ESTRO 2022 - Abstract Book

S247

Abstract book

ESTRO 2022

cannot rely on the traditional PTV-based plan evaluation anymore. Therefore, Korevaar et al. (1) proposed an alternative plan evaluation method, using voxel-wise minimum/maximum (vw-min/max) dose distributions. They calibrated the traditional PTV D98% ≥ 95% with -0.9% and D2% ≤ 107% with 2.3% on average for all tumor sites. The proposed plan evaluation method was validated in another radiotherapy institute and calibrated for various treatment sites to determine target criteria when PTV-based planning is replaced by robust planning. We hypothesized that sampling regular or random setup error directions to create the required vw-min/max dose distributions does not influence the results. Materials and Methods The D98% and D2% of intensity modulated photon plans of (in total 294) lung, breast, esophagus, rectum, prostate, and head and neck cancer patients were retrospectively evaluated on the PTV of the nominal dose distribution and the clinical target volume (CTV) of the vw-min/max dose distribution. The vw-min/max dose distributions were created from a combination of 14 perturbed dose distributions. The perturbed error scenarios were drawn from the cardinal axis and the corners of a cube, projected on a sphere with a magnitude of shift equal to the CTV-PTV margin. Similarly, the vw-min/max dose distributions were constructed from 20 randomly sampled perturbed scenarios on the same sphere. The PTV D98%/D2% and the CTV D98%vw-min/D2%vw-max were compared and calibrated for each tumor site. Results On average, the primary CTV D98%vw-min ≥ 95% was highly correlated with the PTV D98% (R2=0.99, slope=1.0), whereas the D2% ≤ 107% required a +2.4% correction for robust CTV evaluation (i.e. D2%vw-max ≤ 109.4%). Site specific evaluation showed that the primary CTV D98%vw-min ≥ 95%/D2%vw-max ≤ 107% required a correction of -2.6%/2.8% (esophagus), -1.0%/3.1% (breast), -0.2%/2.1% (head and neck), -0.2%/1.3% (prostate), 1.2%/1.2% (rectum), and 4.5%/3.7% (lung) using regularly distributed scenario directions. On average, the correction for the D98% CTV nodes was -0.7%. The results of the regular and random sampling evaluation were on average within 0.4% and varied per tumor site with 0.2%, on average (table 1).

Conclusion A clinical plan robustness evaluation method using vw-min/max dose distributions can replace traditional PTV-based plan evaluation when robust planning is introduced. We externally validated and calibrated the method and the average results for PTV D98% and D107% were, as compared to the results of Korevaar et al., within 0.5% and 0.1% , respectively. Differences in sampling method (regular or random) were negligible.

OC-0287 Advantages of DIBH in IMRT of locally advanced NSCLC systematically investigated with autoplanning

K. Fjellanger 1,2 , L. Rossi 3 , B.J.M. Heijmen 3 , H.E.S. Pettersen 1 , S. Breedveld 3 , I.M. Sandvik 1 , T.H. Sulen 1 , L.B. Hysing 1,2

1 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway; 2 University of Bergen, Institute of Physics and Technology, Bergen, Norway; 3 Erasmus University Medical Center, Department of Radiotherapy, Rotterdam, The Netherlands Purpose or Objective Studies have found encouraging reproducibility and patient compliance of deep inspiration breath hold (DIBH) radiotherapy for locally advanced non-small cell lung cancer (LA-NSCLC). Dosimetric comparisons of DIBH IMRT with free breathing (FB) IMRT have not been published, and DIBH is not routinely used for this patient group. This study uses automated multi- criterial planning with integrated beam angle optimization to systematically compare DIBH IMRT with FB IMRT, avoiding potential planner bias. Materials and Methods 33 LA-NSCLC patients were prospectively included. One 4DCT and three DIBH CTs were acquired for each patient. For FB planning, the OARs and GTV were delineated on the average intensity projection of the 4DCT, and the internal GTV (IGTV) incorporated the GTV positions on all 4DCT phases. For DIBH planning, the OARs and GTV were delineated on one DIBH CT, and the IGTV incorporated the GTV positions on the two repeated DIBH CTs. The CTV was a 5 mm expansion of the IGTV, adjusted for uninvolved organs, and the PTV was a 5 mm isotropic expansion of the CTV. A novel in-house system for multi- criterial planning was used to automatically generate two deliverable 6-beam IMRT plans with optimized beams angles for each patient, one for FB and one for DIBH. The prescribed dose was 60-70 Gy in 2 Gy fractions. Relevant dose-volume parameters were compared using the Wilcoxon signed-rank test ( p ≤ 0.05).

Results