Prognosticating Gross Tumor Volume in Head-and-Neck Cancer - Redefining Gross Tumor Volume Beyond Contouring

Purpose: Precision radiotherapy (RT) requires accurate delineation of gross tumor volumes (GTVs) for targeted dose delivery. Volumetric measurement of this GTV can predict the treatment outcomes. This volume has been limited for mere contouring and its potential as the prognostic factor is less explored. Materials and Methods: The data of 150 patients with oropharynx, hypopharynx, and larynx cancer undergoing curative intensity-modulated RT and weekly cisplatin between April 2015 and December 2019 were retrospectively evaluated. GTV-P (primary), GTV-N (nodal), and GTV-P+N were defined, and volumetric parameters were generated. Volume thresholds were defined as per the receiver operating characteristics, and the prognostic value of these tumor volumes (TVs) with respect to treatment outcomes was analyzed. Results: All patients completed 70 Gy, median chemotherapy cycles were six. Mean GTV-P, GTV-N, and GTV-P+N were 44.5 cc, 13.4 cc, and 57.9 cc, respectively. Oropharynx constituted 45% of cases. Forty-nine percent had Stage III disease. Sixty-six percent had complete response (CR). As per the defined cutoff values, GTV-P <30cc, GTV-N <4 cc, and GTV-P+N <50 cc had better CR rates with P < 0.05 (82.6% vs. 51.9%; 74% vs. 58.4% and 81.5% vs. 47.8%, respectively). At median follow-up of 21.4 months, overall survival (OS) was 60% and median OS was 32.3 months. The median OS in patients with GTV-P <30 cc, GTV-N <4 cc, and GTV-P+N <50 cc was better with P < 0.05 (59.2 vs. 21.4; 59.2 vs. 22.2, and 59.2 vs. 19.8 months, respectively). Conclusion: GTV should not just be limited for contouring but its role as an important prognostic factor has to be recognized.