Abstract P4-12-27: Dosimetric impact of axillary irradiation: Comparison between 3D-conformal radiotherapy and two types of intensity-modulated radiotherapy
Authors: Abbassi Louisa Marie , Arsène-Henry Alexandre , Kirova Youlia M 
Published: March 2020

The challenge with early breast cancer (EBC) management is finding balance between the most effective curative treatments with minimal long-term toxicity. De-escalade treatment modalities are emerging: sentinel lymph node biopsy instead of systematic axillary dissection, chemotherapy indications reduced with molecular signature, reduction of adjuvant trastuzumab duration, partial breast irradiation.

Several trials, such as ACOSOG Z011 and AMAROS trials, demonstrated safety and tolerability of axillary RT instead of surgical approach.

In this context, we aim to evaluate the dosimetric impact of axillary irradiation during adjuvant breast/chest wall radiotherapy (RT) for EBC and comparison between 3 techniques: three-dimensional conformal RT (3D), rotational intensity-modulated RT (IMRT) (VMAT) and helical IMRT with Tomotherapy® (HT).

We selected all consecutive patients treated with RT including lymph node in our institution between February 2017 and November 2018. All patients received either 50Gy in 25 fractions or 40Gy in 15 fractions on the breast/chest wall and lymph node regions (IV, III, II, interpectoral, internal mammary chain+/- I, according to ESTRO Guidelines). In case of conservative surgery, patients received tumor bed boost (16Gy in 8 fractions in both arm); simultaneous integrated boost (SIB) was allowed (51.52Gy on breast and 63Gy on boost in 28 fractions and 42.3Gy on breast and 52.2Gy on boost in 18 fractions respectively). Dosimetric plan were performed on Eclipse TPS (3D and VMAT) or Tomotherapy® TPS (HT). We compared patients with axilla level I irradiation (group A) with patient without (group B); in group B, axilla level I (CTV) was retrospectively delineated.

68 patients, all female, were included in our dosimetric analysis. All patients received 95% of the prescribed dose on breast/chest wall and lymph node regions PTV (except 2 patients with 3D in group A, on the axilla level I).

In B group, the axilla level I mean dose was superior with VMAT (44.7 and 36Gy) and HT (44.6 and 33.3Gy) compared with 3D (27.2 and 13.1Gy) with both treatment schedules respectively. This non-intentional axillary irradiation was heterogenous (table 1). Differences to organs at risk (OAR) doses between groups according treatment schedule are presented in table 2 (no comparison was calculated in 3D because only one patient in each schedules). HT was associated with additional doses to lungs and heart when the axilla level I was treated. This difference was not seen using VMAT.

Our dosimetric work shows in group B that axilla level I receive a non-intentional dose, especially with IMRT. Although this irradiation is heterogenous, it is a non-negligible dose which could potentially have deleterious effect considering the morbidity of axillary treatment(s) already received by the patients.

Willingly axillary irradiation (group A) is associated with increased doses to OAR with HT whereas it is not observed with VMAT. This suggest to wisely choose the IMRT technics according to patient medical history, especially cardiac and pulmonary morbidity.

Table 1: Axilla level I dose according radiotherapy technics, with normo- (left column) and hypo-fractionated (right column) schedule, in group B.3DTomotherapy®VMATV953.11.433.723.431.121.4V8011.14.440.029.440.230.4V5028.516.445.43446.538.3Dmean27.213.144.633.344.736V95: volume of axilla level I that receive at least 95% of prescribed dose.V80: volume of axilla level I that receive at least 80% of prescribed dose.V50: volume of axilla level I that receive at least 50% of prescribed dose.Dmean: mean dose in Gy received by axilla level I region.3D: three-dimensional conformal radiotherapy.VMAT: rotational intensity-modulated radiotherapy

Table 2: Additional dose to organs at risk according to radiotherapy technics, with normo- (left column) and hypo-fractionated (right column) schedules, in group A compared with group B.Tomotherapy®VMATHeart Dmean+2.1Gy+0.4Gy-2.2Gy-0.3GyHeart Dmax+11.7Gy+6.6Gy-10.6Gy-1GyIpsilateral lung V20-1.7%+3.2Gy-2.2%-3.5%Ipsilateral lung V30-1.1%+0.5%-1.7%-0.3%Controlateral lung V5+17.2%+1.1%-17.2%+4.2%Dmean: mean dose.Dmax: maximal dose.V20: lung volume receiving at least 20Gy.V30: lung volume receiving at least 30Gy.V5: lung volume receiving at least 5Gy.

Citation Format: Louisa Marie Abbassi, Alexandre Arsène-Henry, Youlia M Kirova. Dosimetric impact of axillary irradiation: Comparison between 3D-conformal radiotherapy and two types of intensity-modulated radiotherapy [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-12-27.