Dosimetric Effects of Head and Neck Immobilization Devices on Multi-field Intensity Modulated Radiation Therapy for Nasopharyngeal Carcinoma

Abstract

Background: In practice, the dose perturbation effect of head and neck immobilization devices is often overlooked in intensity-modulated radiation therapy (IMRT) for nasopharyngeal carcinoma (NPC). Purpose of this study is to verify and analyze the dosimetric effect of head and neck immobilization devices on NPC multi-field IMRT.Methods: Ten patients with nasopharyngeal carcinoma were randomly selected. Two sets of body contours were established for each patient. One set of body contours did not contain the immobilization device, and the other contour set included the immobilization device. For each patient, dose calculations were conducted for the two sets of contours using the same 9-field IMRT plan, which were recorded as Plan- and Plan+. The dose difference caused by the head and neck immobilization devices was assessed by comparing the dose-volume histogram (DVH) parameter results and by plan subtraction. The gafchromic EBT3 film and anthropomorphic phantom were used to verify the calculated doses.Results: The target coverage and average dose of Plan+ were lower than those of Plan- : the prescription dose coverage rates for PTVnx, PTVnd, PTV1 and PTV2 decreased by 2.4%, 9.9%, 1.5%, and 3.6%, respectively, and the mean doses were reduced by 0.9%, 1.9%, 1.1%, and 1.5%, respectively. Doses in the organs at risk showed no significant differences or slight reductions (the maximum reduction in mean dose was 1.7%). From the EBT3 measurements, the skin dose on the posterior neck was increased by approximately 53%.Conclusion: The attenuation and bolus effects of the head and neck immobilization device reduce dose coverage rate and average dose of the planning target volumes in nasopharyngeal carcinoma and lead to an increase in the skin dose. During treatment planning and dose calculation, the immobilization device should be included within body contour to account for the dose attenuation and skin dose increment.

Dosimetric effects of supine immobilization devices on the skin in intensity-modulated radiation therapy for breast cancer: a retrospective study

“…Patient immobilization devices have become an important tool to guarantee the accurate delivery of highly conformal dose distributions [3]. As the materials used in immobilization devices are not completely X-ray transmissible and can cause attenuation of the delivered dose, the dosimetric effects of immobilization devices should be included in dose calculations [17,24]. Beam attenuation from the couch, additional inserts, and immobilization devices can cause a misrepresentation of the actual dose delivered to the PTV, with a deviation of more than the recommended 3-5% accuracy range reported by Olch [3].…”

“…Beam attenuation from the couch, additional inserts, and immobilization devices can cause a misrepresentation of the actual dose delivered to the PTV, with a deviation of more than the recommended 3-5% accuracy range reported by Olch [3]. Chen reported the attenuation of head and neck immobilization devices, which reduced the dose coverage rate from 1.51 to 9.92% and the average dose from 0.93 to 1.92% of planning target volumes in nasopharyngeal carcinoma [24]. Olson assessed the dose variance from immobilization devices in volumetric-modulated arc therapy (VMAT) head and neck treatment planning and found that the plan calculated without immobilization devices was problematic, showing compromised V 95 , D 100 , and PTV coverage [17].…”

“…In addition, the bolus effect cannot be avoided. Beams, especially posterior oblique orientations beams, passing through the immobilization devices involved in treatment can result in increased unexpected skin doses to ultimately affect the dosimetric effects [1,8,24,25]. Kelly et al utilized radiochromic film and MOSFET [1].…”

Dosimetric Effects on skin of supine Immobilization Device in Intensity Modulated Radiation Therapy for breast cancer: a retrospective study

“…Patient immobilization devices have become an important tool to guarantee accurate delivery of highly conformal dose distribution (20). As the materials used in immobilization devices are not completely X-ray transmission and can cause attenuation of delivering dose, the dosimetric effects of immobilization devices should be included in dose calculations (14,21) . In addition, the bolus-effect cannot be avoided either.…”

“…Chen found the attenuation of the head and neck immobilization devices, which reduce dose coverage rate (reduced by from 1.51% to 9.92%) and average dose (reduced by from 0.93% to 1.92%) of planning target volumes in nasopharyngeal carcinoma (21). Alyssa Olson assessed dose variance from immobilization devices in VMAT head and neck treatment planning, and found that plan calculated without immobilization devices was problematic with compromising V95, D100 and PTV coverage (14).…”