ESTRO 2023 - Abstract Book

S1833

Digital Posters

ESTRO 2023

2. Use the top benchmarked plans to create a new class solution that increases treatment plan quality and decreases variability among planners. Materials and Methods 50 randomly selected CHHiP plans were picked to form the quality review set. A further 40 patient plans were then selected, post release of the tools, to validate the model. All plans were created within Pinnacle (v14.0, Philips Medical Systems) with plan scoring, DVH parameters collected and analysed in MS. Excel. The macro-enabled spreadsheet calculates a combined overall score, for PTV coverage, dose spillage and for doses to all OARs. For every parameter evaluated, an ideal value and associated weight were defined and compared against the plan values during analysis. Plans score more if a DVH value approaches the ideal score, then scores linearly lower as the value deviates. Ideal scores for OAR sparing were derived for each new plan, based on the performance of the review set relative to the current overlap of Bladder and Rectum to the PTV. Average DVH parameters are derived from the previously achieved doses of the review set. In this way, a sparing predictor model was based on prior experiences and is updated for the anatomy of any future case. After scoring the review set, the top 15 performers were then selected to update the Scripted class solution and, along with the scoring tool, released for clinical use. Close monitoring of post release plans ensued, with plan score results recorded in the validation set in comparison to the review set.

Results Prior release, the median scores for the review set were:

• overall score 72.2% • PTVs score 59.8% • OARs score 85.1%.

Post release, the median score for the validation set were: • overall score is 78.2%

• PTVs score 63.4% • OARs score 93.0%.

Conclusion The introduction of the updated class solution and plan score tool showed improved plan quality and decreased plan variability (standard deviation reduced by ≈ 3%). The scoring tools uses data derived from DVH to objectively quantify the quality of dose distribution decreasing the subjectivity associated with plan evaluation. The tools proved successful in increasing normal tissue sparing, providing effective quality control of the VMAT plan for Prostate cancer patients.

PO-2057 Volumetric modulated arc therapy (VMAT) for whole brain radiotherapy with hippocampal sparing

Ł . Szczurek 1

1 International Oncology Center Affidea, Medical Physics, Pozna ń , Poland

Purpose or Objective In recent years, a number of techniques for irradiation of patients with brain tumors enabling protection of the hippocampus have been developed. This advancement significantly improves quality of life of the patients after irradiation in terms of cognitive functions the affected organs are responsible for. Materials and Methods We investigated the feasibility of whole brain radiotherapy (WBRT) with using volumetric modulated arc therapy (VMAT) technique to spare the hippocampi and the other organs at risk (OARs). Ten patients previously treated with WBRT were analyzed. VMAT plans were generated for a prescription dose of 30 Gy in 10 fractions. Accurate localization of the hippocampus and other critical organs was possible based on additional examination using magnetic resonance (MR). Computed tomography (CT) simulation images of each patient’s head were co-registered with the T1-weighted cranial MR images with contrast medium-enhanced. Treatment plans were made on the Monaco Treatment Planning System (TPS) using photons with a nominal accelerating potential of 6MV and the Monte Carlo calculation algorithm. The VMAT technique with 6 double arcs and collimator angle set to 10 and 350 degrees, was used. Dosimetry treatment plans were analyzed according to the recommendations included in the NRGG-CC01 protocol. The brain area was defined as a planning treatment volume (PTV) where the maximum permissible dose, Dmax, was ≤ 37.5 Gy, D98% was above 25 Gy, while V30 Gy concerned the volume of ≥ 95%. Variation acceptable (VA) was defined for Dmax, D98% and V30 Gy within 37.5 and 40 Gy; 22.5 and 25 Gy; 90 and 95%, respectively. In the area of the hippocampus, the criteria for plan approval were defined consecutively for Dmax ≤ 16 Gy (VA from 16 to 17 Gy), and D100% ≤ 9 Gy (VA from 9 to 10 Gy). For the chiasma and the optic nerves, the maximum permissible dose was ≤ 30 Gy (VA from 30 to 37.5 Gy). Results Properly defined objective functions during optimization in the TPS allowed to obtain the recommended dose levels in the PTV and OARs. In PTV, Dmax, D98% and V30 Gy were 35.3 ± 0.5 Gy, 25.02 ± 0.3 Gy and 95.6 ± 0.8%, respectively. For right and left hippocampus Dmax were 14.7 ± 0.9 Gy and 15.8 ± 0.8 Gy, respectively. For both optic nerves and chiasma Dmax were 35.7 ± 0.6 Gy and 34.2 ± 1.1 Gy, respectively.

Conclusion