Abstract Book

ESTRO 37

S516

under both breath-hold and stationary conditions, to perform relative dose comparison. In addition, measurements for case A were repeated during free- breathing conditions. The gamma pass rate (3%, 3mm) was evaluated in addition to the percentage of voxels with more than 5% under dosage. Results The dose difference distributions are shown in Figure 1. The gamma pass rate for the static and breath-hold SFUD deliveries were 95.2%, 86.1 and 90.5% for cases A-C respectively. For the IMPT deliveries the gamma pass rates were 67.9%, 56.4% and 48.1%. For case A free- breathing, the gamma pass rate was 57.5% and 82.2% for the SFUD and IMPT deliveries respectively. The percentage of voxels receiving more than 5% under dosage was 1.7%, 4.8% and 3.9% for the SFUD cases A-C respectively, and 3.6%, 65.5% and 40.1% for the IMPT cases. The free-breathing case A delivery resulted in more than 5% under dosage in 43.1% and 16.5% of the voxels, for SFUD and IMPT plans respectively. Figure 1. The dose difference images (breath-hold minus static) for both (A) single field uniform dose (SFUD) and (B) intensity-modulated proton therapy (IMPT) plans in color wash. Interplay patterns are visible for the free- breathing (FB) case, and for breath-hold IMPT cases. Conclusion SFUD plans are robust to residual breath-hold motion as a small difference in dose was measured between static and breath-hold dose deliveries. Further, breath-hold efficiently mitigated the motion effect as compared to free-breathing for the SFUD deliveries as shown by the increase in gamma pass rate and decrease of under dosage. For IMPT, large motion effects were observed, e.g. by visible interplay patterns and low gamma pass rate. Even though the gamma pass rate of IMPT plans delivered during free-breathing was higher than for breath-hold, the under-dosage was more pronounced. This indicates that breath-hold can mitigate the motion effect to a certain level. Additional measures, such as e.g. rescanning, will be warranted to mitigate the motion effect on IMPT plans. PO-0947 Development of a customized tongue displacement device using a 3D printer in head and neck IMRT C.S. Hong 1 , S.G. Ju 1 , D. Oh 1 , Y.C. Ahn 1 , C.H. Na 2 , D.Y. Kwon 1 1 Samsung Medical Center, Radiation Oncology, Seoul, Korea Republic of 2 Sungkyunkwan University, Department of Medical Device Management and Research- Samsung Advanced Institute for Health Science & Technology, Seoul, Korea Republic of Purpose or Objective To reduce radiation doses to the tongue, a patient- specific customized tongue displacement device (TDD) was developed using a 3D printer (3DP) for radiation therapy (RT) in head and neck (H&N) cancer patients. Dosimetric characteristics of the TDD were evaluated,

comparing them with those of a standard mouth-piece (SMP). Material and Methods The TDD consists of three parts: a mouthpiece, connector with an immobilization mask, and tongue displacer, which can displace the tongue to the contra lateral side of the planning target volume (PTV), facilitating a stick- out position of the tongue (Figure 1). The mouthpiece has a tooth stopper, an airway, and a tongue position guide bar and verification window to improve inter-fractional reproducibility and to provide patient comfort. Patient- specific customization is enabled by changing the size of the TDD, which provides different thicknesses and lengths to cover various patient conditions, including different mouth-opening and oral cavity lengths. To simplify the manufacturing process, the TDD was designed using a computer-aided design program and printed using a fused deposition modeling 3DP with a biocompatible material. Dosimetric evaluation was conducted for seven patients with histologically proven H&N squamous cell carcinoma, who underwent helical Tomotherapy. Two sets of the planning CT were obtained for each patient using TDD and SMP in the supine position. Tomotherapy plans were generated by applying the same dose constraints and prescription (66 Gy to PTV) based on the RTOG protocol. Dosimetric characteristics of the TDD plan were evaluated, comparing them with those of the SMP plan.

Results Significant reduction was observed in the oral tongue volume (%) within the PTV, in patients treated with the TDD compared to those treated with the SMP (14.6% and 8.7% respectively, p=0.016), whereas the median volume was similar for both SMP and TDD (82.6 cm 3 vs 80.5 cm 3 , p=0.578). Significant median dose reduction of 15.9% was observed following TDD use (29.6 Gy, IQR; 27.3, 30.4) compared to SMP use (34.3 Gy, IQR; 33.0, 35.5). The volumes of tongue receiving a dose of 15, 30, 35, 45, and 60 Gy were significantly lower following TDD use (79.9%, 37.8%, 30.6%, 22.7%, and 7.4%, respectively) than SMP use (89.7%, 48.3%, 41.4%, 29.4%, and 13.7%, respectively) (p<0.05) (Table 1).