Breast Cancer

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TU_10_3418 - Acute Toxicity Outcomes and Dosimetric Implications from Incidental Irradiation of Adjacent Tissues in Tangent Field Hypofractionated Breast Radiation Therapy

Tuesday, October 23
2:45 PM - 4:15 PM
Location: Innovation Hub, Exhibit Hall 3

Acute Toxicity Outcomes and Dosimetric Implications from Incidental Irradiation of Adjacent Tissues in Tangent Field Hypofractionated Breast Radiation Therapy
S. R. Alcorn, T. R. McNutt, F. Asrari, V. J. Croog, S. F. Stinson, B. Floreza, A. A. Weaver, and J. L. Wright; Department of Radiation Oncology and Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Baltimore, MD

Purpose/Objective(s): In whole breast radiotherapy (WBrRT), current protocols often specify dosimetric constraints for the contoured breast alone. However, adjacent normal tissues outside of the breast may also receive substantial doses within standard tangent fields. To better characterize the impact of incidental irradiation of these tissues-in-beam (TIB), we analyzed dosimetric parameters of TIB and associated acute toxicity during hypofractionated WBrRT.

Materials/Methods: Tangent field WBrRT plans for patients treated to 40.5 Gy/15 fractions from 4/2016 to 1/2018 were evaluated. Breast tissue excluding skin was contoured, and a TIB structure was created to include all breast and other tissues encompassed within the plan 30% isodose line. Non-breast TIB (nTIB) was calculated as TIB minus breast tissue. Volumes of the breast, TIB, and nTIB receiving 100%-107% of the prescription dose (V100-V107) were calculated in cm3. Acute toxicities were prospectively collected weekly during WBrRT. Radiation dermatitis, pruritus, hyperpigmentation, and fatigue were evaluated per CTCAE v.4. Pain scale (0-10) and presence of burning, twinging, or tenderness components were noted. Relationships between volumetric and dosimetric parameters were assessed using Spearman’s rank-order correlation. Toxicity grade changes of ≥1 unit over the course of WBrRT were calculated, and uni- and multivariable logistic regressions evaluated TIB V100-V107 (in cm3) as predictors of toxicity outcomes.

Results: A total of 137 plans were evaluated, with complete toxicity assessments in 117 patients. Prone positioning was used in 53% of cases. Mean breast and TIB volumes were 687 cm3 (SD 451) and 1565 cm3 (SD 793), respectively. Larger breast volumes were positively correlated with larger nTIB volumes (rho= 0.53, p<0.05) and with increasing nTIB V100 (rho= 0.29, p<0.05). V105 and V107 of >2 cm3 were noted in 63% and 14% of breast volumes but also in 58% and 21% of nTIB volumes, respectively. Table 1 shows toxicity outcomes significantly associated with increasing TIB V100-V107 on univariate analyses. After adjusting for corresponding breast V100-V107, increasing TIB V100 and V105 significantly increased odds of burning or twinging pain (OR 1.004 and OR 1.007, respectively), and TIB V105 also increased odds of hyperpigmentation (OR 1.017), all p<0.05.

Conclusion: For hypofractionated WBrRT, doses to TIB in excess of the prescription were common outside of the breast volume and predicted for development of acute toxicity including pain and hyperpigmentation. These data support inclusion of TIB as a region of interest in treatment planning and protocol design.
Table 1: Toxicity outcomes significantly associated with TIB V100-V107
V100 V105 V107
Hyperpigmentation Hyperpigmentation Hyperpigmentation
Burning or twinging pain Burning or twinging pain
Pain scale score

Author Disclosure: S.R. Alcorn: Research Grant; National Institute of Health. T.R. McNutt: Research Grant; Elekta Oncology Systems, Philips Radiation Oncology Systems, Toshiba. Patent/License Fees/Copyright; Accuray-Tomotherapy, Sun Nuclear. President Elect; AAPM-MAC. F. Asrari: None. V.J. Croog: None. S.F. Stinson: None. A.A. Weaver: None.

Sara Alcorn, MD, MPH

Disclosure:
Employment
The Johns Hopkins Hospital: assistant faculty: Employee

Compensation
NIH: Research Grants

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TU_10_3418 - Acute Toxicity Outcomes and Dosimetric Implications from Incidental Irradiation of Adjacent Tissues in Tangent Field Hypofractionated Breast Radiation Therapy



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