3737 - Implications of the Primary Site and Additional Clinical Target Volume Margin in Target Volume Delineation for Intensity Modulated Radiation Therapy with Moderate Hypofractionation in Head Neck Cancer

S. Sinha¹, S. Ghosh Laskar¹, N. Rahul¹, A. Kumar¹, S. Mohanty¹, S. Kale², M. Anaz², and J. P. Agarwal¹; ¹Department of Radiation Oncology, Tata Memorial Centre, Homi Bhabha National Institute, Mumbai, India, ²Department of Medical Physics, Tata Memorial Centre, Homi Bhabha National Institute, Mumbai, India

Purpose/Objective(s): The recent consensus guidelines advocate a 5+5 mm margin from the Gross Tumor Volume (GTV) to Clinical Target Volume (CTV) for definitive Intensity Modulated Radiation Therapy (IMRT) for Head and Neck Squamous Cell Carcinomas (HNSCC). We analyzed the failure patterns of patients treated with definitive IMRT at our institute. Materials/

Methods: This ambispective study includes HNSCC patients (non nasopharyngeal) treated with definitive IMRT between January 2017 and December 2022 at a tertiary cancer center in India. The CTV primary was delineated by generating a 5mm isotropic margin to the GTV, including other regions with a high risk of subclinical spread and editing from uninvolved anatomical barriers. An isotropic CTV to PTV margin of 5 mm was given. The recurrence scans of patients with local, regional, or locoregional failure were fused with the planning CT scan. The locoregional failures were classified according to the standard definition. A dosimetric analysis was done by generating a CTVP2 and giving it an isotropic 5mm margin to generate PTVP2 on the same plan.
Results: One hundred ninety two patients were included in the study. Tobacco use was seen in 156 (81.2%) patients. The most common primary sites were Oropharynx in 69 (35.9%) (p16 positive=19) and Hypopharynx in 48 (25%) patients. The most common dose prescription for the primary tumor was 66Gy/30 fractions (n=168, 87.5%). Bilateral neck nodes were irradiated in 185 patients. The median follow up of the cohort was 20 months.The cohorts two-year locoregional control, progression-free survival, and overall survival were 60.7%, 58.5%, and 72.7%, respectively. Of the total 80 failures, 23 (28.8%) had local, 13 (16.3%) had nodal, 18 (22.5%) had a locoregional, and 26 (32.5%) had distant failures. On analyzing all local, regional, and locoregional failures (n=55), 44 (80%) had infield, 10 (18.1%) had marginal, and 1 (1.8%) had out of field failures (Table 1). The primary site of the tumor was significant for the type of locoregional failure, with all marginal/ out of field failures in p16 negative oropharynx and hypopharynx primaries (p=0.04). An additional CTVP2 and PTVP2 were generated for 80 patients. The median mean dose received for CTVP2 and PTVP2 with the treated plan was 65.7Gy and 63.9Gy, respectively. The PTVP2 was not receiving a mean dose of 57Gy equivalent in only 2 (1.4%) patients.
Conclusion: A 5mm expansion of GTV to CTV and CTV to PTV, along with modest hypofractionation, is safe for definitive IMRT in HNSCC without additional marginal/ out of field failures. The region encompassed by PTVP2, even if not generated separately, receives the recommended intermediate dose in most plans with standard IMRT undergoing bilateral neck RT treated with a single high risk primary PTV. Abstract 3737 – Table 1: Site-wise pattern of all locoregional failures

	Infield (%)	Marginal (%)	Out of field (%)
Oral Cavity (n=43)	16 (37.2)	0	0
Oropharynx (n=69)	8 (11.6)	6 (8.7)	1 (1.4)
Larynx (n=32)	7 (21.9)	0	0
Hypopharynx (n=48)	13 (27.1)	4 (8.3)	0