Breast cancer is the most common cancer diagnosed worldwide in both sexes, combined and women [1].

Long-term toxicity of breast cancer radiotherapy includes cardiac and lung side effects. Breast oedema, radiation fibrosis, and brachial plexopathy can also be present, but longevity is affected little. Cardiac side effects such as myocardial infarction, ischaemic heart disease, and radiation-induced heart disease can impact the patient’s overall survival. Ironically, cancer treatment rather than cancer determines the patient’s survival in this scenario. So, attempts at reducing the dose received by the heart have been the point of interest of researchers in the past two decades. Various modalities were tried, and ABC is one of the promising tools in place.

Various studies have attempted optimal patient selection for ABC. Ryohei Yamauchi et al. [8] reported that a relative reduction of the mean heart dose correlated with the patient’s body mass index (BMI) DIBH was found to be more beneficial in the patients with low BMI. Soujanya Ferdinand et al. [9] correlated heart volume in field and maximum heart depth with decreased heart dose using ABC. The maximum heart depth was defined as the distance from the heart border to the edge of the field, while the heart volume in field was the volume of the heart within the 50% isodose line. In this study, a single parameter that predicted the smooth running of this programme is the ability of the patient to hold their breath. This depends on the ability of the patient to comprehend the instructions and, in turn, the practice. This was an observation echoed in the study by Eldredge Hindy et al. [10] Gabrielle Peters et al. [11] tried ABC for right breast cancer with regional lymph nodal irradiation. The lung (mean lung dose, V20, V5) and liver (Dmax, V20, V30) parameters decreased significantly with the use of ABC. But heart parameters did not improve significantly.

In this study, the dosimetric parameters of the heart, such as the mean dose, V5, V10, and V30, were compared between the ABC and FB plans. All the parameters were significantly lower in the ABC set of plans. Similar results were seen in the study by Beena Kunheri et al. [12], where parameters such as V5, V10, V15, V20, V25, V30, V35, V40, Dmean, and Dmax were compared. All the values were lower in the ABC group with statistical significance. In a 6-year data by Swanson et al. [13], there was a significant improvement in heart dose parameters with ABC treatment (V5, V10, V15, V20, V25, V30, MHD). A percentage reduction of the dose by 40% was observed. A systematic review by Lloyd M. Smith et al. [14] of the literature based on DIBH versus FB dosimetric comparison showed a percentage reduction in mean heart dose to the range of 38 to 67%. In this study, the reduction was to the tune of 29.8%.

The lung dose received by the patients treated with ABC was also significantly lower with respect to Dmean, V5, V10, and V20. These results concurred with the findings by Eldredge Hindy et al. [10]. Except for the maximum dose, other parameters of the left lung, such as mean dose, V5, V10, and V20, were found to be lower with ABC. The total lung parameters were also significant except for V5. In this study, V5 of the right lung was lower but not statistically significant even though the mean doses were lower and significant in the ABC group. The study by Ferrat Dincoglan et al. [15] showed significantly lower values for ipsilateral and lung parameters, such as V20 and Dmean. V5 was not reported here. On the contrary, V5, V10, V20, and mean doses of left lung and whole lungs were significantly lower in the study by Lin et al. [16].

In this study, the mean dose to the right breast was found to be significantly lower in the ABC group. But V5 and Dmax were comparable. In fact, very few studies have attempted to study the right breast dosimetry. Ferrat Dincoglan et al. [15] reported a reduction in both the maximum and the mean right breast doses.

There was a significant reduction in the mean dose as well as V30 of LADA. There was an absolute difference of 13.66 Gy in the mean dose received by LADA with ABC. As the tangential fields tend to include the left ventricle, LADA invariably receives the major brunt. Dosimetry of LADA was not routinely considered in earlier studies. The study by Beena Kunheri et al. [12] showed a 53% reduction of mean dose to LADA using ABC. Regardless of the kind of DIBH used, a systematic review by Smyth et al. [14] had shown a consensus of reduction in mean LADA dose to a range of 31 to 71%. In this study, a reduction of 29.28% was achieved.

The variation in results and findings of DIBH plans versus free-breathing plans may be attributed to the wide spectrum of eligibility criteria employed in various studies, the technique of radiotherapy attempted (IMRT vs 3DCRT), the technique of DIBH (RPM vs ABC), and the difference in breast volumes between races. But in most of the cases, it was evident that there is a clear benefit of using ABC-mDIBH in breast cancer, especially left sided for cardiac and ipsilateral lung sparing.

Clinical implications of the improvements in dosimetry regarding the cardiac and respiratory were not assessed as a part of the study as long-term follow-up is required. All subsets of left breast cancer patients were included in the study. These subsets include the patients who underwent BCS and MRM and early and locally advanced cases. It is challenging to know which subset of these patients received the maximum benefit from using ABC. The numerical advantage of dosimetry was seen more in MRM patients than in BCS. The study also did not explore the possibility of finding other factors that may predict better dosimetric outcomes, such as the mean heart distance, lung distance, and mean heart volume. 3DCRT was the technique that was used for all the plans. The impact of using IMRT, VMAT, or hybrid IMRT is areas that need further research.