ESTRO 2021 Abstract Book

S1623

ESTRO 2021

The use of breath-hold (BH) irradiation for the treatment of left-sided breast cancer may allow to distance the heart from the target with advantages on the dosimetry of the treatment plan. The precision and the easiness of the of the breath hold procedure remain crucial points for the accuracy of the treatment delivered. This work gives an overview based on the first one hundred patients treated, of the achieved dosimetric endpoints for the target and organs at risk (OARs), the accuracy of patient positioning and the stability of the BH procedure. Materials and Methods This retrospective study includes 109 female patients with early stage pT1c- T2N1 left breast carcinoma who underwent radiotherapy in BH after conserving surgery. Clinical target volume (CTV) and OARs were contoured by experienced radiation oncologists on the BH computed tomography (CT) scans; the planning target volume (PTV) was generated with an isotropic expansion of the CTV with a 3 mm margin in all directions, excluding the first 5 mm inside the body external contour. Patients were treated with a prescribed dose (Dp) of 50 Gy in 25 fractions. Plans were based on a tangential field in field technique with a 6 MV beam of an Elekta Infinity LINAC using two opposing tangential fields, each including 3 to 5 sub-segments. Treatment plans were optimised in Pinnacle 3TM Version 9.10 to achieve minimum 95% of the PTV covered by 95% isodose line and a mean dose (Dm) to the PTV equal to the Dp while keeping the OARs doses as low as achievable. Dose volume histogram (DVH) endpoints for PTV, heart, left lung, right lung and right breast were evaluated. At the linac, the 3D body surface of the patient based on the planning CT was compared with the real time 3D surface acquired by a surface tracking imaging system (AlignRT). The differences between the setup errors determined from the weekly cone beam CT (CBCT) and AlignRT on a selected region of interest (ROIs) were evaluated in the left-right (LR), cranial-caudal, (CC) and anterior-posterior (AP); similarly, the maximum deviation (MDev) between the surface acquired in real time and the reference surface from the planning CT was registered. Results OARs and target dosimetry are reported in Table 1. Heart and left lung doses are lower than the corresponding FB values retrieved from literature (Breast Cancer, doi.org/10.1007/s12282-019-01023-9). For the patient positioning, the median setup differences between AlignRT and CBCT (n=300) on the surface of the ROI selected are 1.5 mm (range 0.0-3.0), 2.0 mm (range 1.0-5.0) mm, and 2.1 mm (range 0.0-3.1), in the LR, CC, AP directions, respectively; the median MDev registered during the treatment delivery is 2.1 mm (range 1.5-3.1).

Conclusion Left-sided breast treatment in BH allows low doses to OARs while keeping adequate target coverage. SGRT enable an accurate patient positioning and can reliably monitor the chest position of patients holding the breath.

PO-1903 Building and comparing treatment quality assessment algorithms among two different clinics A. Scaggion 1 , S. Bresciani 2 , M. Fusella 3 , A. Galla 4 , M.A. Rossato 3 , M. Sepulcri 5 , M. Paiusco 6 , B. El Khouzai 5 1 Veneto Institute of Oncology IOV - IRCCS, Medical Physics Departement, Padova, Italy; 2 Candiolo Cancer Institute - FPO, IRCCS, Medical Physics Division, Candiolo, Italy; 3 Veneto Institute of Oncology IOV – IRCCS, Medical Physics Department, Padova, Italy; 4 Candiolo Cancer Institute - FPO, IRCCS, Department of Radiotherapy, Candiolo, Italy; 5 Veneto Institute of Oncology IOV – IRCCS, Radiation Oncology Department, Padova, Italy; 6 Veneto Institute of Oncology IOV – IRCCS, Medical Physics Department, Candiolo,, Italy Purpose or Objective The Plan Quality Metric (PQM) is a relative scoring system which allows to compare competitive plans. The PQM algorithm composition is not unique nor objective. This work introduces two different methodologies to build a PQM algorithm and compares their outcomes. The work has been focused on prostate treatment scoring and involved two different clinics. Materials and Methods 60 (30+30) intermediate-risk prostate cancer cases treated with VMAT were retrospectively selected among two different institutions. Contouring was performed according to the ESTRO guidelines with CTV-PTV expansion by 7mm in every direction and by 5mm in the posterior direction. Center A prescribes 70Gy in 28 fractions while Center B 70.2Gy in 26 fractions. Contoured OARs were: rectum, bladder, penile bulb and femoral heads. Each center individually developed its own PQM algorithm. Center A algorithm was inspired by published standards [Abu- Gheida,2019] and tailored to the clinical practice. Center B scoring system was built on the basis of QUANTEC constraints and on a statistical ideal dose volume histogram (SI-DVH), defined as the average DVH generated using an ideal dose falloff on a set of historical countered volumes. PQM% scores were computed with PlanIQ (Sun Nuclear Corp., Melbourne, FL). The plan quality of every patient has been quantified through both PQM algorithms and results have been compared on the