The effect of body contouring on the dose distribution delivered with volumetric‐modulated arc therapy technique

Abstract

The purpose of the study was to investigate the dosimetric effect defining the body structure with various Hounsfield unit (HU) threshold values on the dose distributions of volumetric‐modulated arc therapy (VMAT) plans. Twenty patients with prostate cancer and twenty patients with head and neck (H&N) cancer were retrospectively selected. For each patient, the body structure was redefined with HU threshold values of −180false(Body180false), −350false(Body350false), −700false(Body700false), and −980false(Body980false). For each patient, dose‐volumetric parameters with those body structures were calculated using identical VMAT plans. The differences in dose‐volumetric parameters due to the varied HU threshold values were calculated. For the prostate boost target volume, the maximum dose, mean dose, D95%, and D5% with Body180 were higher than those with Body980 by approximately 0.7% (p<0.001). For H&N target volumes, the changes in D95% of the targets receiving 67.5 Gy, 54 Gy, and 48 Gy between Body180 and Body980 were −1.2%, −0.9%, and −1.2%, respectively (p<0.001). The differences were larger for H&N VMAT plans than for prostate VMAT plans due to the inclusion of an immobilization device in the irradiated region in H&N cases. To apply all attenuating materials to dose calculation, the body structure would be defined with −980 HU. Otherwise, systematic error of about 1%, resulting in underdosage of the target volume, can occur.PACS number: 87.55.ne

Statistical process control to monitor use of a web‐based autoplanning tool

“…The doses then began to equalize toward the distal OARs (e.g., brain contour for head and neck, heart contour for chest wall). Particularly for targets near the surface in head and neck cases, this variability in SPC control limits may be attributed in part to differences in dose calculations near the surface and to differing body contours between Eclipse and RayStation 12 . These factors also may explain why the dose differences in chest wall cases and head and neck cases were similar among the various contours, but were not as pronounced in the cervix cases, in which the dose differences were more uniform across contours.…”

“…Particularly for targets near the surface in head and neck cases, this variability in SPC control limits may be attributed in part to differences in dose calculations near the surface and to differing body contours between Eclipse and RayStation. 12 These factors also may explain why the dose differences in chest wall cases and head and neck cases were similar among the various contours, but were not as pronounced in the cervix cases, in which the dose differences were more uniform across contours. In addition, this might also provide an explanation as to why we see the opposite effect of overall dose in Raystation-CCC versus Eclipse-Acuros for cervix plans compared to head and neck and chest wall plans.…”

“…For example, changes in dosimetric settings in the end user's systems, such as the calibration of the planning system or changes to the multileaf collimator (MLC) parameters (e.g., dynamic leaf gap or transmission) may affect dose calculations. Other process changes, intentional or unintentional, that could result in dosimetric changes include changes to the CT‐electron density table used for dose calculations and changes to the threshold values used to determine the body contour 12 . Because RPA plans are calculated twice—once by the RPA system and once by the end user's own TPS—QA systems for RPA should compare the two plans and identify differences or changes over time.…”

“…Other process changes, intentional or unintentional, that could result in dosimetric changes include changes to the CT-electron density table used for dose calculations and changes to the threshold values used to determine the body contour. 12 Because RPA plans are calculated twice-once by the RPA system and once by the end user's own TPS-QA systems for RPA should compare the two plans and identify differences or changes over time.…”