ESTRO 2023 - Abstract Book

S1841

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ESTRO 2023

In MA-VMAT, ETT increased about 80 sec per arc ([number of arc; ETT sec], 2; 252, 3: 328, 4; 409, 5; 450, 6; 617). Details of CGs and achieved values calculated by SA or MA are presented in Table with p-values of t-test. Structure Goal SA MA p-Value Bladder V37Gy < 5 ml 2.96 2.75 0.342 Bowel D1ml < 30 Gy 19.10 17.17 0.001

Femoral head_Lt Max < 27.5 Gy 15.37 11.73 0.000 Femoral head_Rt Max < 27.5 Gy 16.89 11.83 0.000 Neural Band D50% < 40 Gy 38.46 37.92 0.001 penile bulb D0.1ml < 29.5 Gy 15.95 11.59 0.000 Prostate V40Gy > 90 % 94.49 96.75 0.004 PTV Conform(35 Gy) 0.67 0.78 0.000 V29Gy > 99.9 % 100.00 100.00 1.000 V36.25Gy > 95 % 96.00 95.88 0.346

Rectum

D20% < 44 Gy D40% < 20 Gy V36Gy < 2 ml Max < 30 Gy

10.28 10.26 0.885 6.19 6.45 0.052 1.72 1.63 0.093 16.76 14.61 0.000 0.58 0.54 0.010 40.63 40.26 0.009

Skin

Testes Urethra

Max < 2 Gy

D20% < 44 Gy

Box-plots of values of CGs were presented in Figure. Average ETT of SA (258 sec) and MA (369 sec) were statistically different (p < 0.0001). SA satisfied all CGs as MA had done. Critical structures such as rectum, bladder and PTV had statistically similar mean values. CGs of those structures which were not strictly restricted had better values in MA compared to SA, however 4 to 6 arcs plan not always had better values compared to 2 or 3 arcs plan.

ETTs of SA were decreased from that's of MA 28.5% in average (range 2.6 - 54.7 %). Averages of EET grouped by arc were decreased to that of SA by 9, 26, 32, 40 and 54% as numbers of arc increased from 2 to 6. Conclusion SA-VMAT had sufficient ability to treat prostate cancer in our CG perspective compared to MA-VMAT. It also had time (and energy) sparing feature. On average 28% of ETT had decreased. As number of arc increased, ETT increased. SA-VMAT may be a sustainable and good clinical solution.

PO-2065 Retrospective VMAT planning study to evaluate benefit of SpaceOAR in patients with hip prostheses

I. Gagne 1 , A. Alexander 2 , K. Scarth 3

1 BC Cancer - Victoria, Medical Physics, Victoria, Canada; 2 BC Cancer - Victoria, Radiation Oncology, Victoria, Canada; 3 Dalhousie University, Radiation Oncology, Halifax, Canada Purpose or Objective The goal of this study was to quantify the dosimetric benefit of SpaceOAR Hydrogel (SOH), in the setting of simulated unilateral and bilateral hip prostheses, in particular with regard to rectal V55Gy, the hypofractionated equivalent of rectal V70Gy, regardless of the VMAT prosthesis avoidance technique used. Materials and Methods An anonymized dataset of 13 prostate cancer patients’ radiation planning scans taken before and after SOH placement were used to create radiation plans with simulated unilateral and bilateral hip prostheses as well as no hip prosthesis. The