Radiation Physics

PV QA 3 - Poster Viewing Q&A 3

TU_5_3166 - Total Body Irradiation (TBI) with organ sparing utilizing both Volumetric Modulated Arc Therapy (VMAT) and electronic compensator

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

Total Body Irradiation (TBI) with organ sparing utilizing both Volumetric Modulated Arc Therapy (VMAT) and electronic compensator
G. Luo1, E. C. Osmundson2, and G. X. Ding1; 1Vanderbilt University Medical Center, Nashville, TN, 2Department of Radiation Oncology, Vanderbilt University Medical Center, Nashville, TN

Purpose/Objective(s): TBI is commonly delivered via APPA fields with patient in the upright position. Two major disadvantages of this delivery technique include limited potential for dose sparing of organs at risk, and the need for ill patients to maintain the upright treatment position, unaccompanied, for long periods. This study presents a gradient matching technique for TBI using a standard Linear Accelerator (LINAC) that permits significant organ sparing when necessary while enhancing patient comfort and safety using supine positioning.

Materials/Methods: We defined the Planning Target Volume (PTV) by cropping 5 mm from the external body contour to account for skin. Our treatment planning LINAC was selected based on its excellent mechanical performance and wide availability. The PTV is divided into two parts of upper and lower body at the level of mid-thigh. Upper body will be planned with head first position, and lower body with feet first position. For upper body, four isocenters are placed to cover head and neck, chest, abdomen and pelvis, respectively. VMAT technique using two to three arcs for each isocenter is utilized providing the potential for organ sparing. Lower body is treated using standard APPA fields. Between each isocenter, at least 5 cm overlap is reserved to achieve a gradient dose match. Treatment planning begins with lower body fields. Electronic compensators are used to achieve a uniform dose distribution, and the superior overlap region is manually brushed to achieve a uniform dose drop off towards the upper body fields. Subsequently the lower body fields are calculated and normalized to obtain reasonable dose coverage. Finally, all arcs treating upper body are optimized simultaneously in one plan, taking the calculated lower body as the base dose plan. During optimization, emphasis is given toward uniform dose distribution, adequate coverage and organ sparing to lungs or other identified organs at risk (e.g. kidneys). All treatment planning is performed using a commercially available system.

Results: Using this technique we observed that 100% of prescription dose covers 90% of PTV, and 95% of prescription dose covers 96% of PTV. The mean dose for PTV is 104% of prescription dose. The 3D maximum dose is 124%, which is located inside PTV and outside of organs at risk. The volume of PTV receiving more than 110% of prescription dose is 11%, whereas the volume receiving more than 120% of prescription dose is less than 0.01%. The mean dose to lungs and kidneys are 55% and 53% of the prescription dose, respectively.

Conclusion: VMAT based TBI delivery provides excellent dose distribution to intended targets, permits dosimetric sparing of organs at risk, and may improve patient safety and comfort through supine positioning. The gradient matching techniques make the delivered dose at field junctions insensitive to patient positioning uncertainties. Also, optimization can be applied to limit dose to organs at risk other than lungs and kidneys if clinically desirable.

Author Disclosure: G. Luo: None. E.C. Osmundson: None. G.X. Ding: Research Grant; NIH. Royalty; National Research Council of Canada.

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TU_5_3166 - Total Body Irradiation (TBI) with organ sparing utilizing both Volumetric Modulated Arc Therapy (VMAT) and electronic compensator



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