Daily Variation of Planning Target Volume Position in Prostate Cancer Patients Treated With Helical Radiotherapy

Background and Purpose Prescribing an accurate dose of radiation when treating prostate cancer using skin marks alone is very challenging due to the mobility of the prostate from a variety of factors. This study measured the daily variation of the Planning Target Volume (PTV) position in patients with prostate cancer treated with Image-Guided Radiotherapy (IGRT). Materials and Methods From May 2011 to June 2011, 38 IGRT sessions from four patients diagnosed with localized prostate cancer were registered. IGRT was performed using a helical tomotherapy machine with a megavoltage computed tomography system. PTV variation was measured daily by comparing tomography coordinates to the corresponding skin marks on patients. Results Differences in displacement were found between X (left-right), Y (anterior-posterior), and Z (up-down) axes. Displacements were greater in X with a range of 33 mm (range, -20 to 13 mm), followed by Z with 18.6 mm (range, -6.1 to 12.5 mm), Y with 11.2mm (range, -5.8 to 5.4 mm), and roll (clockwise-counterclockwise) with 0.7° (-1.6 to 3.1°). Large prostate shifts were found between 18.4% to 52.6% in X, 2.6 to 7.9% in Y, and 21.1% to 94.7% in Z. Conclusion Large displacements and large prostate shifts were detected in all patients. IGRT contributed to reduced exposure of excessive radiation to OAR, which would not have been possible with skin mark guides alone. Adequate IGRT should be employed to make daily adjustments to the PTV location