ESTRO 36 Abstract Book

S550 ESTRO 36 2017 _______________________________________________________________________________________________

M. Kertevig 1 1 Rigshospitalet, Department of Oncology- Section for Radiotherapy, Copenhagen, Denmark Purpose or Objective Throughout the course of radiotherapy in head and neck patients tumor shrinkage occurs. This may influence the dose to organs at risk (OAR) around the tumor area, as tumor shrinkage can lead to different dose distribution in the patient than originally calculated. As Volumetric Modulated Arc Therapy (VMAT) technique is frequently used for head and neck patients, it is relevant to study the impact of VMAT treatment on tumor coverage and OAR, during a course of radiotherapy, and the necessity to adaptive plans during the course of treatment. Material and Methods A retrospective study has been carried out on 13 consecutive patients who have been treated with VMAT for head and neck cancer. The Cone-beam Computed Tomography (CBCT) from the first treatment fraction was compared with the CBCT from the last fraction. Dose and volume comparison was performed for planning target volume (PTV), spinal cord, brainstem and both parotid glands. A paired t-test was used to test for significance and p -values <0.05 were considered statistically significant Results The mean volume of PTV on the CBCT from the first treatment fraction was 283.98 cm 3 compared with the CBCT from the last fraction which was 270.33 cm 3 . The mean volume of the PTV decreased significantly with 5% ( p = 0.003), due to tumor shrinkage during the course of treatment. The mean D 95 to the PTV decreased by less than 1% from 62.34 Gy to 61.88 Gy. The mean D max to the spinal cord increased by 1% from 41.33 Gy to 41.78 Gy and to the brainstem by 3.8% from 32.11 Gy to 33.33 Gy. The mean dose to the left parotid gland decreased with less than 1% from 22.08 Gy to 22.06 Gy. In contrast, the mean dose to the right parotid gland was significantly increased by 6.5% ( p = 0.033) (table 1). There were no significant differences in the mean dose to either PTV ( p = 0.12) spinal cord ( p = 0.27), brainstem ( p = 0.22) and left parotid gland ( p = 0.98), which means that treatment with VMAT, had negative effect on dose to spinal cord, brainstem and left parotid gland with a 95% probability for this patient cohort. Even though the dose to the right parotid gland increased significantly, the dose to all OAR remained within the defined constraints. In addition, the tumor coverage remained sufficient throughout the treatment. This need to be studied further with larger sample sizes together with a dose study for all the OAR in the head and neck region to fully determine the necessity to adapt the patients plan, especially since it might be possible to reduce the dose to the parotid glands for patients suffering from xerostomia.

“similar” to those defined by the Radiation Oncologist.

PO-1006 Evaluation of an auto-segmentation software for definition of organs at risk in radiotherapy M.D. Herraiz Lablanca 1 , S. Paul 1 , M. Chiesa 1 , K.H. Grosser 1 , W. Harms 1 1 St. Claraspital, Radioonkologie, Basel, Switzerland Purpose or Objective The aim of this work is to evaluate the capability of a commercial software performing automatic segmentation of relevant structures for radiotherapy planning, as well as the time saving of it use on a daily Basis. Material and Methods The software Smart Segmentation Knowledge Based Contouring (Version 13.6) from Varian Medical System was evaluated according to segmentation quality and time saving. For that purpose, 5 consecutive prostate and breast patients were contoured manually and automatically using the software, recording the time needed in both, manual and automatic contouring with corrections. This task was performed by the RTTs, since they are responsible of the OARs contouring in our department. Segmentation quality was qualitatively scored in four levels: 'excellent”(1), 'good”(2), 'acceptable”(3) and 'not acceptable”(4) and quantitatively evaluated calculating five parameters: Relative difference in volume, DICE similarity coefficient, Sensitivity Index, Inclusiveness Index, Mass Center Location. Results Mean values of the qualitative evaluation and acceptance are summarized in Table 1. The acceptance of the structures automatically contoured is higher for breast cancer than for prostate patients, as well as the mean time saving, that is above four minutes for breast and around 1 minute for prostate.

Good agreement was found between manual and automated segmentation for heart with a mean difference in volume of 7%, DICE of 0.87 and deviation of mass center less than 2mm in all directions and for liver with a mean difference in volume of 11%, DICE of 0.91 and deviation of mass center less than 2mm in all directions. Poor acceptance was found in complex structures as penile bulb, small bowel, sigma and rectum wall anterior and posterior. Conclusion Smart Segmentation software is a useful tool for the delineation of relevant structures for breast although did not generate useful delineation for neither the mammilla nor the esophagus. For relevant structures for prostate as penile bulb, small bowel, sigma and rectum wall anterior and posterior the software was not good enough. Further analysis will be performed including more patients.

Poster: RTT track: Treatment planning and dose calculation / QC and QA

PO-1007 The effect of VMAT on tumor coverage and organs at risk for head and neck cancer patients

Conclusion