ESTRO 2021 Abstract Book

S1441

ESTRO 2021

PO-1716 Neoprene wet suit hood bolus in total scalp helical tomotherapy: does it work? G. Lazzari 1 , D. Becci 2 , A. Bruno 3 , E. Verdolino 3 , F. Itta 4 , D. Mola 4 , G. Silvano 5

1 San Giuseppe Moscati Hospital , Radiation Oncology Unit, Taranto, Italy; 2 San Giuseppe Moscati Hospital , Physic Unit , Taranto, Italy; 3 San Giuseppe Moscati Hospital, Physic Unit, Taranto, Italy; 4 San Giuseppe Moscati Hospital , Physic Unit, Taranto, Italy; 5 San Giuseppe Moscati Hospital , Radiation Oncology Unit , Taranto, Italy Purpose or Objective Helical Tomotherapy ( HT) is a has been demonstrated a valuable technique for total scalp irradiation in several cancers. Issues on role and type of customized bolus for adequate dose coverage to the entire scalp surface still remain unsolved. Herein we report our experience on the use of a neoprene wet suit hood as Consolidative RT in HT was prescribed to a 76 years old woman suffering of cutaneous lymphoma in the scalp with four residual nodular masses after 4 R-CHOP cycles. A neoprene 3 mm thick wet suit hood as bolus was worn on hairless scalp under a customized thermoplastic mask. To assess the dose distribution on scalp coverage, the treatment was first evaluated on an anthropomorphic head phantom with gafchromic film sheet. Then the treatment was planned on the patient’s planning 1mm thick CT scan images without (A) and with (B) the neoprene hood. The dose distribution on human skin scalp was evaluated with four Mosfet dosimeters (MDs) placed under the cup during the first fraction. Two CTVs were defined in IMRT-SIB plan. CTV1 consisted of the entire scalp from the skull base to the frontal skin up to the orbit roof. The external body included the neoprene border in plan B. For PTV1 the prescribed dose was 36 Gy/1.8 Gy. CTVs2 consisted of the 4 residual lesions spread in the scalp (GTV1- ) plus 2mm as margins except on the external border. The prescribed dose to each PTVs2 was 40 Gy/2 Gy. Directional blocking dictates were given to spare brain and eyes as the main OARs. Results On phantom, the gafchromic film showed a superficial dose of 104 % of the PD within the first 4 mm depth in more than 95 % of the PTVs . On patient plan, the advantage with bolus consisted of an increase from 34.6 (A) to 36.7 Gy (B) for PTV1 and 37.6 (A) to 40.8 Gy for PTV2(B). DVH improved showing PTV 95 % > 98% for PTV1 and PTVs2; D2% were 107 % in A vs 103% in B. The mean dose to the brain was 12.5 Gy vs 11 Gy while the lens mean dose was 3 Gy vs 2 Gy. The mean surface dose as measured by the 4 MDs was 190 cGy (95%) of the PD 200 cGy. In the set -up, the lateral mean shift was -0,7 mm, longitudinal -0,5mm, vertical 1 mm. Conclusion Neoprene wet suit hood worn as customized bolus in total scalp HT works well leading to a better homogeneity and delivery dose, coverage of the entire scalp surface as well as robustness to minimize set up errors. bolus in a total scalp HT. Materials and Methods PO-1717 In vivo shielding disc setup verification for breast IOERT C. Cases Copestake 1 , G. Oses 1 , A. Herreros 1 , J. Saez 1 , C. Gomà 1 , C. Castro 1 , E. Escudero 1 , M. Mollà 1 1 Hospital Clinic de Barcelona, Radiotherapy, Barcelona, Spain Purpose or Objective Intraoperative electron radiotherapy (IOERT) is a radiotherapy technique which aims to treat limited volumes during oncological surgery. Although the setup/placement of applicator and shielding disc are correlated with the risk of PTV underdosage and organ at risk (OAR) irradiation, in-vivo verification of this parameter is scarcely reported. The aim of our study is to report and analyze the amount of misalignment between the applicator and the disc using radiochromic films during IOERT and its possible causes. Materials and Methods In vivo verification of disc setup was performed for 19 early-stage breast cancer patients receiving IOERT with a LIAC (Sordina) electron accelerator. To perform the in vivo disc positioning check several radiochromic film templates were created according to the different diameters of the shielding disc. Radiochromic films were sterilized by hydrogen peroxide plasma and sutured on the outer face of the disc. The size of the chosen applicator was based on the tumor size. After removal of the tumor the disc (at least 1 cm bigger than the applicator) was placed between the treatment volume and the chest wall and sutured to the pectoral muscle. The tissue surrounding the excision cavity was temporarily sutured within the area defined as the CTV to ensure complete irradiation of the tumor bed margins. The percentage of the irradiation area outside the disk was recorded (Figure 1) and various parameters (applicator angulations, prescription depth, tumor location and breast size) were analyzed to find possible correlations with the misalignment.