Optimized Planning Target Volume for Intact Cervical Cancer

Purpose: To model interfraction clinical target volume (CTV) variation in patients with intact cervical cancer and design a planning target volume (PTV) that minimizes normal tissue dose while maximizing CTV coverage. Methods and Materials: We analyzed 50 patients undergoing external-beam radiotherapy for intact cervical cancer using daily online cone-beam computed tomography (CBCT). The CBCTs (n = 972) for each patient were rigidly registered to the planning CT. The CTV was delineated on the planning CT (CTV0) and the set of CBCTs ({CTV1–CTV25}). Manual (n = 98) and automated (n = 668) landmarks were placed over the surface of CTV0 with reference to defined anatomic structures. Normal vectors were extended from each landmark, and the minimum length required for a given probability of encompassing CTV1–CTV25 was computed. The resulting expansions were used to generate an optimized PTV. Results: The mean (SD; range) normal vector length to ensure 95% coverage was 4.3 mm (2.7 mm; 1–16 mm). The uniform expansion required to ensure 95% probability of CTV coverage was 13 mm. An anisotropic margin of 20 mm anteriorly and posteriorly and 10 mm superiorly, inferiorly, and laterally also would have ensured a 95% probability of CTV coverage. The volume of the 95% optimized PTV (1470 cm3) was significantly lower than both the anisotropic PTV (2220 cm3) and the uniformly expanded PTV (2110 cm3) (p < 0.001). For a 95% probability of CTV coverage, normal lengths of 1–3 mm were found along the superior and lateral regions of CTV0, 5–10 mm along the interfaces of CTV0 with the bladder and rectum, and 10–14 mm along the anterior surface of CTV0 at the level of the uterus. Conclusion: Optimizing PTV definition according to surface landmarking resulted in a high probability of CTV coverage with reduced PTV volumes. Our results provide data justifying planning margins to use in practice and clinical trials.