ESTRO 2020 Abstract book

S60 ESTRO 2020

plan-of-the-day protocol were included. Radiotherapy was combined either with concurrent chemotherapy, or with neo-adjuvant chemotherapy and concomitant hyperthermia. Selected patients had filled in a combined EORTC QLQ-C30 and QLQ-CX24 questionnaire at baseline, and at least one combined questionnaire in the acute phase of the treatment (week 4, week 5, 1 week after RT, 4 weeks after RT or 3 months after RT). Maximum deterioration from baseline was recorded for the question regarding diarrhea symptoms, and was scored as an event when a deterioration of two points or more was registered. Dose-volume parameters were collected for the rectum and bowelbag. Dose Surface Maps (DSMs) were constructed for the rectum, which is a technique to visualize a dose distribution on the surface of a tubular organ in a standardized way. Clinical variables and dose-volume parameters were included in the analysis. Permutation testing was performed for the DSMs to find areas of significance ( p < 0.05). Results Within the entire cohort, a diarrhea incidence of 59% was found. The use of concomitant chemotherapy resulted in a significant increase in reported diarrhea with an odds ratio of 2.5 (CI: 1.1-5.9). The dose-volume parameters V 5Gy -V 25Gy of the rectum were significantly associated with diarrhea, but no significant relation was found for the bowelbag. Conversely, the PTV volume was significantly related with diarrhea with an odds ratio of 1.17 (CI: 1.01- 1.37) per 100 cc increase in PTV volume. The results of the rectal DSMs are shown in Figure 1, where a significantly higher dose to the inferior part of the rectum is visible.

OC-0112 Patient-Specific Heart Constraint lowers mean heart dose for patients receiving breast RT K. West 1,2 , R. Ward 1,2 , D. Latty 2 , T. Wang 1,3,4 , S. Cross 5 , V. Gebski 1,6 , K. Stuart 1,3,4 1 Crown Princess Mary Cancer Centre, Westmead Hospital, Wentworthville, Australia ; 2 Blacktown Cancer and Haematology Centre, Blacktown Hospital, Blacktown, Australia ; 3 Westmead Breast Cancer Institute, Westmead Hospital, Wentworthville, Australia ; 4 Sydney Medical School, University of Sydney, Sydney, Australia ; 5 Nepean Cancer Care Centre, Nepean Hospital, Kingswood, Australia ; 6 NHMRC Clinical Trials Centre, University of Sydney, Sydney, Australia Purpose or Objective In 2014, our institution implemented a Patient-Specific Heart Constraint (PSHC) and a mean heart dose (MHD) constraint of 4Gy for patients receiving breast RT with a simultaneous boost (SIB). The PSHC uses a calculation method for determining MHD before optimising IMRT fields. The aim of this study was to determine whether the introduction of a PSHC would reduce MHD, whilst maintaining optimally-dosed treatment plans. Material and Methods Consecutive patients who received BCS and whole breast with SIB adjuvant RT were retrospectively identified and divided into two cohorts, pre- and post-implementation of the PSHC. Patients were simulated in the supine position. Prescribed dose to the breast PTV was 50Gy in 25 fractions; the SIB PTV was 57Gy in 25 fractions. Plans were generated using a hybrid IMRT technique, where 30Gy was delivered through a static tangential field arrangement, and the remaining 27Gy was delivered through IMRT fields. The calculation of the PSHC used the formula: MHD (open tangent fields) x 2. The following values were collected: D95% SIB PTV, RTOG conformity index (CI), homogeneity index (HI); MHD, V25Gy and V5Gy for heart, left anterior descending coronary artery (LAD) and ipsilateral lung mean doses. Results Data from 264 patients treated between June 2013 and July 2015 were collected. There were 138 patients in the pre-PSHC cohort and 126 patients post-PSHC. The plans showed no difference in the CI of the SIB PTV D95% (pre- PSHC mean 1.82 (±0.56), post-PSHC 1.79 (±0.45), p=0.634 (95% CI -0.09-0.15); HI was slightly worse post-PSHC but not clinically relevant (pre-PSHC 6.1 (±1.35), post-PSHC 6.46 (±1.5), p=0.04 (95% CI -0.7, -0.01). The SIB D95% coverage was reduced post-PSHC but was above the 54.15Gy limit (pre-PSHC 55.64Gy (±0.59) post-PSHC 55.31Gy (±0.48) p<0.001 (95% CI 0.19-0.45)). Organ at risk dose comparisons are displayed in Table 1. The PSHC benefit was enhanced further with DIBH for heart V5Gy (p=0.034; 95%CI 0.42-9.85), but not for other parameters. IMRT fields contributed less to MHD post-PSHC (Figure 1). Table 1. Comparison of heart, left anterior descending coronary artery and ipsilateral lung doses pre- and post- implementation of a patient-specific heart constraint Figure 1: Contribution to MHD from IMRT fields per patient in chronological order, for women receiving whole breast with SIB irradiation. This is based on the formula: Total MHD/(Open MHD x 1.67), where Total MHD is the sum of both the Open and IMRT components of heart dose received. The Open MHD component (representing MHD from the 30Gy delivered tangentially) is multiplied by 1.67 to predict MHD50 should 50Gy be delivered tangentially. The factor of 1.2 takes into account the boost contribution (57/50 = 1.14) with added buffer up to 60Gy (60/50 = 1.2)

Conclusion PTV volume and concomitant chemotherapy were found to be significant for acute diarrhea symptoms, possibly attributable to bowel irradiation and chemotherapy- related radiosensitizing effects. Additionally, the results suggest that a reduction of inferior rectum dose could decrease patient-reported diarrhea symptoms. Diarrhea typically originates from the bowel, yet the DSMs in this study indicate that radiation-induced damage to the rectum contributes to these complaints. Further studies are necessary to validate the results.

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