Effect of daily setup errors on individual dose distribution in conventional radiotherapy: an initial study

Avoidance sectors to reduce dosimetric impact of an irreproducible pannus on setup uncertainty in prostate SBRT VMAT: A case study

This work investigates whether the use of an avoidance sector in a two-arc volumetric modulated arc therapy (VMAT) prostate stereotactic body radiotherapy (SBRT) plan reduces dosimetric variations due to an irreproducible pannus. A morbidly obese patient with favorable-risk prostate cancer elected treatment with SBRT. The patient was treated with the avoidance arcs across the pannus to eliminate reproducibility issues created by daily pannus variability in set up. For post-treatment assessment, the case was planned using Varian Eclipse™ treatment planning system (TPS) with two VMAT arcs with and without 100° avoidance sectors across the pannus. The dose was re-calculated using the external body contour from four daily treatment cone-beam computer tomography scans, and on two virtual body contours created by expanding the pannus region of the external contour by 5 and 10 mm. Dose differences between planned and re-calculated rectal wall mean dose and the V_24Gy were numerically larger in the absence of the avoidance sector for all fractions and for both simulated pannus variations, with maximum changes of 2.6% and 1.3%. Maximum point dose variations in the PTV, CTV, rectum, bladder, and femoral heads were 105 cGy or less for all cases, with and without the avoidance sector. The use of an avoidance sector across this large, asymmetrical pannus did not inhibit achieving dose constraints and provided a reduction in dose variability which was nominal in this case for 10 mm variations. Avoidance sectors can be safely implemented in cases with obvious reproducibility concerns in the setting of prostate VMAT SBRT.

Impact of pitch angle setup error and setup error correction on dose distribution in volumetric modulated arc therapy for prostate cancer

In volumetric modulated arc therapy (VMAT) for prostate cancer, a positional and rotational error correction is performed according to the position and angle of the prostate. The correction often involves body leaning, and there is concern regarding variation in the dose distribution. Our purpose in this study was to evaluate the impact of body pitch rotation on the dose distribution regarding VMAT. Treatment plans were obtained retrospectively from eight patients with prostate cancer. The body in the computed tomography images for the original VMAT plan was shifted to create VMAT plans with virtual pitch angle errors of ±1.5° and ±3°. Dose distributions for the tilted plans were recalculated with use of the same beam arrangement as that used for the original VMAT plan. The mean value of the maximum dose differences in the dose distributions between the original VMAT plan and the tilted plans was 2.98 ± 0.96 %. The value of the homogeneity index for the planning target volume (PTV) had an increasing trend according to the pitch angle error, and the values of the D 95 for the PTV and D 2ml, V 50, V 60, and V 70 for the rectum had decreasing trends (p < 0.05). However, there was no correlation between differences in these indexes and the maximum dose difference. The pitch angle error caused by body leaning had little effect on the dose distribution; in contrast, the pitch angle correction reduced the effects of organ displacement and improved these indexes. Thus, the pitch angle setup error in VMAT for prostate cancer should be corrected.