International Journal of Radiation Oncology*Biology*Physics
Clinical InvestigationCone Beam CT Imaging Analysis of Interfractional Variations in Bladder Volume and Position During Radiotherapy for Bladder Cancer
Introduction
Recent advances in radiotherapy (RT) treatment planning and delivery technology have enabled the ability to produce highly conformal RT treatment plans with high accuracy not seen before. Despite these technical advances in RT practice, one potential barrier to accurate RT delivery remains interfractional target volume position and/or size variations that may occur during RT.
Current RT treatment processes are based on static radiologic images obtained prior to commencing RT. Population-based margins are placed around gross tumor volumes (GTV) defined on these images to account for adjacent regions of microscopic disease extension (clinical target volume [CTV]) and day-to-day variations in patient and/or target volume position (planning target volume [PTV]), with the assumption that during RT, the predefined GTV will be contained within the PTV. This assumption may not be entirely true in bladder cancer RT.
Despite the demonstrated efficacy of RT for bladder cancer 1, 2, 3, technical challenges remain to accurately deliver RT to this tumor. The bladder is a dynamic organ whose position can vary significantly depending on the degree of filling, making bladder cancer a good example of how interfractional target volume size and position variations can make accurate daily RT delivery challenging. Using diagnostic CT scans obtained midway through RT for bladder cancer, earlier studies demonstrated that despite the addition of margins around the bladder to define the PTV, changes in bladder target volumes occur during RT that are significant enough to warrant making plan adjustments to ensure adequate PTV coverage in a significant proportion of study participants (4).
The recent development of on-board imaging devices has enabled daily assessments of RT target volume position and size prior to RT. One such device is the kilovoltage cone beam CT scanner (CBCT) 5, 6. The purpose of this protocol was to quantify interfractional variations in bladder size and position that occur during RT for muscle-invasive bladder cancer.
Section snippets
Patient selection
Accrual to the protocol commenced after institutional research ethics board approval was granted. Inclusion criteria for the protocol were presence of biopsy proven muscle-invasive transitional cell bladder carcinoma; undergoing a regimen of concurrent chemoradiotherapy or RT alone in which at least 20 RT fractions were prescribed; and no prior chemotherapy. Signed informed consent was obtained. Ten patients were recruited. Patient, tumor, and treatment details are summarized in Table 1. Each
Variations in COM and perimeter margin position
The largest magnitude of bladder COM shift was observed in the y coordinate (anterior-posterior) (mean, −0.32 cm). The anterior bladder wall position shifted the most (mean shift, −0.58 cm). Variations in the COM shifts in the x, y, and z coordinates and the positional shifts of the other bladder walls (posterior, inferior, superior, right, and left lateral) are summarized in Table 2. Figure 2a and b summarize mean temporal changes in COM coordinates and bladder wall positions for all 10
Discussion
The recent development of the ability to deliver highly conformal RT dose distributions to identified PTVs has necessitated increased attention to the issue of daily target volume localization, especially when treating mobile targets which may deform in their configuration at different times during the course of RT (7). New in-room imaging tools such as megavoltage and kilovoltage CT scanners directly integrated with linear accelerators facilitate the ability to determine target volume position
Conclusions
Significant highly individualized interfractional variations occur in bladder size and position during a course of RT for muscle-invasive bladder cancer, suggesting the need for strategies to address these variations and ensure accurate daily RT delivery.
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2021, Clinical OncologyCitation Excerpt :Marker placement in the normal bladder at the boundary of the tumour may not always be representative of changes in GTV position, and so the further away from the GTV border, the less representative it becomes [36]. Different degrees of invasion could also vary tumour contractibility, as it can be presumed that bladder wall fixation and stretch may be different for T2 tumours compared with T4 tumours [35]. It remains unclear whether there is any correlation between GTV size change and bladder filling status.
Anisotropic Bladder Planning Target Volume in Bladder Radiation Therapy
2019, Practical Radiation OncologyCitation Excerpt :Turner et al. recommend using a margin of at least 2 cm to account for interfraction bladder volume variability.6 The region of the bladder with the largest magnitude of volume variability is the dome because of the superior and anterior displacement of the bladder wall in response to filling.5,7,8 Even for less invasive tumors, this recommendation seems standard.
This work was funded by a grant from Varian Medical Systems.
Conflict of interest: DY holds a research grant from Varian Medical Systems.