Assessing the fraction of dose originating from the penumbra region for plans of varied complexity

• Purpose: The absorbed dose delivered to the patient that originates from a penumbra region have larger uncertainties compared to the dose delivered from the center of the treatment field. 1,2 The magnitude of these uncertainties could be important input when evaluating absorbed dose to organs at risk (OARs) and target volumes. The aim of this work was to examine the fraction of penumbra dose in three-dimensions (3D) for volumetric modulated arc therapy (VMAT) plans of different complexities. Methods and Materials: A C# software was developed in our department, able to calculate the fraction of penumbra dose in relation to the total dose in each voxel and visualize it as a 3D distribution (penumbra map). Twelve treatment plans of previously treated patients have been selected to represent different types of treatment geometry and different treatment sites (prostate, head & neck, lung and gynecological cancer). The clinical plan used for the actual treatment of the patient was reoptimized in Eclipse TPS (Varian Medical Systems, Palo Alto, CA), to create one plan with reduced complexity (“simple plan”) and one of higher complexity (“complex plan”). These additional plans were generated by altering the monitor unit constraint and the aperture shape controller level, while retaining a similar dose distribution. The fraction of penumbra dose was calculated for both the clinical and the newly generated plans creating in total 36 penumbra maps. Results: The generated penumbra maps were able to highlight anatomical regions with high fraction of penumbra dose (e.g., above 50%). These regions were mostly observed outside the planning target volume (PTV), regardless of treatment site and complexity level. Generally, the fraction of penumbra dose, both inside and outside the PTV, was increased with increased plan complexity. The average fraction of penumbra dose for the body, PTV and an example OAR are shown in Table 1 across all analyzed plans. For certain cases, the absorbed dose in parts of OARs originated from penumbra regions to a considerable degree, even for the least complex version of the plan. Examples are shown in Figure 1 featuring the rectum in a prostate plan and the parotid glands in a head & neck plan. Conclusion: The proposed method functions as a tool for assessing the extent to which the dose to certain voxels is influenced by the penumbra. A link was established between increased complexity and increased mean fraction of penumbra dose. Therefore, this approach could be used to quantify the complexity of the treatment plan in 3D

Subject headings

NATURVETENSKAP  -- Fysik -- Annan fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Other Physics Topics (hsv//eng)

MEDICIN OCH HÄLSOVETENSKAP  -- Hälsovetenskap -- Annan hälsovetenskap (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Health Sciences -- Other Health Sciences (hsv//eng)

Keyword

VMAT

penumbra

dose uncertainty

Publication and Content Type

vet (subject category)

kon (subject category)