A patient immobilization device for prone breast radiotherapy: Dosimetric effects and inclusion in the treatment planning system.

PURPOSE: To assess the dosimetric impact of a patient positioning device for prone breast radiotherapy and assess the accuracy of a treatment planning system (TPS) in predicting this impact. METHODS: Beam attenuation and build-up dose perturbations, quantified by ionization chamber and radiochromic film dosimetry, were evaluated for 3 components of the patient positioning device: the carbon fiber baseplate, the support cushions and the support wedge for the contralateral breast. Dose calculations were performed using the XVMC dose engine implemented in the Monaco TPS. All components were included during planning CT acquisition. RESULTS: Beam attenuation amounted to 7.57% (6MV) and 5.33% (15MV) for beams obliquely intersecting the couchtop-baseplate combination. Beams traversing large sections of the support wedge were attenuated by 12.28% (6MV) and 9.37% (15MV). For the support cushion foam, beam attenuation remained limited to 0.11% (6MV) and 0.08% (15MV) per centimeter thickness. A substantial loss of dose build-up was detected when irradiating through any of the investigated components. TPS dose calculations accurately predicted beam attenuation by the baseplate and support wedge. A manual density overwrite was needed to model attenuation by the support cushion foam. TPS dose calculations in build-up regions differed considerably from measurements for both open beams and beams traversing the device components. CONCLUSIONS: Irradiating through the components of the positioning device resulted in a considerable degradation of skin sparing. Inclusion of the device components in the treatment planning CT allowed to accurately model the most important attenuation effect, but failed to accurately predict build-up doses.

Improved cone-beam computed tomography in supine and prone breast radiotherapy. Surface reconstruction, radiation exposure, and clinical workflow.

BACKGROUND AND PURPOSE: Cone-beam computerized tomography (CBCT) enables three-dimensional information of the scanned region and provides soft tissue images with good spatial resolution. Our aim was to optimize image acquisition settings for prone and supine breast radiotherapy with respect to contour accuracy, clinical practicalities, and radiation dose. PATIENTS AND METHODS: CBCT images were acquired for both prone and supine anthropomorphic phantoms and a female cadaver in supine and prone set-up. CBCT protocols were investigated by altering the tube current, exposure time, range of projection views, field of view (FOV), and starting angle. For clinical practicalities, the frequency of the use of an offset CBCT isocenter was evaluated at 558 205°-CBCTs (37 patients; 13 prone and 24 supine) and 1272 360°-CBCTs (102 patients; 13 prone and 89 supine). RESULTS: Prone and supine breast CBCT images acquired with a bowtie filter, a small FOV, a range of projection views equaling 180°, a tube current of 20 mA and an exposure time of 32 ms, demonstrated adequate contour accuracy and an elimination of the offset CBCT isocenter procedure, while this occurred in 40.7 % for the old full-rotation protocol. Furthermore a 4.3-fold dose reduction was observed for the Computed Tomography Dose Index (CTDIw) compared to the preset Chest M20 protocol. CONCLUSION: The established 180° protocol demonstrated acceptable contour accuracy, eliminated the CBCT isocenter offset procedure and reduced patient radiation exposure.

Evaluation of a glassless photographic film scanner for high-gradient radiochromic film dosimetry.

This study evaluates the performance of the Nikon Coolscan 9000 ED film scanner for high-gradient radiochromic film dosimetry. As a reference for comparison, analogue experiments were performed on the Epson Expression 10000XL flatbed scanner. Based on these results, a dosimetric protocol was established for the Nikon scanner and its overall performance for high-gradient dosimetry was evaluated. The Nikon scanner demonstrated a high sensitivity for radiochromic film dosimetry, resulting in more contrast in the digitized image. The scanner's optics also demonstrated excellent stability and did not necessitate warm-up scans prior to data acquisition. Moreover, negative effects of temperature changes of the film inside the scanner were shown to be limited. None of the digitized images showed significant disturbances by moiré-patterns, by virtue of the absence of a glass plate for film positioning. However, scanner response was found to vary considerably across the reading area, requiring an optical density-dependent correction procedure to be incorporated into the scanning protocol. The main limitation of the Nikon Coolscan 9000 ED transmission scanner remains its film size restriction to 6.2 × 20 cm2. Nevertheless, its excellent characteristics render it the preferential tool for high-gradient radiochromic film dosimetry in applications limited to small film sizes, such as dosimetry in the build-up region.