ESTRO 2020 Abstract Book

S772 ESTRO 2020

Results Minimum requirements for PTVs coverage and maximum allowed overdosages were always satisfied for both tumors and both algorithms, therefore not reported below. For both prostate and HN GPS plans: no statistically and clinically significant improvements were found on average for GPS plans over IOA plans, as visible in figure 1. For prostate IOA plans: rectum D 15% and D mean were 3.1% and 5.7% lower and left and right femoral heads D mean were 4.1% reduced (both p<0.05, as visible in figure 1). Mean PSV was improved by 2.9% and computation time was reduced from 24 to 11 min (both p<0.05). CBS was 4.7% in favor of IOA plans, but without statistical significance, and IOA plan was selected over GPS plan for 13/20 patients. For HN IOA plans: spinal cord D 1cc and Esophagus D 1cc were 9.9% and 7.9% lower, respectively, and oral cavity D 35% and D 65% were both lower by 12% (all p<0.05, as visible in figure 2). PSV was 1% worsen, but not statistically significant, and computation time was reduced from 21 to 14 min (p<0.001). CBS was 6.7% in favor of IOA plans but without statistical significance, and for 13/20 patients IOA plans were selected.

usefulness of this method for cervical cancer brachytherapy. Material and Methods The robust optimization model implemented, was based on the worst dose distribution method for considering the dose in the presence of source position uncertainties. For each iteration on optimization, apart from the nominal scenario (dose distribution with sources in the planned positions), we assumed that all sources move ±2 mm along the x, y, and z directions. In these 7 scenarios (D0-D6), the worst dose distribution for each voxel was identified as the one with the lowest dose in the target and the highest dose outside the target. The iterative objective function was calculated using the worst dose distribution. Twenty patients with radical cervical cancer who had completed the treatment were retrospectively selected. The conventional optimization plans (COPs) and robust optimization plans (ROPs) were compared. Dose Volume Histogram (DVH) parameters were used for the comparison, SPSS 19.0 software (IBM, Armonk, NY) was used for the statistical analyses, and p values of less than 0.05 were considered statistically significant. The robustneof the COPs and the ROPs was evaluated using the DVH bands. Results The dosimetric results obtained in the nominal scenario and in the worst dose distribution for the COPs and ROPs are reported in Table 1. It was noted that the results obtained for the ROPs are worse than the ones obtained for COPs. In the nominal scenario, the mean CTV D100%of the ROPs is 3.08% lower than the COPs, while the mean CTV V150% is 4.25% higher ( p < 0.05 ). For the bladder, rectum, and small intestine, the mean D2cc of the ROPs is 0.20 Gy, 0.19 Gy, and 0.12 Gy higher than the COPs ( p < 0.05 ), respectively. In the scenarios with shift source position, the mean CTV D100% of the ROPs is 1.47% lower than the COPs, while the mean CTV V150% is 3.77% higher. The mean CTV D90%of the ROPs is 0.35% higher than the COPs ( p > 0.05); however, the difference has a negligible difference. The mean D2cc of the bladder, rectum, and intestines is 0.12Gy, 0.17Gy, and 0.06Gy higher than the COPs ( p < 0.05 ), respectively.

Worst dose distribut ion

Nominal scenario

Parame ter

p

p

COPs ROPs

COPs

ROPs

CTV, D 100% CTV, D 90% CTV, V 150%

3.89±0. 30 6.00±0. 00 52.59±2 .13 4.62±0. 61 4.04±0. 69 3.05±1. 03

3.77±0.2 8 6.00±0.0 0 54.83±2. 44 4.82±0.7 2 4.23±0.7 2 3.17±1.0 5

0.00 3

3.40±0. 21 5.68±0. 07 50.42±2 .70 5.27±0. 81 4.58±0. 82 3.42±1. 19

3.35±0.2 0 5.70±0.0 7 52.32±2. 58 5.39±0.8 1 4.75±0.8 5 3.48±1.1 9

0.02 1 0.09 3 0.00 0 0.00 1 0.00 1

-

0.00 0 0.04 5 0.00 0 0.03 6

Bladder D 2cc Rectum D 2cc Sigmoid D 2cc

Conclusion For both prostate and head and neck IOA plans were selected in the blind scoring by clinician in 65% of cases; it was faster than GPS and certain dosimetric parameters were improved. PO-1449 A robust optimization algorithm for brachytherapy X. Wang 1 , P. Wang 1 , J. Li 1 , L. Clara Orlandini 1 1 Sichuan Cancer Hospital & Institute- Sichuan Cancer Center- School of Medicine- University of Electronic Science and Technology of China, Department of Radiation Oncology, Chengdu, China Purpose or Objective This paper presents a robust optimization algorithm for brachytherapy treatment planning and discusses the

0.00 6

There was no remarkable difference in the DVH bands of the ROP and the COP in any of the patients studied.. Figure 1 reports the DVH bands for a representative patient.