GEOMETRIC UNCERTAINTIES IN RADIOTHERAPY BIR PDF

Tel: ; Fax: ; gro. Abstract Purpose To estimate radiation therapy planning margins based on inter- and intrafractional uncertainty for pediatric brain and head and neck tumor patients at different imaging frequencies. The pretreatment offsets were used to calculate the interfractional setup uncertainty SU , and posttreatment offsets were used to calculate the intrafractional residual uncertainty RU. SU data was used to simulate four intervention strategies using different imaging frequencies and thresholds.

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Tel: ; Fax: ; gro. Abstract Purpose To estimate radiation therapy planning margins based on inter- and intrafractional uncertainty for pediatric brain and head and neck tumor patients at different imaging frequencies.

The pretreatment offsets were used to calculate the interfractional setup uncertainty SU , and posttreatment offsets were used to calculate the intrafractional residual uncertainty RU.

SU data was used to simulate four intervention strategies using different imaging frequencies and thresholds. Results The SM based on all patients treated on this study was 2. The average SU for a 2-mm threshold based on no imaging, once per week imaging, initial five images, and daily imaging was 3. Considering patients who undergo daily pretreatment CBCT, the SM is larger for those treated in the prone position or smaller for those treated under anesthesia because of differences in the RU.

Because RT has wide ranging side effects in pediatric patients 1 — 5 , investigators have focused on reducing normal tissue irradiation. One method is to reduce the target volume margins that are used in RT planning. The volume and margin definitions of the International Commission on Radiation Units and Measurements Reports 50 and 62, 6 , 7 have been adopted in pediatric clinical trials for brain and head and neck tumors—namely, gross tumor volume GTV , clinical target volume CTV , and planning target volume PTV.

The purpose has been to use three-dimensional treatment planning and delivery systematically in clinical trials and study ways to reduce treatment effects using disease-specific target volume margins. Only the CTV should receive the prescription dose; however, because of temporal variation in the position, shape, and size of the CTV, an internal margin IM must be added. In addition, because of uncertainties in the daily patient positioning, patient intrafractional motion, dose calculation, and beam delivery, a setup margin SM is also required.

The need to estimate and minimize the IM and SM in a variety of clinical settings and for children of all ages has become increasingly important as highly conformal radiation therapy, including intensity modulated radiation therapy IMRT and proton therapy, have entered the mainstream.

The PTV margins currently specified for children with brain and head and neck tumors enrolled on institutional and cooperative group trials are largely empiric.

As the definition of the GTV is refined and the CTV is systematically reduced for specific diseases, the margin chosen for the PTV will become an increasingly important means to minimize dose to normal tissues and the potential risk factor for treatment failure.

Given the importance and complexity in determining the appropriate PTV margin, a number of studies have proposed patient population-based formulas for the patient-related portion of the setup margin 8 — 11 to allow for systematic quantification of setup uncertainties based on statistical methods. Among the sites under study, intracranial 12 , 13 and head and neck 14 — 17 sites are of particular interest; however, only two small studies have been published focusing on pediatric localization 18 , We developed a protocol to quantitatively assess localization and refine PTV margin definitions for pediatric patients with head and neck and intracranial tumors.

The goal was to estimate the patient-related components of the SM and provide guidelines for target volume definitions in clinical trials. In this report, we estimated setup uncertainty SU and residual uncertainty RU. The SU represented interfractional positioning differences, and the RU represented intrafractional patient motion. The acquired data were modeled using different imaging regimens 20 , 21 to determine the influence of imaging frequency on the SM.

The cohort included 54 male and 46 female patients with a median age of 7. Only two patients were older than 21 years of age at the time of irradiation. They were included because they were diagnosed with pediatric brain and musculoskeletal tumors. General anesthesia GA was required for 46 median age, 4.

The treatment position was prone for 25 median age, 8. For 62 patients, the treatment method was considered three-dimensional conformal radiation therapy. IMRT was used for 38 patients. The patients were treated with 1. The median number of treatment fractions was 29 range, 7— This results in a photon beam with a mean energy of approximately keV.

The details of the modifications are explained by Faddegon et al. Dose calculations from the TPS were verified with ion chamber measurements. Daily image quality and weekly output and energy checks are routinely performed on the IBL system to ensure proper functionality. For purposes of analysis, the SU and RU were meant to correspond to the interfraction and intrafraction variability, respectively.

On a daily basis, the therapists positioned the patient according to external visual markings before acquiring the CBCT. The therapist examined the registration and modified when necessary following the instructions of the treating radiation oncologist on the first day of treatment.

The bony anatomy in the vicinity of the target was critical to the registration process. Coordinates were obtained automatically corresponding to the lateral, longitudinal, and vertical offsets. If the calculated magnitude of the three-dimensional offset vector was less than 2 mm, no shift was applied.

When necessary, the procedure was repeated until the offset vector met criteria for no intervention. All offsets were recorded in a database at the time of treatment and used to determine SU. Figure 1 is a flow chart that outlines the process.

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However, the use of daily online IGRT negates some previous errors and replaces them with different, albeit smaller, sources of uncertainty. The ubiquity of intensity modulated techniques, especially VMAT and other rotational methods of delivery, also pose problems to application of these recommendations. This talk will explore the reasoning behind the need for an update to the report before discussing each source of geometric uncertainty in turn: delineation error, fusion error, target deformation, rotational error, intra-fraction motion, surrogate error, matching error and technical delivery error. Each source will be characterised, and practical, pragmatic methods of determining its magnitude will be explored. Watch the video and complete the online self-reflection form.

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This event will provide radiotherapy professionals with the information they need to understand best practice in the application of clinical IGRT. Image-guidance protocols for common clinical sites will be discussed and the evolving role of adaptive radiotherapy techniques described. Where daily online IGRT is in use, the accepted methodology for margin calculation can be difficult to apply. We will present the new report from the BIR into this topic, and discuss how residual errors such as delineation error, deformation error, rotational variation, matching error amongst others can be measured and incorporated into a CTV-PTV margin formula. These images will be used by the BIR to share news about the event and to publicise upcoming events. Images may be used in press releases, printed publicity and published on the website and social media.

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