Dosimetric comparison between using daily cone beam CT and planning CT in volumetric modulated arc therapy technique for prostate cancer therapy

Abstract

The variations of shape and size of organs inside, especially bladder and rectum, in prostate cancer patient are mainly factors of external beam radiation therapy that may impact to disease control and normal tissue toxicity. The purpose of this study was to investigate the dosimetric comparison between using daily CBCT and planning CT in Volumetric modulated arc therapy (VMAT) technique for prostate cancer therapy. The HU and electron density between CBCT and planning CT were verified by using Catphan® 600 phantom. Treatment planning system was verified by dose distribution comparison between plans on CBCT and planning CT images of Anthropomorphic RANDO® phantom. The dose on both images at central axis plan were transferred to Sun Nuclear Patient software to compare in terms of gamma index. Image registration software were also verified. The CBCT image were registered to CT image and the phantom was move to the known couch shifted. The CBCT was repeated and registered with CT. The registration errors were repeated. In clinical part, the total of seven cases with 299 daily CBCT images dataset were performed in this study. The patients were selected from selection criteria as follows with patients whom were diagnosed prostate cancer and already finished the course of VMAT prostate cancer treatment with 6 MV, 79.2 Gy prescription dose, 44 total factions. The structures of Clinical Target Volume (CTV), Planning Target Volume (PTV), bladder, and rectum on daily CBCT were contoured. The CTV was delineated in whole prostate including seminal vesicle and 8 mm margin in all directions except posterior that used 5 mm margin were expended to PTV following by RTOG 0815. The original plans were transferred into daily CBCT images and recalculated by EclipseTM TPS. The volume difference of CTV V100%, PTV V95%, bladder and rectum at V75Gy, V70Gy, V65Gy and V60Gy were analyzed. For results, the HU and electron density was not significant between two imaging modalities. Treatment planning system and image registration showed good agreement on acceptable criteria. The volume differences of CTV at V100% on average from all 7 patients were only -0.1 ± 3.6% that were not significantly different while the volume differences of PTV at V95% was -6.7 ± 5.3%. The bladder volume difference were -23.6 ± 22.8% on average, while the average volumes difference of rectum was more than planning CT of 16.8 ± 58.6%. In conclusion, during course of prostate cancer treatment, organs inside always daily change. The changing is not impacts to dosimetric effect of CTV that means the CTV to PTV margin is enough expansion to disease control. However, the bladder and rectum show significant difference and large variations, especially rectum that have impact on normal tissue toxicity increased.