ESTRO 36 Abstract Book

S474 ESTRO 36 _______________________________________________________________________________________________

Results IMC coverage was equivalent between DIBH and FB plans (Table 1). Twenty-one patients (70%) had ≥5% reduction in ipsilateral lung V20 with DIBH (Figure 1). A reduction in lung metrics was observed with an absolute reduction in mean ipsilateral lung V20 by 7.8% (range: 0.0-20.0%; Table 1). There was a mean reduction of 42.3cc (range: 0-178.9) in the volume of liver receiving 50% of the prescription dose. The differences in cardiac doses were statistically significant, but unlikely clinically significant (Table 1). Seven patients with stages IA-IIIC right-sided breast cancer treated between April to October 2016 received a DIBH scan as a result of a FB V20Gy ≥30%, treatment volumes included IMCs in all cases. In this cohort a ≥5% difference between plans was observed for all patients with a mean FB V20 of 34.8% (range: 30.0-43.4%), which was reduced by an absolute value of 11.8% (range: 5.4-19.1%) with DIBH.

(∆R o ) was determined at the distal and proximal edge of the tumor respectively. Secondly, for each angle and patient, the 90th percentile of ∆R u and ∆R o were calculated. Finally, median and inter-quartile ranges for these beam-specific range uncertainties were evaluated. Results Figure 1 shows the median and inter-quartile range of ∆R u and ∆R o for the 13 different angles. For both ∆R u and ∆R o, the range errors of the lateral beams (around 90 ⁰ ) are significant lower (paired T-test, p < 0.05) than the anterior and posterior beams. Moreover, there is considerable inter-patient differences in range uncertainties.

Figure 1. Median and inter-quartile range of the range error ∆R in overshoot and undershoot at the beam’s proximal and distal edge of the tumor respectively. The angle of 90 ⁰ is the lateral beam to the ipsilateral lung. Conclusion Variation in anatomy during the course of irradiation causes variation in range, possibly leading to tumor under- coverage and high dose in normal tissue. These range uncertainties depend on patient and beam angle, and are smaller for the lateral beams. Taking these beam-specific range uncertainties into account, could improve the robustness of proton treatment plan against anatomical variations. PO-0870 DIBH produces a meaningful reduction in lung dose for some women with right-sided breast cancer J.L. Conway 1 , L. Conroy 1 , L. Harper 1 , M. Scheifele 1 , W. Smith 1 , T. Graham 1 , T. Phan 1 , H. Li 1 , I.A. Olivotto 1 1 Tom Baker Cancer Centre, Radiation Oncology, Calgary- Alberta, Canada Purpose or Objective To determine whether deep inspiration breath hold (DIBH) produced a clinically meaningful reduction in pulmonary dose in comparison to free breathing (FB) during adjuvant loco-regional radiation (RT) for right-sided breast cancer. Subsequently, to prospectively evaluate DIBH in right- sided breast cancer cases with a FB V20Gy ≥30%. Material and Methods Thirty consecutive women with breast cancer treated with tangent pair RT following breast conserving surgery were included. ESTRO guidelines were used to contour right- sided IMC nodes on DIBH and FB scans, with care taken to ensure comparability between scans. A four-field, modified-wide tangent plan was developed on each scan to include the right breast and full regional nodes with a minimum dose of 80% to the IMC CTV. The junction between the supraclavicular and tangent fields was at the inferior extent of the ossified medial clavicle. Treatment plans were calculated in Eclipse using Acuros algorithm version 11. FB and DIBH plan metrics were compared using Wilcoxon-signed rank testing. Commencing in March 2016, as per a new institutional policy based on the above results, all right-sided breast cancer patients with a FB ipsilateral lung V20 ≥30% had a DIBH treatment plan developed prior to compromising on IMC coverage. If the absolute difference in lung V20 was ≥5% between plans, the DIBH plan was used. The junction was moved superiorly in only one case.

Conclusion DIBH reduced mean ipsilateral lung V20 by 7.8% and mean lung dose by 3.4Gy. For some patients, the volume of liver receiving ≥25Gy can also be reduced with DIBH. DIBH should be available as a treatment strategy to reduce right lung V20 without compromising on IMC coverage for patients with right-sided breast cancer during loco- regional RT. This strategy can be advantageous when the ipsilateral FB V20 ≥30%, a value that prompts many radiation oncologists to exclude IMCs. Within a small prospectively evaluated cohort reflective of a change in institutional policy, we have observed an absolute reduction in mean ipsilateral lung V20 by 11.8% with DIBH. PO-0871 Study of the effect of heterogeneous setup random errors in treatment margins D. Sevillano 1 , A.B. Capuz Suárez 1 , R. Colmenares Fernández 1 , R. Morís Pablos 1 , M.J. Bejar Navarro 1 , D. Prieto Moran 1 , F. García-Vicente 1 1 Hospital Ramón y Cajal, Medical Physics, Madrid, Spain