ESTRO 37 Abstract book

S1042

ESTRO 37

to compare all treatment options available at our site to the one used in routine clinical practice. Material and Methods 10 patients treated for breast cancer with lymph node involvement were selected. Prescription dose was 63 Gy to the tumor bed, 51.8 Gy to the whole breast and 50.4 Gy to the lymph nodes in 28 fractions. HT_FW_5, VMAT and PT plans were designed and compared to the clinically approved plan (HT_FW_2.5) using dosimetric indices for OAR (D max for the spinal cord, D mean for the heart, both lungs and contralateral breast) and PTV (D 95% , D 2% and Homogeneity Index). A paired Student’s t-test ( α=0.05 ) was used to cross-check all plans. For all plans, a homemade dosimetric score was computed (S OAR and S PTV ). Based on previous data gathered through the years, OAR results were divided into four quartiles: Q1 (S OAR =1), Q2 (S OAR =2), Q3 (S OAR =3) and Q4 (S OAR =4). PTV scores are based on the D 95% . If D 95% ≥ 98% of the prescribed dose S PTV =1, if D 95% ≥ 95% S PTV =2, if D 95% ≥ 92% S PTV =3, and if D 95% < 92% S PTV =4. For both OAR and PTV, 1 is the best and 4 the worst achievable score. Combined with the dosimetric indices, these scores are useful to improve our clinical practice. Results Results reported in Table 1 show that HT_FW_5 and VMAT plans ensure an equivalent or even better PTV coverage than the initial clinically approved plan (HT_FW_2.5), but at the cost of a deterioration of the OAR sparing. However, PT_PBS plans show that an excellent PTV coverage can be maintained along with significantly lower doses to the heart, contralateral lung and contralateral breast. Conclusion HT_FW_5 and VMAT plans allow reducing significantly the radiation time for the treatment of breast cancer with lymph node involvement compared to the initial clinically approved plan (HT_FW_2.5). It could benefit to a specified patient population, even if compromises would have to be made between PTV coverage and OAR sparing. In addition, results showed that PT_PBS treatment should be considered in the near future as it showed great potential benefit to lower the risk of side effects. EP-1916 Treatment techniques and plan quality for breast irradiation with simultaneous integrated boost H. Svensson 1 , M. Hällje 1 , R. Chakarova 1 , D. Lundstedt 2 , M. Gustavsson 1 , P. Karlsson 2 1 Sahlgrenska University Hospital, Medical Physics and Biomedical Engineering, Göteborg, Sweden 2 Institute of Clinical Sciences- Sahlgrenska Academy- University of Gothenburg, Department of Oncology, Göteborg, Sweden Purpose or Objective The study aims to develop dose planning and plan evaluation strategies applicable to different patient groups undergoing hypo-fractionated whole breast irradiation with simultaneous integrated boost. Material and Methods Dose of 40 Gy/15 frac/2.67 Gy to the breast and 48 Gy/15 frac/3.2 Gy to the lumpectomy cavity was ordinated. Ten patients with left-sided breast cancer, ten

patients with right-sided breast cancer, both with CT scans under free breathing were studied as well as ten consecutive patients receiving left-sided breast cancer treatment during Deep Inspiration Breath Hold (DIBH). For each patient four plans were created: one conventional plan with tangential fields (3DCRT), one 3DCRT-VMAT hybrid (80:20) and one 3DCRT-IMRT hybrid (70:30) [1] as well as one VMAT plan with restricted arcs. Tolerance criteria for target and organs at risk from RTOG 1005 were utilized when analyzing the DVHs obtained for the different plans. Additional parameters, e.g. ventricle mean dose, near max heart dose, mean lung dose, gEUD, NTCP for ipsilateral lung and others, are included. The Plan Quality Metric (PQM) strategy [2] was applied identifying 14 sub-metrics to evaluate the plan quality and variation for the different treatment techniques. Three levels of performance were defined for each sub-metric using linear and non-linear scoring. Target coverage accounted for 56% of the total score. The percent for heart, lung and contralateral breast was 20, 16 and 8%, correspondingly. Results In general, the RTOG tolerance criteria have been fulfilled for all plans. High target coverage has been achieved with each technique and the mean values for PTV V95% and PTVT V95 were larger than 96% and 98.5%, correspondingly. PTV conformity indexes (CI) were comparable for the hybrid and VMAT plans and superior over the corresponding conventional plans (Table). The mean doses to heart and lungs in the Table indicate a smaller increase of relative risk for lung cancer and cardiac mortality than reported in [3]. Plans using 3DCRT- IMRT technique received the highest score. The relative performance of the four techniques tended to persist even on individual plan level (Figure). The small difference between the PQM scores for the different techniques may be related to the domination of the target coverage in the total score. Choice of sub-metrics and score distribution allows stratification of breast cancer patients into groups for further optimization of risk/benefit balance.