ESTRO 37 Abstract book

ESTRO 37

S490

S. Cilla 1 , G. Macchia 2 , A. Ianiro 1 , M. Ferro 2 , A. Pierro 3 , I. Djan 4 , M. Ntreta 5 , G. Siepe 5 , M. Buwenge 5 , V. Panni 5 , A. Zamagni 5 , M. Ferioli 5 , A. Angelini 6 , S. Cammelli 5 , G. Sallustio 3 , V. Valentini 7 , A. Morganti 5 , F. Deodato 2 1 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Medical Physics Unit, Campobasso, Italy 2 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Radiation Oncology Unit, Campobasso, Italy 3 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Radiology Department, Campobasso, Italy 4 Oncology Institute of Vojvodina- S. Kamenica - University of Novi Sad, Radiotherapy Department, Novi Sad, Serbia 5 Department of Experimental- Diagnostic and Specialty Medicine - DIMES- University of Bologna- S.Orsola- Malpighi Hospital, Radiation Oncology Center, Bologna, Italy 6 Department of Experimental- Diagnostic and Specialty Medicine - DIMES- University of Bologna- S.Orsola- Malpighi Hospital, Medical Physics Unit, Bologna, Italy 7 Policlinico Universitario "A. Gemelli"- Università Cattolica del Sacro Cuore, Radiation Oncology Department, Roma, Italy Purpose or Objective We explored the potential of volumetric modulated arc therapy (VMAT) to spare the critical erectile structures in complex SABR treatments of prostate cancer with a simultaneous integrated boost (SIB) to the dominant intraprostatic lesion (DIL). Material and Methods Five patients were selected for replanning. DILs were defined using T2-weighted, dynamic contrast-enhanced and diffusion-weighted MRI. Nerve structures were contoured and expanded by a uniform 2 mm margin. The prostate (or the prostate plus 1 cm of seminal vescicles for intermediate-risk patients) and the DIL were expanded uniformly by 3 mm to create the planning target volumes (PTVprost and PTVdil, respectively). Gold seeds were implanted in the prostate for image-guided purposes. PTVprost and PTVdil dose prescription was 35 Gy an 40 Gy, respectively, in 5 fractions. The doses were prescribed to cover >95% of PTVs. Steep dose fallout outside PTVs will be prioritized. Dose constraints present in current literature, corrected for EQD2 late effects, were used for rectum, bladder and other OARs. All VMAT plans were generated in a dual-arc modality. Original clinical plans were optimized with a standard 10mm width MLC (ST-VMAT). Plans aiming to nerve-sparing were generated using 10mm width MLC (NS10-VMAT) and 5mm width MLC (NS5-VMAT). Dose-volume histograms for PTVs and OARs were compared between the plans using a 2- tailed paired t-test. Results Planning with two strategies (with and wi thout nerve sparing) demonstrated comparable planning target coverage for ST-VMAT, NS10-VMAT and NS5-V MAT (PTVprost D95%: 98.6%, 98.0% and 98.3%; PTVdil D95%: 98.6%, 98.0% and 98.3%). Dmean for both PTVs were found not statistically different. D98% was found >90% for all patients and techniques. No significant differences were found in sparing rectum, bladder, penile bulb, urethra and femoral heads between the three plans. With regard to vessel-sparing, VMAT plans aiming to nerve sparing provided a significant reduction of dose irradiation. NS10-VMAT and NS5-VMAT provided a mean

the initial ones (71.9±5.7). The coverage of the TVs was increased, maintaining the same level of sparing for rectum and half rectum with a better sparing of the bladder (mean dose: 30.5 vs 36.6 Gy, p=0.024; D40%: 31.6 vs 40.5 Gy, p=0.014). The only parameters increased were the maximum dose inside the TVs (82.4 vs 82.0 Gy, p=0.002) and the V74.1Gy inside the rectum (0.8% vs 0.4%, p = 0.003), but at levels that are clinically acceptable. The standard deviation was decreased for each dosimetric parameter. Most significant findings are illustrated in the figures below.

Fig 1: Difference of the metric of the initial and the re- planned plans

Fig. 2: Resume of the statistical test performed on the couples of metrics Conclusion The re-planned treatments showed a better coverage of the TVs, with the same or increased level of OARs sparing. Moreover, the reduced standard deviation implies a better reproducibility of the treatment plans. Based on these results, PlanIQ can represent a valuable tool aiding planners in a goal-oriented optimization to design the best deliverable treatment. However, since the Quality Algorithm is created by the planner or the physician himself and it cannot cover each point on the Dose-Volume Histograms, the metrics should be carefully chosen in order to not introduce a bias in the plan scoring.

PO-0914 Nerve-sparing prostate stereotactic ablative radiotherapy (SBRT) using SIB-VMAT technique