ESTRO 2021 Abstract Book

S1366

ESTRO 2021

PO-1645 Dosimetric comparison of radiotherapy techniques for left breast, axilla and supraclavicular fossa A. Gupte 1 , A. Sasidharan 1 , B. Kunheri 1 , A. Kumar 2 , S. Reddy 1 , H. Nair 1 , P. K U 1 , A. R 1 , D. Dutta 1 1 Amrita Institute of Medical Sciences, Radiation Oncology, Kochi, India; 2 Amrita Institute of Medical Sciences, Medical Physics, Kochi, India Purpose or Objective Evaluation of dosimetric parameters with four adjuvant radiation therapy (RT) planning techniques for left breast, axilla and supraclavicular fossa. Materials and Methods Ten node positive left sided breast cancer patients underwent breast conservative surgery were planned with forward planned IMRT with tangential fields (F-IMRT) and BH technique (F-IMRT-BH) for left breast, axilla and supraclavicular fossa. CT simulation scans done in free breathing (FB) and with moderate deep inspiratory breath hold (mDIBH) with Active Breath Coordinator (ABC). Tomotherapy Helical (TH) and Tomtherapy Direct (TD) plans were generated with FB scan. F-IMRT plans with field-in-field (FIF) technique was generated with both FB scan and mDIBH scan. CTV (Breast, SCF & Axilla) were contoured as per ESTRO guidelines . TD and TH plans were generated using precision treatment planning station (V2.0.1.1). All plans were optimized using 2.5cm jaw and modulation factor of 2.2. Pitch for TH and TD plans were 0.26 and 0.251. For PTV-breast 6 angles and SCF 4 angles were given. 3D conformal plans with F-IMRT FIF technique were generated in XIO planning station (V5.10.02). Tangential fields [medial tangent (MT) and lateral tangent (LT)] were given with photon energy 6X, while SCF field beams were combination of 6X & 15X. FIF beam (MT subfield) used to reduce hotspots and heart dose. Results Four techniques of radiation planning were F-IMRT, F-IMRT-BH, TD and TH. Significant difference in conformity index (CI) for “PTV COMBINED” amongst the 4 groups (p=<0.001). TH had better CI compared to F-IMRT-BH (p=0.001) and F-IMRT (p=0.010). Homogeneity Index (HI) was similar between Tomotherapy plans and were statistically superior over both F-IMRT plans. V107% mean (±SD) values in F-IMRT-BH, F-IMRT, TD and TH were 4.0 (± 2.5), 4.05 (± 4.4), 0.05 (± 0.1) and 0.19 (± 0.5) suggesting lesser “hot” in TH/TD plans. Heart mean (±SD) dose in F-IMRT-BH, F-IMRT, TD, TH was 383.4 (± 151.3) cGy, 488.8 (± 165.6) cGy, 348.2 (± 139.3) cGy and 599.9 (± 78.7) cGy (p-0.001). Mean (±SD) heart dose was greater in TH group (p-0.001). V20Gy in F-IMRT- BH, F-IMRT, TD, TH technique was 14.26 ± 1.4%, 16.1 ± 2.5%, 10.2 ± 1.2% and 14.19 ± 2.4% respectively (p- <0.001). Mean (±SD) dose to contralateral breast in F-IMRT-BH, F-IMRT, TD, in TH technique was 25.7 ± 9.5, 20.2 ± 40.7, 50.50 ± 19.8 and 267.8±36.5 (p- <0.001). Max dose (±SD) to left anterior descending artery (LAD) in F-IMRT-BH, F-IMRT, TD, TH was 3595.50 ± 589.1cGy, 3744 ± 186.3cGy, 3231.50 ± 688.4 cGy and 2908 ± 248.5 cGy (p-0.017). Conclusion For left breast, axilla and supraclavicular fossa RT, Inverse IMRT with TD plan achieve better HI and reduced dose to ipsilateral lung compared to F-IMRT-BH. TH plans achieve better CI, reduced maximum dose to LAD artery compared to F-IMRT plans, but have higher low dose spill to other OARs. Optimal suitable technique will depend upon patient related factors and expectations. 1 Aarhus University Hospital, Danish Center for Particle Therapy, Aarhus, Denmark; 2 Maastricht University Medical Centre+, Department of Radiation Oncology (MAASTRO), GROW - School for Oncology, Maastricht, The Netherlands Purpose or Objective Proton radiotherapy is faced with a number of specific uncertainties such as the relative importance of patient setup errors, changes in patient anatomy and range uncertainties. A major contributor to range uncertainties is the conversion of CT number to proton stopping power ratio (SPR), which is unique for particle therapy. Robust optimization is often used in contemporary proton treatment planning to ensure optimal dose distribution even with setup- and range uncertainties. This method provides an acceptable dose distribution by optimizing the dose in worst case scenarios of uncertainty. A typical range uncertainty around 3.5% is often utilized. Unfortunately, no consensus exists as to where the 3.5% should be added/subtracted. The aim of this study is to investigate the effect on the dose distribution when the range uncertainty is added to either the CT numbers or the stopping power ratios. Materials and Methods Treatment plans were created for head-and-neck (HN), liver and prostate cancer patient. The prescription dose was 70Gy for all three patients for the purpose of comparison. The HN plan consisted of five beams, the liver and the prostate plan consisted of two beams, which is consistent with clinical use. Each of the three treatment plans were recalculated adding positive and negative range uncertainties of 3.5% either to the CT number or SPR to compare the two uncertainty evaluation strategies. Plan optimization was performed without the use of robust optimization to clearly see the difference between the two strategies. New dose distributions were evaluated by subtracting the CT number manipulated dose distribution from the SPR manipulated dose distribution for both negative and positive range uncertainties. Differences in dose distribution were plotted in a histogram. Here all voxels with a dose of 0Gy in the original plan were excluded from the comparison. The difference in target dose for the two uncertainty evaluation strategies was also calculated. To investigate differences between the results for the three treatment sites, the amount of air/lung (<-300HU), soft ([-300,200]HU) and bone (>200HU) tissue that the beams travelled through was extracted. Results There was a small dose difference between dose evaluations where the range uncertainty was added to the CT PO-1646 The consequence of different definitions of range uncertainty I. Sojat 1 , K. Jensen 1 , V.T. Taasti 2