Abstract P3-12-03: Analysis of radiation dose in the gap region between the supraclavicular target volume to the internal mammary target volume in women receiving regional nodal irradiation
Authors: Bazan JG , Dicostanzo D , Healy E , Beyer S , White JR 
Published: May 2019

Abstract
Purpose: Consensus guidelines for regional nodal irradiation (RNI)/postmastectomy radiation (PMRT) clinical target volumes (CTV) have slight variations amongst leading national organizations. In the US, the Radiation Therapy Oncology Group (RTOG) defines the caudal edge of the supraclavicular (SCV) CTV as the junction of the brachiocephalic and axillary vessels while the internal mammary nodal (IMN) CTV starts at the superior aspect of the medial first rib. This leaves an anatomical gap between the two target volumes. The European Society of Radiation Oncology (ESTRO) does not recommend leaving a gap between the SCV CTV and IMN CTV. We set to analyze radiation dose and patterns of failure in this region.
Materials and Methods: We identified consecutive patients treated with RNI/PMRT at our institution from 2013-2016. Patients with metastatic or recurrent disease were excluded. All patients received 50 Gy/25 fractions to the breast/chestwall+regional nodes (including IMN PTV) +/- boost to the lumpectomy cavity/mastectomy scar using 3D conformal radiotherapy (3DCRT) or intensity modulated radiation therapy (IMRT). We retrospectively contoured the vessels from one slice below the caudal border of the SCV PTV contour to one slice cranial to the first IMN PTV contour. We calculated the mean dose and the relative V40Gy, V45Gy, and V47.5Gy of the gap region.A gap failure was defined as a first recurrence in this region with or without simultaneous loco-regional recurrence (LRR) or distant metastases (DM). We used the cumulative incidence method to calculate the gap recurrence rate with DM, LRR, and death, as competing risks.
Results: 230 patients were included with median age 52 years, predominantly stage III disease (60%), and most treated with preoperative (51%) or postoperative (41%) systemic therapy. Breast cancer subtype was ER+/HER2- in 138 patients, triple negative in 44 patients, and HER2+ in 48 patients. The median (IQR) mean dose, V40Gy, V45Gy, and V47.5 Gy in the gap region were: 20.3 Gy (14.8-26.2 Gy), 6% (1.3%-20.0%), 0.6% (0%-7.0%), and 0% (0%-1.3%). The mean dose to the gap region was slightly higher in patients treated with IMRT (N=68) compared to 3DCRT (N=162): 25.3 Gy (SD 7.5 Gy) vs. 19.5 Gy (SD 8.0 Gy), p&lt;0.0001. With median follow-up of 32 months, there were 2 recurrences in the gap region, both of which occurred with simultaneous distant metastases. No patients had isolated recurrences in the gap region. The 3-year cumulative incidence of recurrence in the gap region was 0.8%. The predominant pattern of failure was DM (N=31) with a 3-year rate of 14.4% followed by LRR (N=6, 4 with simultaneous distant metastases) with a 3-year rate of 3.1%.
Conclusion: In a clinical practice in which we routinely contour and treat the IMN PTV and SCV PTV with a gap region between those two volumes, we found that the mean radiation dose to this region is low, at about 50% or less compared to the prescription dose. Despite this, recurrences in this region are exceedingly uncommon and have not yet occurred in the absence of simultaneous DM. While the follow-up is limited, these data support the current guidelines of not routinely targeting this region.
Citation Format: Bazan JG, Dicostanzo D, Healy E, Beyer S, White JR. Analysis of radiation dose in the gap region between the supraclavicular target volume to the internal mammary target volume in women receiving regional nodal irradiation [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-12-03.