Validation of the CTV Margin for Laryngeal and Hypopharyngeal Cancer

Modality-specific margins for the clinical target volume (CTV) were reported for laryngeal and hypopharyngeal cancer in previous work. CTV margins of 4.3 mm, 6.1 mm and 5.2 mm were needed to achieve adequate tumor coverage for gross tumor volumes (GTV) delineated on respectively CT, MR and 18-FDG-PET images. This CTV margin was determined using hematoxylin-eosin (HE) stained whole mount slices of total laryngectomy specimens. Because of the clinical consequences of this finding we decided to validate this HE-based tumor delineation using immunohistochemical pan-cytokeratin (CKAE1/3) staining as gold standard. CTV margins were derived using data from twenty-seven patients with T3/T4 laryngeal/hypopharyngeal cancer that underwent CT, MRI and PET scans before total laryngectomy. From each patient, four subsequent slices containing tumor tissue were selected yielding a total of 108 slices. From each slice, two 4μm-thick whole-mount sections were obtained and stained with HE and with CKAE1/3. The tumor was delineated both on the HE and the CKAE1/3 stained sections by a dedicated head-and-neck pathologist using a microscope. The stained sections were three-dimensionally reconstructed and rigidly registered to each other. Registration between CKAE1/3 and HE stained slices was performed by manually selecting 3-5 corresponding landmarks. The HE-based tumor delineations were compared with the CKAE1/3-based tumor delineations. The maximum distance between HE and CKAE1/3 tumor delineations was determined for each patient. The 99th percentile distance for microscopic disease detected outside the tumor delineated on the HE sections was 1.7 mm (range 0.9-3.4 mm). This distance includes the registration error between the HE and the CKAE1/3 stained slices and is comparable to the inter-observer variation reported for delineations on HE sections. No indications for isolated islands of tumor cells were detected on both HE and CKAE1/3 stained slices. The CTV margin for laryngeal and hypopharyngeal tumors of 4-6 mm depending on the imaging modality has been validated using CKAE1/3 staining as gold standard. No indication was found for an additional margin needed to cover isolated islands of tumor cells.