Effect of bladder volume changes and verification of CTV on CBCT for rectal cancer patients
Abstract: Background and purpose: During radiotherapy, rectal cancer patients show inter-fractional internal motion that effects the delivered dose distribution. The purpose of this work is to study I) the inter-fractional bladder volume change, II) the effect of bladder volume change on bowel dose distribution, III) the effect of bladder optimization on the relationship between bladder volume and bowel dose and IV) the possibilities and difficulties of validating the clinical target volume (CTV) using a surrogate on cone-beam computed tomography (CBCT) scans. An additional purpose is to find the most appropriate surrogate of the CTV to use in the validation. Material and methods: Twenty-eight rectal cancer patients treated with preoperative radiotherapy in the period February 2015 to January 2016 were included in the study. All treatments were delivered with RapidArc, using two arcs. Each patient had a planning CT scan and weekly CBCT were acquired the first three, four, five and six weeks for 23, 1, 1 and 3 patients, respectively. The bladder and the bowel was delineated on the CBCT scans and transferred to the CT scan Treatment plans not including a bladder optimization were re-optimized, to investigate possibilities with lowering the bowel dose. In accordance with the latest local guidelines, the re-optimized treatment plans included a bladder optimization and delivered the dose with three arcs instead of two. A surrogate of the CTV was delineated on the CT and compared with corresponding structure on the CBCT. The surrogate was defined as the rectum where rectum and mesorectum could be distinguished and as the mesorectum elsewhere. Caudally from the rectum, the surrogate was defined by the circumference of the levator ani. No surrogate was delineated cranial from the rectum. The surrogate was divided into an upper, mid and lower section during the validation. Results: The median bladder size was significantly smaller on the CBCT than on the CT. Out of the 28 patients, 13 patients had bladders consistently smaller and larger on the CBCTs. Six patients had consistently larger bladder volumes on the CBCTs than on the CT. For 9 patients, the relationship between the bladder volume on the CT and CBCTs varied. The bladder volumes on the CBCTs ranged from 0.1 to 3.5 times the bladder volume on the corresponding CT, with the majority (81/96) in the range between 0.3 and 2.0. The change in bladder filling did not result in a violation of the bowel constraint V45Gy<195 cm3 for any patient. The re-optimized treatment plans resulted in lower bowel doses, without compromising PTV coverage. The bladder optimization was not proven (p=0.46) having an effect the relationship between V45Gy and bladder volume change. The image quality was sufficient for a validation of the surrogate on 94/95 CBCT scans. The CBCT scan where the surrogate could not be validated suffered from major artifacts due to internal gas. The delineated surrogate did not extend as far cranially as the gross tumor volume (GTV) or the primary CTV for 12/28 and 19/26 patients, respectively. The variation of surrogate was within 5 mm in the mid and upper section on 40/70 and 56/90 CBCTs, respectively. Cranial-caudal shifts in the position of the sigmoideum and internal gas challenged the validation of the surrogate. Conclusion: There is a correlation between bladder volume and bowel radiation dose. However, the dose constraint for the small bowel was not violated for any of the patients in this study. The relation between bowel dose and bladder volume is highly individual and not proven being effected by bladder optimization. The bowel dose can be reduced by optimizing the plans according to the latest local guidelines. The CTV can be validated on CBCT using a surrogate. The surrogate can be defined somewhat general caudal from the cranial border of rectum but not cranially from rectum. The variation of the surrogate is usually within 5 mm.
AT THIS PAGE YOU CAN DOWNLOAD THE WHOLE ESSAY. (follow the link to the next page)