# Interobserver Variation Within Planning Target Volume and Organs at Risk in a Patient with Oropharyngeal Carcinoma: A Contouring Study with Anatomical Analysis

## TL;DR

This study shows that senior radiation oncologists have significant differences in how they outline cancer treatment areas and nearby organs, highlighting the need for better standardization in radiotherapy planning.

## Contribution

The study quantifies interobserver variability in contouring practices among experienced oncologists for oropharyngeal carcinoma.

## Key findings

- Substantial discrepancies were found in PTV size, with nearly two-fold differences observed.
- High agreement was seen in including certain lymph node levels, but variability was notable in others like the cervical triangle group.
- The greatest variability in OARs was observed in the delineation of the left and right inner ear.

## Abstract

This study examined differences in the contouring of planning target volumes (PTVs) and organs at risk (OARs)—a critical component of modern radiotherapy planning—among senior radiation oncologists. Ten participants independently contoured a case of oropharyngeal carcinoma, revealing substantial discrepancies, including up to nearly two-fold differences in PTV size. These findings underscore the ongoing need to refine and standardize contouring practices to ensure greater consistency in radiotherapy planning and ultimately improve the quality and safety of patient care.

Background: Despite the availability of contouring guidelines and advanced imaging modalities, interobserver variability (IOV) in the delineation of the planning target volume and organs at risk remains a critical factor influencing treatment quality in radiotherapy. The aim of this study was to examine variations in contour delineation with respect to anatomical landmarks, as well as differences in the inclusion of lymph node levels within the PTV. Methods: Ten senior radiation oncologists from six different institutions participated in the study and contoured PTV1, PTV2 and 16 OARs in a patient with oropharyngeal carcinoma. Interobserver variation was quantified by volume statistics such as mean, standard deviation (SD) and ranges, as well as using coefficient of variance (CoV) and conformity index (CI). Results: High agreement was observed in the inclusion of the ipsilateral lymph node levels Ib–IVa and VIIa+b, whereas notable discrepancies were identified in the delineation inclusion of the cervical triangle group and lateral supraclavicular nodes. Regarding OARs, the greatest variability was observed in the delineation of the left and right inner ear, with volume ranges of 0.12–2.84 cm3 and 0.11–2.38 cm3, respectively. Conclusions: This study reaffirms the presence of significant interobserver variability in the delineation of PTVs and OARs in patients with oropharyngeal carcinoma. Especially inclusion of elective lymph node levels and definition of margins around the gross tumor volume are substantial factors for IOV. By emphasizing structured anatomical assessment as a standard approach, variability can be minimized, treatment consistency enhanced, and ultimately, patient outcomes improved.