Abstract
Particle therapy (PT) has shown positive therapeutic results in local control of locally advanced pancreatic lesions. PT effectiveness is highly influenced by target localization accuracy both in space, since the pancreas is located in proximity to radiosensitive vital organs, and in time as it is subject to substantial breathing‐related motion. The purpose of this preliminary study was to quantify pancreas range of motion under typical PT treatment conditions. Three common immobilization devices (vacuum cushion, thermoplastic mask, and compressor belt) were evaluated on five male patients in prone and supine positions. Retrospective four‐dimensional magnetic resonance imaging data were reconstructed for each condition and the pancreas was manually segmented on each of six breathing phases. A k‐means algorithm was then applied on the manually segmented map in order to obtain clusters representative of the three pancreas segments: head, body, and tail. Centers of mass (COM) for the pancreas and its segments were computed, as well as their displacements with respect to a reference breathing phase (beginning exhalation). The median three‐dimensional COM displacements were in the range of 3 mm. Latero–lateral and superior–inferior directions had a higher range of motion than the anterior–posterior direction. Motion analysis of the pancreas segments showed slightly lower COM displacements for the head cluster compared to the tail cluster, especially in prone position. Statistically significant differences were found within patients among the investigated setups. Hence a patient‐specific approach, rather than a general strategy, is suggested to define the optimal treatment setup in the frame of a millimeter positioning accuracy.PACS number(s): 87.55.‐x, 87.57.nm, 87.61
Highlights
Pancreatic cancer has a high incidence and mortality rate worldwide
As a matter of fact, the abdominal region is affected by both inter- and intrafraction anatomical changes, which need to be considered in order to grant the optimal dose distribution.[15]. The impact of these anatomical variations is strictly related to size, shape, and anatomical site of the lesion
While using three common immobilization devices for patients in prone and supine positions, we found pancreas ranges of motion that were globally lower than what reported in previous studies, where no immobilization systems were applied, and maximal motion values on the order of 20 mm were found in the SI direction.[21,23,29,30] In keeping with our results, a prior study investigating the effectiveness of immobilization systems reported superior–inferior motion of 4.4 mm for abdominal organs with the compressor device, proving a significant motion reduction.[38]. In the current study we found median motion in SI direction below 2.5 mm, and similar motion in the LL direction
Summary
Pancreatic cancer has a high incidence and mortality rate worldwide. It is considered to be amongst the 10 most common tumors affecting the population of the United States, with the number of expected deaths in 2015 equaling approximately 80% of the estimated new cases.[1]One of the main reasons for such a high death rate and unfavorable prognosis is related to late diagnosis; the first symptoms appear after approximately a decade of pancreatic carcinogenesis, when the tumor has reached an advanced stage.[2,3] As a consequence, only 20% of pancreatic cancer patients are candidates for surgery,(4) as the majority of pancreatic cancer patients show locally advanced or metastatic disease. Novel radiation therapy techniques with accelerated ion beams have shown positive therapeutic results in local control of locally advanced pancreatic cancer.[6,7,8,9]. The use of ion beams in particle therapy (PT) enables dose distributions with higher physical and radiobiological selectivity compared to conventional photons. Such an increased dose conformity to the target may yield benefits in terms of enhanced efficacy and reduced side effects of the treatment.[7,10,11,12,13,14] this geometrical advantage is associated with a strong sensitivity to inconsistencies between the treatment plan and the actual treatment scenario. All these features make the pancreas a challenging treatment site, requiring appropriate techniques for describing, handling, and mitigating pancreas motion
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