Abstract
e17056 Background: Intensity modulated radiation therapy (IMRT) has been widely used in the treatment of head and neck cancer and has the potential of escalating the dose with less morbidity. Integral dose is a concern in radiotherapy especially with IMRT as it may lead to an increase in second malignancies. Setup error is another important issue in radiotherapy which affects the final treatment outcome. In this study, the effect of setup error on non-target integral dose (NTID) for head and neck cancer treated with IMRT has been studied. Methods: Twelve patients of carcinoma of nasopharynx treated by IMRT with simultaneous integrated boost technique were included. The dose to the gross tumor volume (PTV70), CTV1 (PTV59.4) and CTV2 (PTV54) were planned for 70 Gy, 59.4 Gy and 54 Gy respectively in 33 fractions. All the patients were planned with nine equally spaced beams. The IMRT plan was generated in Eclipse treatment planning system using sliding window technique. Keeping it as the base plan, the patient setup error was simulated in the treatment planning system for 3 mm, 5 mm and 10 mm by shifting the isocenter in all three dimensions viz. anterior, posterior, superior, inferior, right lateral and left lateral. NTID was calculated from the following relation NTID = (non-target tissue volume X mean non-target tissue dose)/1000. Paired sample t-test was performed to compare NTID for different shifts with no shift (statistical significance: p < 0.05). Results: The mean NTID for no shift was 120.75 ± 22.85 (median: 116.59). The mean NTID with isocenter shift along the anterior direction were 119.87 ± 22.80, 119.24 ± 22.80, 117.97 ± 22.52 for 3 mm, 5 mm and 10 mm respectively. Similarly, the NTID for the posterior shifts were 121.59 ± 22.96, 122.08 ± 23.07 and 123.26 ± 23.34 for 3 mm, 5 mm and 10 mm respectively. The anterior and posterior setup error showed statistically significant difference (p < 0.0001) for NTID compared to no shift whereas lateral, superior and inferior setup errors were found to be statistically insignificant. Conclusions: Setup error is an important issue in radiotherapy. Our study shows that setup error along the posterior direction significantly increases NTID. Hence, the setup error along the posterior direction may increase the risk of second malignances in IMRT. No significant financial relationships to disclose.
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