A patient immobilization device for prone breast radiotherapy: Dosimetric effects and inclusion in the treatment planning system. Issue 6 (June 2016)

• Record Type: Journal Article
• Authors: De Puysseleyr, A.; De Neve, W.; De Wagter, C.
• Is Part Of: Physica medica. Volume 32:Issue 6(2016)
• Page Start: 758
• Page End: 766
• Publication Date: 2016-06
• Subjects: Radiotherapy -- Patient positioning device -- Treatment planning system -- Build-up dose; Medical physics -- Periodicals; Biophysics -- Periodicals; Biophysics -- Periodicals; Imagerie médicale -- Périodiques; Radiothérapie -- Périodiques; Rayons X -- Sécurité -- Mesures -- Périodiques; Physique -- Périodiques; Médecine -- Périodiques; 610.153
• DOI: 10.1016/j.ejmp.2016.04.013
• Languages: English
• ISSNs: 1120-1797

Abstract

Highlights: Beams intersecting the AIO patient positioning device are attenuated considerably. Build-up doses increase substantially in beams intersecting the positioning device. Attenuation is accurately modeled by inclusion of the device in the planning CT. This approach fails to precisely predict build-up doses. Abstract: 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% (6 MV) and 5.33% (15 MV) for beams obliquely intersecting the couchtop–baseplate combination. Beams traversing large sections of the support wedge were attenuated by 12.28% (6 MV) and 9.37% (15 MV). For the support cushion foam, beam attenuation remained limited to 0.11% (6 MV) and 0.08% (15 MV) 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.