ESTRO 36 Abstract Book

S555 ESTRO 36 _______________________________________________________________________________________________

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 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.

Conclusion This study showed that VMAT treatment plans were relatively robust during the treatment course. In this patient cohort small changes in dose to OAR were not significant, despite a reduction in PTV. PO-1008 Feasibility of stereotactic ablative radiotherapy for locally-advanced non-small cell lung cancer K. Woodford 1 , V. Panettieri 1 , T. Tran Le 1 , S. Senthi 1 1 The Alfred Hospital, Alfred Health Radiation Oncology, Melbourne, Australia Purpose or Objective Stereotactic ablative radiotherapy (SABR) has enabled a curative treatment for elderly patients or those with significant comorbidities diagnosed with early-stage non- small cell lung carcinoma (NSCLC) who would have otherwise gone untreated. As a result population-based survival has improved. If SABR could be utilized in the treatment of locally-advanced NSCLC in the same way, the public health impact would be greater, as twice as many patients are diagnosed with advanced disease. We assessed the feasibility of SABR for locally-advanced NSCLC. Material and Methods Twenty three patients with N2 and/or N3 locally-advanced lung cancer were retrospectively replanned. Targets and organs-at-risk (OAR) were delineated using 4DCT and replanned with RapidArc delivery (AcurosXB Vn13.6). Three planning approaches were assessed; conventional approach (1.0cm ITV to PTV expansion, prescribed to 100%); SABR approach (0.5cm ITV to PTV expansion, prescribed to 80%) and a hybrid approach (0.5cm ITV to PTV expansion, prescribed to 100%). We assessed the feasibility of three dose regimes, with PTV doses all having a biologic equivalence of 60Gy in 30 fractions (α/β=10). The planning aim was to determine the least number of fractions to deliver an acceptable plan. Acceptable was defined as ≥95% target coverage by the prescribed dose whilst maintaining the OAR tolerances below. Marginally acceptable was defined as 90-95% target coverage with lung V20 <30% and other OAR tolerances met. Descriptive statistics were used. We assessed doses to the PRVs (2mm expansion) of each OAR to determine the IGRT requirements for each strategy. Results Fourteen patients had N2 involvement whilst nine had N3 involvement. Mean ITV size was 207.7cc (range 31- 706.1cc). The hybrid approach generated acceptable plans in 48% of patients (11/23), while the conventional and SABR approaches achieved 26% (6/23) and 4% (1/23) respectively. If acceptable was defined by >90% target coverage by the required dose and lung V20 was less than 30%, 70% (16/23) of patients had acceptable plans with the hybrid approach. Those that failed the hybrid