Evaluation of planning target margins in head and neck cancer patients undergoing radiotherapy: a multicentric study in Bangladesh.

Abstract

In radiation therapy, the planning target volume (PTV) is a geometrical concept used to account for the variations of target position during treatment in terms of systematic (Σ) and random (σ) errors. It ensures that the prescribed radiation dose is delivered to the target with a high degree of confidence. We report the results of a multi-institutional study investigating PTV margin calculation for head-and-neck cancer patients across four radiotherapy departments in Bangladesh, considering the specific challenges and resource constraints of the local context. Forty patients from four different Institutions were enrolled in this multicentric prospective study. All patients underwent planning computed tomography (CT) simulation on supine position, using a head-shoulder thermoplastic mask for immobilization. Contrast-enhanced CT images were acquired with a 3–5 mm slice thickness. All institutions used their locally developed treatment planning technique and inverse planning objectives for intensity-modulated radiotherapy (IMRT) or volumetric modulated arc therapy (VMAT) optimization, together with their imaging protocols (electronic portal imaging device (EPID)-based megavolt (MV), EPID-based kilovolt (kV) or cone-beam CT imaging (CBCT)). For each patient, shifts in the lateral (LR), antero-posterior (AP) and supero-inferior (SI) directions were recorded for each daily fraction. PTV margins in each direction and three-dimensional isotropic margins (PTV3D) were calculated using the Stroom and the van Herk’s equations. The comparisons among institutions were performed using the non-parametric statistical Kruskal-Wallis test, followed by a Dunn’s test. Results were compared with those published in literature for head-and-neck cancer radiotherapy. 1400 setup images from CBCT or EPID were analysed. Using the van Herk recipe, in order to ensure ≥ 95% of tumor coverage in 90% of patients, the isotropic PTV3D were calculated as 7.1, 4.7, 8.0 and 7.4 mm for the institutions A, B, C and D, respectively. Institution B reported significant smaller values for all metrics (p < 0.05) that was attributed to the use of a hybrid kV-CBCT in the set-up protocol of this Institution, not used in the other three Centers. A total of 16 studies were identified from the literature, providing a weighted mean PTV3D margin of 6.8 mm. This multicentric study represents a significant step forward in addressing the challenges of PTV margin calculation for HNC patients in a low- and middle-income country setting. The findings highlight substantial variability in systematic and random errors across institutions, emphasizing the critical importance of customizing PTV margins to reflect local practices, available resources, and technological constraints. The study also demonstrates that accurate image-guided radiation therapy (IGRT) protocols, can significantly reduce PTV margins, potentially minimizing toxicity without compromising treatment efficacy, highlighting the need to prioritize investments in advanced IGRT technologies.