ESTRO 2020 Abstract book

S106 ESTRO 2020

PD-0192 One-year experience with total body irradiation (TBI) using forward IMRT and on-line image guidance R. Van Leeuwen 1 , D. Verwegen 1 , P.G. Kollenburg 1 , M. Swinkels 1 , R.W. Van der Maazen 1 1 Radboudumc, Radiotherapy, Nijmegen, The Netherlands Purpose or Objective Total body irradiation (TBI) is a treatment used in the conditioning of patients prior to hematopoietic stem cell transplantation. Dependent on patient characteristics and conditioning regimen (chemotherapy), low dose (e.g., 1x2Gy) or high dose TBI (e.g., 6x2Gy) can be given. We developed a forward IMRT technique for radiotherapy treatment planning using a CT scan and Pinnacle treatment planning software (TPS, Philips, Best, NL), enabling a homogeneous dose distribution and sparing of critical organs (lungs) to lower doses. Material and Methods For our high-dose TBI, patients were scanned and treated in side position with bent knees fixed in a vacuum mattress. Low-dose TBI patients were treated in supine position. Treatment was performed on an Elekta Agility linac (Stockholm, Sweden) at 350 cm (source to isocenter) enabling a maximal treatment length of 160 cm. For treatment planning, we adjusted the Pinnacle beam model to better resemble absolute dose and beam profile at long SSDs. Using the CT scan and digitally reconstructed radiographs from both the Anterior (A) and Posterior (P) directions, various beams were created: (1) a beam following the patient contour that was used to position the patient using the light field, (2) a beam slightly larger than the projection of the lungs (Fig. c) for positioning the patients with respect to the beam using a mobile megavoltage (MV) imager (Cablon, Leusden, NL), (3) an open beam with collimator angle 45 degrees (Fig. a), (4) beams shielding the left (Fig. b) and right lung, and (5) various beams to homogenize the dose. Beam weight optimization was performed to achieve the objectives that depend on the fractionation, e.g.: Lung: mean 10 Gy; kidney/brains: mean 12 Gy; rest of body: mean 12 Gy. In vivo dosimetry was performed using MOSFET dosimeters (Best medical, Ottawa, Can.) at the level of the left lung (A and P) and the abdomen (P only).

Conclusion In a single optimization, RapidPlan produced treatment plans with similar target coverage and improved OAR sparing compared to manually optimized plans. The potential for OAR dose reduction was largest for low and medium doses, likely because the higher doses have been given more attention in the manual planning, and the OARs are close to or overlapping with targets that have higher priority. KBP can improve plan quality and spare resources in the treatment planning process for high-risk prostate cancer, and has been routinely used in our clinic since July 2019.

Results From October 2 nd , 2018 to October 2 nd , 2019, 12 patients were treated with our new technique totalling 34 treatment fractions. Treatment planning dose objectives could well be achieved although creating a homogeneous dose distribution in protruding extremities (right upper arm, upper legs) was a challenge and treatment planning took a considerable amount of time (up to 16h). Before treatment, positioning of the patients using the linac light field and MV imager (MV image in Fig. d) could be performed within clinically relevant limits. Results of in vivo MOSFET dosimetry was within the set specifications (+/- 10%): mean deviations: Lung: 6%; Abdomen: -1%. Conclusion

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