Abstract Book

S1077

ESTRO 37

Conclusion Multi-patient analysis showed KBP to be non-inferior with expected post-implementation OAR sparing gains in all observed disease sites. EP-1980 Influence of different DVH algorithms on dose constraints evaluation for SBRT M. Zani 1 , M. Esposito 1 , C. Carbonini 1 , S. Clemente 1 , C. Fiandra 1 , M. Fusella 1 , C. Garibaldi 1 , F. Giglioli 1 , C. Marino 1 , E. Moretti 1 , S. Russo 1 , A. Savini 1 , L. Strigari 1 , S. Strolin 1 , C. Talamonti 1 , E. Villaggi 1 , M. Stasi 1 , P. Mancosu 1 1 SBRT Working Group, AIFM- Italian Association of Medical Physics, Italy, Italy Purpose or Objective High target coverage and maximum dose sparing to the organ at risks (OARs) have to be reached to optimize radiotherapy (RT) plans. For serial organs the maximum dose is the parameter that has to be taken into account as dose constraint. In stereotactic treatments (SBRT) of spinal metastasis, plan optimization is generally guided by the spinal cord tolerance dose with a target coverage that can vary substantially from case to case. Therefore, an accurate estimation of the OAR maximum dose is, for this case, more crucial that for other SBRT treatments. Aim of the present work was: (i) to identify significant differences in the dose to OARs as calculated by the dose-volume histograms (DVH) produced by different treatment planning systems (TPS) and (ii) to outline if these differences can have an influence on the planning process. This work was carried out in the framework of large scale multicentric study. Material and Methods wo spinal cases were planned by 39 centers (6 different delivery techniques, and 9 planning systems for a total of 96 plans) consisting, respectively, of a single dorsal metastasis (plan1), and of two separated cervical metastases (plan2). A 1.25 mm slice thickness CT scan was used for planning. The prescription dose (PD) was 30 Gy in 3 fractions, AAPM TG101 OAR constraints were applied. In particular, for plan1 two constraints for the spinal cord were the limiting ones: V18Gy<0.35cm 3 and V21.9Gy<0.03cm 3 , while, for plan2, a constraint on the oesophagus dose maximum was also considered (V25.2Gy<0.03 cm 3 ). A maximum voxel size of 1.5x1.5x1.5mm 3 and dose calculation algorithms of category 4 were required. From the data sent by each center DVHs were generated using a single workstation with MIM 6.5 (MIM Software Inc. Cleveland US). The bin width was set to 0.1 Gy. Results In 46 cases a difference greater than 0.5 Gy was found between the maximum dose value obtained by the original locally created DVH.txt files and the MIM recomputed DVH values: data are reported in figure1 for plan1 (a) and plan2 (b), as a function of the treatment planing system. The MIM estimated dose was higher than the "original” in the 50% cases whit a maximum discrepancy of 2.5 Gy (Dmax to oesophagus for plan2). In 7 out of the 96 plans collected and analyzed, MIM recomputed value did not respect at least one constraint, while in the original plan these constraints violations were not present. A summary of the plans not respecting OAR constraints is shown in table 1.

fully-automated treatment planning. For equivalence, all plans were normalized to PTV D95%=100%. Dose metric differences were computed for standard parameters across the disease sites; two-tailed paired t-test quantified statistical significance (p<0.01). Results An excerpt of the results is shown in Table 1 (dose metrics relative to PTV prescription). Statistically significant OAR improvements were observed in all disease sites: rectum and bladder in prostate/prostatic fossa; total and ipsilateral lung in left lung; larynx, cochlea, pharynx, and cricopharyngeus in head-and-neck (Fig. 1). The prostatic fossa ∆D1%=1.2% was deemed acceptable given OAR sparing gains. The PTV ∆D1% and ITV ∆D99% increase for hypo-fractionated lung was by design (at physician request) due to ITV dose variance in the pre-KBP lung sample. Prosta te (81 Gy) Prostati c fossa (70.2 Gy) Left Lung (48 Gy) Right Lung (48 Gy) Head and neck (70 Gy)

Structur e Dose paramet er Percent age differen ce p-value Structur e Dose paramet er Percent age differen ce p-value Structur e Dose paramet er Percent age differen ce p-value Structur e Dose paramet er Percent age differen ce p-value Structur e Dose paramet er Percent age differen ce p-value

PTV ∆D1% 1.2% (p<0.0 1)

PTV ∆D1% 14.0% (p<0.0 1)

PTV ∆D1% 18.7% (p<0.0 1)

PTV ∆D1% 0.3% (p=0.0 5)

PTV

High

∆D1% 0.1% (p=0.84)

Rectu m ∆V40G y -4.3% (p<0.0 1) Rectu m ∆V65G y -1.1% (p<0.0 1) Rectu m ∆V75G y -0.8% (p<0.0 1) Bladde r ∆V40G y -2.0% (p<0.0 1)

ITV ∆D99% 5.0% (p<0.0 1)

ITV ∆D99% 6.9% (p<0.0 1)

Rectum ∆V40Gy -3.3% (p<0.0 1)

Larynx ∆Dmean -10.5% (p<0.01)

Total Lung ∆V50% -0.1% (p<0.0 1)

Total Lung ∆V50% 0.0% (p=0.4 5)

Rectum ∆V65Gy -1.8% (p<0.0 1)

Pharynx ∆Dmean -3.4% (p<0.01)

Left Lung ∆V50% -0.2% (p<0.0 1)

Rectum ∆V70.2 Gy -3.3% (p<0.0 1)

Right Lung ∆V50% 0.1% (p=0.4 1)

Cochlea ∆Dmean -7.8% (p<0.01)

Left Lung ∆Dmea n -0.1% (p=0.3 9)

Right Lung ∆Dmea n 0.1% (p=0.6 5)

Cricopharyn geus ∆Dmean -24.3% (p<0.01)

Bladder ∆V40Gy -2.9% (p<0.0 1)