ESTRO 36 Abstract Book

S845 ESTRO 36 _______________________________________________________________________________________________

target volume because of high radiation exposure to the heart. This planning study aimed to quantify the reduction of cardiac and pulmonary radiation dose with moderate deep inspiration breath hold (DIBH) technique compared with free breathing (FB) for irradiation of left-sided breast including IMN. Material and Methods Ten patients underwent CT simulation scans during FB and DIBH, which was performed with the SpiroDyn’RX (Dyn’R) spirometer. The clinical target volumes (CTV) included the breast, the ipsilateral IMN, the supraclavicular lymph nodes area and the tumor bed site. Contouring was performed by the same physician, following ESTRO consensus guidelines, on both CT scans. Prescribed doses were 50 Gy in 25 fractions to the breast, the IMN and the supraclavicular area, followed by 16 Gy to the tumor bed site. Treatment plans were calculated by the same physicist with Pinnacle 9.10 (Philips) TPS for both CT scans. Three isocentric beams were used for the 50 Gy volumes: two wide Step and Shoot (S&S) tangents for irradiation of the breast and IMC, and an anterior conformal beam for treatment of the supraclavicular nodes. Three oblique S&S beams were used for tumor bed boost. The resulting averaged dose-volume histograms (DVH) were generated and compared. Mean dose to the heart (D mean, heart ) and heart volume receiving 25 Gy or more (V 25Gy, heart ), mean left lung dose (D mean, lung ) and lung volume receiving 20 Gy or more (V 20Gy, lung ) were evaluated and compared. Results The average DVHs for FB and DIBH are shown on figure 1.

breast cancer.

EP-1569 A comparison of SRS plan quality when using VMAT vs non-coplanar static conformal fields. R. Brass 1 , L. Howard 1 , M. Gilmore 1 1 The Clatterbridge Cancer Centre, Physics, Bebington, United Kingdom Purpose or Objective To produce a VMAT multi-arc solution in Eclipse for SRS patients with at least equivalent plan quality to previously used static conformal field (SCF) technique. To establish a plan quality tool based on acceptable plan quality metrics for SRS patients. Material and Methods 10 clinical SRS plans were chosen to create a cohort with a variation of indications, target volumes, shapes and positions within the brain (see Table 1). Patients with multiple targets were excluded from the study. The plan quality parameters used were the Paddick conformity index (CI) [Paddick 2000] , Paddick gradient index (GI) [Paddick & Lippitz 2006] and normal tissue overdose factor (NTOF: ratio of volume of normal tissue receiving prescription isodose to volume of prescription isodose) along with various dose-volumes (e.g. V5Gy); baseline values were calculated for the SCF plans. Several arc configurations were considered, ranging from 1 full arc at 0° couch angle to 1 full arc plus 3 half arcs at couch angles of 45 o , 90 o & 315 o . One of the more complex cases was used to develop a VMAT planning solution by increasing the number of half-arcs used until gains in plan quality became negligible. The Normal Tissue Objective (NTO) parameters in Eclipse were then optimised to further reduce the dose to OARs and normal brain tissue. The rest of the cohort was planned using this final solution and plan quality measures calculated and compared to SCF baselines. A final VMAT solution was decided upon consisting of 1 full arc at couch zero plus 2 half arcs at 45 o and 315 o .

With similar target volumes coverage, average D mean, heart was reduced from 9.7 +/- 2.1 Gy [range: from 6.1 to 12.7 Gy] to 5.1 +/- 2 Gy [range: from 3.2 to 8.9 Gy] in DIBH plans compared to FB. Averaged V 25Gy, heart was reduced from 13.5 +/- 4.3% in FB plans to 4.7 +/- 3.6% in DIBH plans. Figure 2 shows a systematic reduction of V 25Gy, heart over the 10 patients, superior to 50% for 8 patients.

The NTO fall off parameter was tweaked while fixing the distance from target , start dose and end dose parameters at values suitable for SRS. The final fall off value for the VMAT solution was set to 0.50. Fall off dictates how quickly the dose should decrease outside the target volume, with a higher value indicating a more rapid decrease. The 0.50 value used here compares to 0.15 used at our centre for VMAT plans.

Averaged mean D mean, lung was reduced from 18.5 +/- 3.2 Gy in FB plans to 17.5 +/- 2.8 Gy in DIBH plans and V 20Gy, lung by 7.8% in DIBH plans, but were not systematically inferior. Conclusion Without compromising target coverage, DIBH treatment plans provided an averaged D mean, heart reduction of 4.6 Gy, and a 1.0 Gy reduction of D mean, lung on average. Related to the reduction of mean cardiac dose and the demonstrated decrease of cardiovascular toxicities, DIBH may be the preferable treatment technique when radiotherapy of the internal mammary lymph nodes is required for left sided