ESTRO 2023 - Abstract Book

S1794

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ESTRO 2023

The planning objectives were fulfilled with all the planning techniques for both target coverage and organs-at-risk sparing. The EAR for CB tumor induction, estimated with the most accurate model, was 4.8 ± 1.3, 5.5 ± 1.3, 5.5 ± 1.2 and 5.5 ± 1.3 (per 10,000 patients per year) with 3D-CRT, FiF, HMRT and HVMAT, respectively (p=0.097). For the CL, these figures were 11.9 ± 1.6, 11.9 ± 1.6, 11.9 ± 1.4 and 10.7 ± 1.5 (per 10,000 patients per year), respectively (p=0.189). For the IL, these figures were 32.2 ± 7.2, 29.0 ± 6.8, 29.9 ± 6.6 and 27.7 ± 7.0 (per 10,000 patients per year), respectively (p=0.222). Concerning the risk of second cancer induction for contralateral breast and both lungs, the differences between the four techniques were never significant. For the CB and CL, the mean absolute difference did not reach 1 case of 10000 patient- years. For the IL, the mean absolute difference was up to 5 case of 10000 patient-years. In particular, HVMAT presented the lower risk of second cancer induction to all structures, although not statistically significant. Conclusion The overall estimation of EAR indicated that the radiation-induced cancer risk of breast radiation therapy was not statistically different for all irradiation techniques. The new advanced automated planning algorithms allow an EAR reduction for VMAT plans at the level of well-consolidated tangential 3D-CRT or FinF techniques, keeping all the VMAT dosimetric improvements unchanged. On the basis on this analysis, it would be safe to say that the adoption of the automated HVMAT technique poses no increase in EAR, and should be considered safe also for younger patients. F. Cucciarelli 1 , M. di Benedetto 1 , E. Arena 1 , E. Galofaro 1 , M.V. Agbaje Olufemi 1 , C. Di Carlo 1 , L. Vicenzi 1 , F. Fenu 1 , M. Parisotto 2 , M. Valenti 2 , G. Mantello 1 1 Azienda Ospedaliero Universitaria delle Marche, Radiotherapy Department, Ancona, Italy; 2 Azienda Ospedaliero Universitaria delle Marche, Medical Physics, Ancona, Italy Purpose or Objective Accelerated partial breast irradiation (APBI) represents a valid alternative treatment for selected patients (pts) with early breast cancer (EBC). Aim of our study is to evaluate the reproducibility and tolerability of APBI with volumetric modulated Arc therapy (VMAT) and daily image-guided radiotherapy (IGRT) in a selected subgroup of pts. Materials and Methods From March 2021 to July 2022, we enrolled 63 EBC pts aged > 60 years underwent lumpectomy and sentinel lymph node (SLN) biopsy, with at least 4 titanium clips on the tumor bed. Prevalent histology was ductal and the molecular subtype was Luminal A. T stage was T1-T2, tumor grade was G1-G2 with negative margins and SLN. A computed tomography scan with two-millimeter slice thickness was obtained. The clinical target volume (CTV) was drawn with an isotropic 3- dimensional margin of 1.5 cm around the surgical clips to include radiological abnormalities and seromas, if present, and to exclude the pectoral muscle when possible. The CTV was limited to 3 mm from the skin surface. An isotropic margin of 0.5 cm was added to obtain the planning target volume (PTV). The ipsilateral and contralateral lung, heart, left coronary artery (LAD) for the left breast, and contralateral breast were contoured as organs at risk (OARs). A total dose of 26 Gy in 5 once-daily fractions was prescribed. These constraints were adopted for VMAT optimization: PTV coverage V95% = 95%; maximum dose (Dmax) on PTV < 107%, for ipsilateral lung V10 < 20%, contralateral lung V5 < 10%, heart V3 < 10% for left breast and < 8% for right breast, LAD mean dose (Dmean) ≤ 5 Gy, contralateral breast Dmax < 1 Gy and Dmean ≤ 3 Gy. Daily cone beam CT (CBCT) was performed for pts set-up. Varian's intrafraction motion review ability to automatically detect fiducial markers was utilized using KV images triggered every 60 degrees of gantry rotation during treatment. Marker displacement from expected position was used to evaluate the reliability of CBCT pts positioning. Toxicities were evaluated using Common Terminology Criteria of Adverse Events (CTCAE) version 5.0 scale at the end of RT and 1 month after RT. The first pts treated were also evaluated at 6 months after RT. Results All dose constraints were widely respected and PTV coverage was V97% = 97%. The median displacement of the marker was greater for the cross- plane axis affected by respiratory motion but remained at approximately 3 mm. RT was well tolerated. None of the pts reported side effects during RT; during follow-up, grade 1 breast pain occurred in 7 patients, and grade 1 skin induration and hyperpigmentation occurred in 5 patients. Conclusion APBI with VMAT technique was well tolerated and had a very low toxicity profile. Intrafraction marker detection showed no significant bias with CBCT imaging and may prevent off-target irradiation when Auto Beam hold is enabled. PO-2020 Reproducibility and tolerance of accelerated partial breast irradiation in selected patients

PO-2021 Importance of Optimisation Parameters for Lung SABR Plan Robustness against Machine Delivery Errors

A. Thorne 1 , A.L. Palmer 1 , G.J. Budgell 2

1 Portsmouth Hospitals University NHS Trust, Radiotherapy Physics, Portsmouth, United Kingdom; 2 The Christie NHS Foundation Trust, Radiotherapy Physics, Manchester, United Kingdom Purpose or Objective Lung SABR planning aims for steep dose gradients at the PTV edge to achieve peak doses within the GTV and low dose in normal tissue. High gradients mean that small errors in MLC position below QA tolerance levels may lead to clinically significant deviations in plan dosimetry [1, 2]. MLC position errors may occur on both MLC banks, doubling the nominal error, resulting in isotropic reduction in dose not adequately accounted for in the PTV margin. The effect of optimisation parameter selection on plan robustness against machine delivery error has not hitherto been investigated.

Materials and Methods