020. 13 (1). 3. Prezado, Y., Divide and conquer: spatially fractionated radiation therapy. Expert Reviews in Molecular Medicine, 2022. 24 : p. 12. 4. Favaudon, V., et al., Ultrahigh dose-rate FLASH irradiation increases the differential response between normal and tumor tissue in mice. Sci Transl Med, 2014. 6 (245): p. 245ra93.

3593

Target volume standardisation and reduction in head and neck tumours: Chances and risks

Ester Orlandi

University of Pavia, Department of Clinical, Surgical, Diagnostic, and Pediatric Sciences, Pavia, Italy. CNAO - National Center for Oncological Hadrontherapy, Clinical Department, Pavia, Italy

Abstract:

Standardizing the delineation of target volumes is crucial for reducing inter-observer variability and ensuring consistency and accuracy in treatment plans. This practice enables clinicians to deliver radiation effectively to tumor sites, minimizing local recurrences, and sparing healthy tissues, thereby reducing the risk of severe toxicities. Efforts to reduce inter-observer variability in delineating the clinical target volume (CTV) for head and neck cancer have been ongoing for decades. Two main approaches have emerged: one based on anatomical extension of the gross tumor volume (GTV), and another involving geometric expansion of the GTV. Recently, international consensus guidelines have integrated the strengths of both approaches for CTV delineation. However, in a recent Belgian study (Bollen H 2023), the implementation of consensus guidelines for the primary tumor's clinical target volume (CTVp) significantly increased delineation uniformity among radiation oncologists. However, this led to a substantial decrease in CTV receiving high dose radiation therapy, highlighting the need for a correct interpretation of the guidelines. Measures to address this included joint delineation review sessions and systematic training via online educational platforms Despite advancements in highly conformal radiotherapy techniques, the burden of toxicity remains significant. One current strategy addressing this issue involves reducing the volume treated electively in the lymph nodes (CTVne) and the CTVp, particularly in oropharyngeal, nasopharyngeal carcinoma and primary unknown cases. Oropharyngeal cancer. Multiple retrospective studies have examined the efficacy of irradiating the ipsilateral neck in early-stage lateralized oropharyngeal cancer (OPC) with minimal disease burden on one side of the neck (N0-N1), showing promising outcomes. Despite the absence of randomized controlled trials, guidelines now recognize the potential to restrict radiation to the ipsilateral neck in specific scenarios, such as lateralized OPC with N0-N1 nodal stage or N0-N2a tonsil tumors that do not invade the soft palate or base of tongue. Burr et al. retrospectively analyzed 134 patients with p16+ oropharyngeal squamous cell carcinoma. They treated patients by eliminating the margin from the gross tumor volume (GTV) to clinical target volume (CTV), using a direct expansion of GTV to planning target volume (PTV) by 2–3 mm. After a median follow-up of 56.2 months, 10.4% of patients experienced recurrence (14 cases). Among these cases, 10 patients had an "in field" recurrence, with only 3 and 1 patients showing marginal or geographic recurrence, respectively (Burr et al., 2019). These findings suggest that smaller margins may be sufficient