ESTRO 36 Abstract Book

S987 ESTRO 36 2017 _______________________________________________________________________________________________

EP-1829 Dose delivery accuracy in total body irradiation delivered with Step and Shoot IMRT T. Berlon 1 , L. Specht 1 , P.M. Petersen 1 , L.S. Fog 1 1 Rigshospitalet, Clinic of Oncology- Department of Radiotheraphy, Copenhagen, Denmark Purpose or Objective In total body irradiation (TBI) delivered with step and shoot IMRT (SS IMRT), the dose conformity is considerably improved compared with the more widely used TBI delivered with open fields. This conformity is achieved through the use of multiple fields defined by multileaf collimators (MLCs). We aim to quantify the accuracy with which TBI patients treated at our clinic were positioned, and to determine the effect any positioning errors may have had on the delivered dose. Material and Methods Images acquired as a routine part of the patient treatment with the Theraview ™ (Theraview Technology, Leuden, The Netherlands) imaging system were used to determine the positioning shift in the cranio-caudal direction. Images for 11 consecutive patients, each receiving 6 fractions, were analysed and the shifts recorded (figure 1). For 3 of the patients, only images for 5 of the 6 fractions were available.

requires a dose limitation in healthy organs at risk (contralateral lung, heart, spine) Our objectives were: -- To evaluate the mean dose (Dm) received by the contralateral lung in SBRT for thoracic lesions. -- To analyze if PTV volume and/or total dose (TD) are related to the Dm achieved in the contralateral lung. Material and Methods A total of 26 pulmonary lesions treated with SBRT were evaluated. Simulation was performed with CT 4D respiratory gating and customized immobilization devices. PTV was designed with an isotropic margin of 0.5 cm from the GTV. Treatment was delivered with Linear Accelerator ( CLINAC, Varian), and verification done with internal fiducial markers surrogates. Results Total Dose (TD) (Gy): range 20 – 45 Gy. Most cases (18; 69%) received a TD of 45Gy. Dose/fraction: range 10-20 Gy/fr. The most frequent fractionation was 15 Gy (20; 77%). PTV volumen (cc): range between 10.18 - 99.33cc, with a mean: 36.14cc ; median: 26.65cc Healthy lung Dm (Gy): range 0.23 - 9.8 Gy; mean: 1.75Gy; median: 1.25Gy The increase in PTV volume did not associate an increase in the average dose to the contralateral lung. Fig 1 An increase in total dose not involved an associated increase in the dose to the healthy lung. Fig. 2

The plans were then recalculated implementing the shifts using the algorithm used for the clinical plans (Eclipse ™, Varian Medical Systems, Palo Alto, AAA algorithm, v 13.6). The mean and maximum doses for the lungs, kidneys, brain and the (body-lungs-5mm) structure were extracted and the difference between the planned and the recalculated doses determined.Results The mean doses change by a maximum of 0.6% (lungs), 0.6 (kidneys), 0.5% (brain) and 0.2% (body-lungs-5mm). The greatest difference between the maximum doses are 8.0% (lungs), 4.8% (kidneys), 2.6% (brain) and 12.0% (bodylungs- 5mm). The standard deviation of the difference between the calculated and recalculated doses are greater for the maximum doses than the mean doses (figure 2). Given that the minimum and maximum doses for SS TBI are typically in the range 90-110% of the prescribed dose, the differences in maximum dose should lead to care being taken when positioning patients for SS TBI.

Conclusion The Mean Dose received by the contralateral healthy lung is minimal. No relationship was found between the increase in total dose and increased in contralateral lung Dmean No relationship was found between the volume of PTV and Dmean reached in contralateral lung The parameters PTV and TD do not appear to relate to the dose received to the contralateral lung. We can conclude that SBRT technique can be applied safely largely preserving the healthy lung.

Conclusion Patient positioning for a total of 63 fractions of SS TBI is such that the mean delivered doses differs from the planned by less than 0.6%. However, the maximum doses