Abstract
PurposeThe purpose of this study is to evaluate inter- and intra-fraction organ motion as well as to quantify clinical target volume (CTV) to planning target volume (PTV) margins to be adopted in the stereotactic treatment of early stage glottic cancer.Methods and materialsStereotactic body radiotherapy (SBRT) to 36 Gy in 3 fractions was administered to 23 patients with early glottic cancer T1N0M0. Patients were irradiated with a volumetric intensity modulated arc technique delivered with 6 MV FFF energy. Each patient underwent a pre-treatment cone beam computed tomography (CBCT) to correct the setup based on the thyroid cartilage position. Imaging was repeated if displacement exceeded 2 mm in any direction. CBCT imaging was also performed after each treatment arc as well as at the end of the delivery. Swallowing was allowed only during the beam-off time between arcs. CBCT images were reviewed to evaluate inter- and intra-fraction organ motion. The relationships between selected treatment characteristics, both beam-on and delivery times as well as organ motion were investigated.ResultsFor the population systematic (Ʃ) and random (σ) inter-fraction errors were 0.9, 1.3 and 0.6 mm and 1.1, 1.3 and 0.7 mm in the left-right (X), cranio-caudal (Y) and antero-posterior (Z) directions, respectively. From the analysis of CBCT images acquired after treatment, systematic (Ʃ) and random (σ) intra-fraction errors resulted 0.7, 1.6 and 0.7 mm and 1.0, 1.5 and 0.6 mm in the X, Y and Z directions, respectively. Margins calculated from the intra-fraction errors were 2.4, 5.1 and 2.2 mm in the X, Y and Z directions respectively. A statistically significant difference was found for the displacement in the Z direction between patients irradiated with > 2 arcs versus ≤ 2 arcs, (MW test, p = 0.038). When analyzing mean data from CBCT images for the whole treatment, a significant correlation was found between the time of delivery and the three dimensional displacement vector (r = 0.489, p = 0.055), the displacement in the Y direction (r = 0.553, p = 0.026) and the subsequent margins to be adopted (r = 0.626, p = 0.009). Finally, displacements and the subsequent margins to be adopted in Y direction were significantly greater for treatments with more than 2 arcs (MW test p = 0.037 and p = 0.019, respectively).ConclusionsIn the setting of controlled swallowing during treatment delivery, intra-fraction motion still needs to be taken into account when planning with estimated CTV to PTV margins of 3, 5 and 3 mm in the X, Y and Z directions, respectively. Selected treatments may require additional margins.
Highlights
Laryngeal cancer is the most frequent cancer of the upper respiratory tract (28 %) affecting over 13,500 patients a year in the United States and causing about 3700 deaths [1]
When analyzing mean data from cone beam computed tomography (CBCT) images for the whole treatment, a significant correlation was found between the time of delivery and the three dimensional displacement vector (r = 0.489, p = 0.055), the displacement in the Y direction (r = 0.553, p = 0.026) and the subsequent margins to be adopted (r = 0.626, p = 0.009)
In the setting of controlled swallowing during treatment delivery, intra-fraction motion still needs to be taken into account when planning with estimated clinical target volume (CTV) to planning target volume (PTV) margins of 3, 5 and 3 mm in the X, Y and Z directions, respectively
Summary
Laryngeal cancer is the most frequent cancer of the upper respiratory tract (28 %) affecting over 13,500 patients a year in the United States and causing about 3700 deaths [1]. The standard treatment for early glottic cancer is surgery (open or endoscopic) or radiotherapy (RT); both approaches provide excellent oncological outcomes with overlapping local control rates at ≈ 90% for T 12–4 and ≈ 80 % for T2 lesions [2,3,4,5]. RT may allow a slightly better quality of voice than surgery [6] and the recommended doses for T1 lesions of the true vocal cords (TVC) are 63-66 Gy in 28–33 fractions to the whole larynx. At a median follow-up of 30 months, the local control rate was 100% in 30 patients with cT1a lesions, without significant acute or long-term toxicity. The voice handicap index (VHI) improved significantly over time, from 33 (baseline) to 9.5 after 6 weeks and to 10 at 18 months
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