Gross Tumor Volume and Metastases Evaluations
Prior to course 1, imaging examinations by magnetic resonance imaging (MRI) and/or computed tomography (CT) were performed to quantify the gross tumor volume (GTV, in cm3) of the primary NB tumor. Imaging examinations were repeated after course 2 of antibody infusions to obtain a new GTV following induction chemotherapy. These volumes were available from the institutional NB study database. A “tumor volume ratio” was calculated by dividing the GTV prior to course 1 by the GTV following course 2. A greater treatment response led to smaller GTV following course 2 (smaller denominator) and therefore a higher tumor volume ratio. In the absence of response, both the numerator and denominator of the tumor volume ratio are the initial GTV, which led to a tumor volume ratio of 1. For patients with acomplete response to treatment (2nd GTV essentially 0 cm3), the denominator of the tumor volume ratio was credited as “1 cm3” to prevent an undefined fraction. To summarize, a higher tumor volume ratio equated to a greater response to treatment. A tumor volume ratio of 1 equated to a complete lack of response to treatment.
Additionally, tumor burden outside of the primary tumor was evaluated using Curie scores obtained by meta-iodobenzylguanidine (MIBG) scans.21 A Curie score ratio (Curie score prior to course 1 divided by Curie score following course 2) was calculated and used for analysis rather than raw Curie scores.