A Contouring Strategy and Reference Atlases for the Full Abdominopelvic Bowel Bag on Treatment Planning and Cone Beam Computed Tomography Images.

Purpose: To establish a practical contouring strategy with reference atlases for the abdominopelvic bowel bag on treatment planning computed tomography (TPCT) and cone beam computed tomography (CBCT) images. Methods and Materials: A scoping literature review was done to evaluate the existing definitions and contouring guidelines for bowel bag and small bowel planning-at-risk volume-like structures. A comprehensive definition was proposed for the abdominopelvic bowel bag that expanded the Radiation Therapy Oncology Group Pelvic Normal Tissue Consensus definition. Seven patients with TPCT and first-treatment-day CBCT images were selected from an institutional database to represent a range of normal anatomy and CBCT image quality. The TPCT and CBCT images were contoured using the proposed definition. During contouring, the Radiation Therapy Oncology Group definition's list of inclusion and exclusion structures was expanded. For areas with limited visibility of the bowel bag on either TPCT or CBCT, a set of operational definitions was developed based on consistently visible reference structures. Results: A literature review showed that previously existing bowel bag definitions predominantly focused on the pelvic region and did not provide a complete and practical description of the full abdominopelvic contour relative to structures consistently visible in all radiation therapy images. The proposed contouring strategy had 4 components: a definition, a list of inclusion and exclusion structures, 15 tabulated operational definitions, and a set of atlases. The bowel bag was defined as the peritoneal cavity and retroperitoneal duodenum and ascending and descending colon, as visualized at the time of image acquisition. The operational definitions formalized the location of the peritoneal fascial planes through a simple look-up table. The proposed contouring strategy and reference atlases were successfully used on both TPCT and CBCT images. Conclusions: This study produced a practical contouring strategy and reference atlases to enable reproducible delineation of the full bowel bag on TPCT and CBCT images. The strategy is a necessary first step toward consensus contouring with reduced observer variability, which is a prerequisite for evaluation of cumulative dose and its correlation with toxic effects, adaptive planning strategies, and automated contouring potential.

Robustness of three external beam treatment techniques against inter-fractional positional variations of the metal port in breast tissue expanders.

INTRODUCTION: Temporary breast tissue expanders contain a metal port that varies in position throughout the course of radiation treatments. The purpose of this study was to quantify the robustness of the three most common external beam treatment techniques (tangential three-dimensional conformal radiation therapy [3DCRT], volumetric modulated arc therapy [VMAT], and helical tomotherapy) against our measured inter-fractional positional variations of the port. METHODS: For eight breast cases, a clinical plan was created for each of the three techniques. The dosimetric effect of our previously measured inter-fractional port errors was evaluated for two classes of error: internal port errors (IPEs) and patient registration errors (PREs). For both classes of error, daily variable and systematic errors were modeled, and their cumulative effects were compared against the originally planned doses. RESULTS: For systematic IPE, the 1%-99% range in point dose differences inside a 5-mm target abutting the implant was the highest for tangential 3DCRT, and it was within 6% and 9% when calculated with Monte Carlo and collapsed cone calculation engines, respectively. Daily variable PRE resulted in mean changes of -3.0% and -3.5% to V100%Rx of the target for VMAT and tomotherapy, respectively. For nearby organs, daily variable PRE resulted in changes to V20Gy of the ipsilateral lung of less than 2% in all three techniques, while V5Gy of the heart increased by as much as 6% in VMAT and 10% in tomotherapy. CONCLUSIONS: When IPEs were modeled, dose variability was the largest in tangential 3DCRT, leading to areas of underdosage in the shadow of the port. When PREs were modeled, the target coverage and nearby organs were affected the most in VMAT and helical tomotherapy. In reality, port positional errors result from a combination of IPE and PRE, suggesting that VMAT and tomotherapy are more robust when patient registration errors are minimized, despite the presence of IPE.

Magnitude and dosimetric impact of inter-fractional positional variations of the metal port of tissue expanders in postmastectomy patients treated with radiation.

BACKGROUND AND PURPOSE: Postmastectomy breast reconstruction involves the insertion of a temporary tissue expander, which contains a metal injection port. The purpose of this study was to determine the magnitude and dosimetric impact of the inter-fractional positional variations of the port for patients treated with radiation. MATERIALS AND METHODS: For nine breast cases treated on Tomotherapy, the deviation of the port in the daily MVCT from its reference position was measured in the three cardinal directions. The dosimetric effects of the measured errors were evaluated for two classes of error: Internal Port Error (IPE) and Patient Registration Error (PRE). For each class, dose accumulation was done for daily measured errors and a systematic error. RESULTS: Inter-fractional positional errors of the port were small, with 87% of the deviations below 5 mm, but errors larger than 1.5 cm were observed. The cumulative effect of the daily measured and systematic IPE decreased target coverage by as much as 2.8% and 3.5%, respectively. The cumulative effect of the daily measured PRE decreased target coverage by an average of 3.5%. The cumulative effect of a systematic PRE significantly decreased target coverage by an average of 16%. CONCLUSION: The presence of IPE over the course of treatment had minimal clinical impact while PRE had a greater impact on clinically-relevant regions. The robustness of treatment delivery can be improved by assigning the port its appropriate density during planning despite contouring uncertainties due to metal artefacts, and by prioritizing anatomical alignment over port alignment during daily registration.