With the ageing population and screening programs adopted worldwide, both the incidence and prevalence of breast cancer (BC) are projected to increase over the next decades. Since radiotherapy is one of the key modalities in BC treatment, the absolute number of new BC patients in need of external beam radiotherapy is expected to increase in the immediate future in nearly all European countries.1
Breast conserving surgery in combination with adjuvant radiotherapy has become the standard of care in BC management.2,3 Large retrospective population-based studies nowadays show that breast conserving therapy (BCT) may have an even better outcome in terms of BC-specific and overall survival compared to mastectomy.4 BCT, when compared to more radical surgery, has a positive impact on the patient’s quality of life many years after treatment, especially in terms of body image, sexual activity, and better physical and role functioning.5
Clinicians and researches alike are paying particular attention to reducing acute and late treatment toxicities in a growing number of BC survivors.6 Acute skin toxicity is very common and ranges from mild erythema to moist or dry skin desquamation, with the peak reaction occurring one to two weeks post treatment.7,8 Late skin reactions include skin fibrosis, skin dyspigmentation, and telangiectasia. Acute heart or lung toxicities are rarely seen in BC adjuvant radiation treatment, but late sequelae may be life threatening, with acute coronary event and lung cancer being two possible complications.9
Advances in BC radiotherapy – among them being moderate hypofractionation schedule (HF), intensity modulated radiotherapy (IMRT), and prone or lateral decubitus position – have all the potential to reduce the rates of acute and long-term radiotherapy-related side effects of BCT.6,7,10,11,12,13,14 The observed normal tissue toxicity rates and breast cosmetic outcome depend on treatment and patient-related factors such as the type and number of surgical procedures, systemic treatment, breast size and shape, race, age, comorbidities, smoking, individual sensitivity to ionizing radiation, choice of fractionation and radiation dose, skin bolus, inter-fraction time interval, volume irradiated, and radiotherapy delivery modality.15 It is a widely accepted fact that patient-related factors, such as higher body mass index (BMI) and larger breasts7,10,16 increase the risk of ≥ grade 2 (G2) dermatitis, regardless of the fractionation regimen.11,17
We conducted a review to summarise the clinical toxicity profile of adjuvant radiotherapy in women with breasts of various sizes, and to assess the dosimetric studies of different treatment planning techniques which compared the target coverage (also related to breast size) and dose to thoracic organs at risk with a focus to cardiac subvolumes.
A comprehensive literature search for clinical and dosimetric findings was carried out using PubMed/ Medline from January 1990, with 30 September 2017 being the last search date. Only English literature was considered, using the following key words: “breast cancer” and “radiotherapy”. Subheadings were searched with “organ size”, “3D-conformal radiotherapy (3D-CRT)”, “intensity modulated radiotherapy (IMRT)”, “hybrid-IMRT”, and “volumetric-modulated arc therapy (VMAT)”, “organs at risk”, “treatment planning”, “Heart/radiation effects”, “Coronary Vessels/radiation effects”, and “dosimetric comparison”. Additional relevant references were found in reference lists published with the articles. Clinical studies were selected independently of the number of the patients included. We also searched for treatment planning studies with at least 2 different treatment modalities (
All selected articles were reviewed in full-text versions and were further divided into clinical or treatment planning articles. In clinical studies, we searched for acute skin toxicity, heart and lung toxicity, secondary malignancy risk, and for possible strategies to modify the toxicity, again taking into account the different breast size categories. Treatment planning studies were reviewed in detail and selected only if the delineated organs at risk included at least one additional heart substructure, namely coronary arteries or cardiac chambers.
The aim of the literature search was to select all clinical and treatment planning studies of adjuvant breast radiotherapy, taking into account the different breast size categories. The search retrieved 6074 articles, 5980 of which were excluded from the review because the content of the article did not match the search criteria, the content was irrelevant to the review topic, or the records were duplicated. Ultimately, 94 articles were found relevant to this study.
Clinical studies do not define different breast sizes uniformly. Some of the studies differentiate between breast volumes using measures such as clothing and bra size, where a cup size ≥ D categorises woman as having large breasts.7,18 In a study by Pignol
Randomised clinical trials and retrospective clinical data from standard tangential two-dimensional radiotherapy with wedges (2D-RT)
In 2008, Pignol
Six other clinical studies reported a comparison of the clinically adverse events in regard to the three groups of breast sizes. Four compared 2D-RT
Selected studies evaluating IMRT versus 2D-RT or 3D-CRT. Patients were further stratified by small, medium or large-sized breasts. 2D-RT = standard 2D wedged plan; CTCAE v. 3.0 = common terminology criteria for adverse events for acute radiation dermatitis, version 3.0; CF = conventional fractionation; HF = hypofractionation; DIBH = deep inspiration breath hold; F-IMRT = forward planned intensity modulated radiotherapy; H-IMRT = hybrid intensity modulated radiotherapy; NCI CTC v. 2.0 = National Cancer Institute common toxicity criteria; version 2.0; RTOG = Radiation Therapy Oncology Group criteriaStudy Number of patients Type of study Technique Total dose and Fractionation CF/HF Breast size (median breast volume)cm3 Scoring system G1 or G2 (%) (whole group) G2 or G3 (%) (whole group) G3 or G4 (%) (whole group) General comments Freedman 200627 131 Case-control retrospective study F-IMRT 2D-RT 46–50 Gy in 23–25 fractions + boost 10–16 Gy Breast size was grouped as small (34 A, B; 36 A), medium (34 C; 36 B, C; 38A, B, C), or large (any D or size ≥ 40) CTCAE v. 3.0 30 (IMRT) 28 (2D-RT) 70 (IMRT) 72 (2D-RT) 0 (IMRT) 0 (2D-RT) IMRT is associated with a decrease in severity of acute desquamation compared with a matched control group treated with conventional radiation therapy. Harsolia, 200725 172 Retrospective study F-IMRT 2D-RT CF median dose 45 Gy + 16 Gy boost 1.326 (IMRT) 1.489 (2D-RT) Breast volume divided into groups: 1.000 cm3 (small), 1.000–1.599 cm3 (medium), 1.600 cm3 (large) NCI CTC v. 2.0 41 (IMRT) 85 (2D-RT) 1 (IMRT) 6 (2D-RT) Lower rates of ≥ G2 toxicity with IMRT regardless of breast size. Freedman 200910 804 Retrospective study F-IMRT 2D-RT 46–50 Gy in 23–25 fractions + boost 10–18 Gy Bra size, (at least 63% with small and medium sizes) CTCAE v. 3.0 52 (IMRT) 75 (2D-RT) More large-breasted patients in IMRT group. Shah 201223 335 Prospective study IMRT 2D-RT IMRT: HF -inversely planned IMRT CF - forward planned IMRT Median dose 45 Gy + boost 16 Gy or 42.56 Gy without a boost 1.378 for the whole group CTCAE v. 3.0 1 (CF-IMRT) 23 (HF-IMRT) 12 (2D-RT) IMRT is associated with reduced toxicities compared with 2D radiotherapy. Breast volume divided into groups: 1.000 cm3 (small), 1.000-1.599 cm3 (medium), 1.600 cm3 (large) In large-breasted patients, CF-IMRT was associated with reduced acute toxicities, while HF-IMRT was not. Hardee 201228 97 Prospective study H-IMRT 3D-CRT IMRT: hybrid IMRT using a mixture of 3D tangent fields and dynamic multileaf collimator (MLC) IMRT fields in a 2:1 ratio 46 Gy in 23 fractions + 14-Gy boost or 42.72 Gy in 16 fractions; all in prone position Breast size was classified as small (A cup, <750 cm3), medium (B-C cups, 750–1.499 cm3), and large (D cup or larger,≥ 1,500 cm3) RTOG 5.1% ≥ G2 Hypofractionated breast radiotherapy is well tolerated when treating patients in the prone position, even among those with large breast volumes. Breast IMRT significantly improves dosimetry but yields only a modest but confirmed benefit in terms of toxicities. De Langhe 201432 377 Prospective study Prone or supine position with INV-IMRT or prone with F-IMRT or prone position with DIBH (n = 22) or supine F-IMRT ± DIBH 40.05 Gy in 15 fractions + boost 10 Gy in 4 fractions (90–75% of patients) or 50 Gy in 25 fractions for 65% of patients with bra cup size ≥ D Breast size was classified A, B, C and ≥ D cup CTCAE v. 3.0 57.3 (≥ G2) CF, supine IMRT, concomitant hormone treatment, high BMI, large breast, smoking during treatment, and genetic variation (in MLH1 rs1800734): all were associated with ≥ G2 toxicity.
Harsolia
Increased rates of acute dermatitis, acute and chronic oedema, and chronic hyperpigmentation, irrespective of the treatment technique (prone
Acute skin toxicity, especially moist desquamation, is associated with late complications of radiotherapy, namely telangiectasia and late subcutaneous fibrosis, as shown in a ten-year update of a Canadian breast IMRT trial and other studies.31,33 Five-year results of simple IMRT (F-IMRT) support the use of BC adjuvant radiotherapy technique that improves homogeneity: the benefit of IMRT was confirmed in a multivariate analysis for both overall cosmesis (p = 0.038) and skin telangiectasia (p = 0.031), although there was no difference in breast shrinkage, breast oedema, tumour bed induration, or pigmentation.24
Moderate HF schedule is an established adjuvant treatment in lymph node-negative BC after breast conserving surgery with no differences in disease-related outcomes and with a favourable toxicity profile.34,35,36,37 Moreover, shorter treatment schedules are a cost-effective approach for both the patient and healthcare providers.38 The advantages of a hypofractionated schedule over conventional fractionation,
A higher daily fraction size (> 1.8–2.0 Gy) and hot spots (> V105 %) may contribute to so-called ‘triple trouble’ or an unequal distribution of the biological effective dose (BED), although the risk is probably insignificant.39,40 To avoid any of the possible complications (greater fibrosis or late normal tissue effects) with HF, it is generally recommended to limit the volume of hot spots and not to exceed 107 % of the prescribed dose.40 Some authors suggest that patients with a large breast size that precludes achieving the maximum dose of > 107 % should be offered a dose/fractionation that is biologically less intense, for example 45 Gy in 25 daily fractions at 1.8 Gy daily with an addition of a boost dose.41
Similarly as with the CF schedule, high BMI, an increasing PTV volume with a cut off value as small as 500 cm332,35,42, and excessive radiation dose in the target volume (
The dose-volume predictors for acute and late radiation-induced toxicities are established for the lung and heart as a whole structure.43,44,45 Recent studies have evaluated the dose to the whole heart and the proportional increase in cardiac events after BC radiotherapy. An estimated linear increase of 7.4 % and 4.1 % was found per every Gy mean dose to the whole heart for major coronary events and cardiac mortality, respectively.9,45 A systematic literature review on modern radiation doses to heart and lung in BC radiotherapy showed an estimated absolute 30-year risk for cardiac mortality of 1 % for smokers and 0.3 % for non-smokers.9 Patient-related factors (age and smoking), systemic therapy, the fractionation schedule (total dose and daily fraction), and dose-volume parameters of radiation treatment plan such as mean dose to the whole lung and V20, all constitute risk factors for pneumonitis and lung fibrosis.15,43
In a recent retrospective clinical study of 4688 WBRT-treated BC patients, it was reported that larger breast separation (> 22 cm) was one of the factors significantly increasing the mean heart dose (MHD) for CF by 1.5 % per 1 cm and in HF by 1.7 % per every 1 cm increase, respectively.46 It has been demonstrated that the dose to the heart can be significantly reduced in both CF and HF by means of breathing adaptation and prone positioning.13,46
Hannan
In BC radiotherapy, differences in body habitus may influence doses to organs at risk, but it is not known if small differences in radiation exposure at the time of the first radiation course significantly influence the risk of a secondary non-breast cancer. Compared to the general population, BC patients have an increased risk of secondary non-breast cancers, five or more years after BC diagnosis with and without radiation therapy (RR 1.12; 95 % confidence interval [CI] 1.06–1.19).49 But probably less than 3.5 % of secondary malignancies in BC survivors are attributable to radiation therapy.50 The total dose of radiation, premenopausal age (< 40–45 years) and the irradiated volume of normal tissue all increase the risk for secondary lung, oesophageal, or thyroid cancer, and secondary sarcomas.49,51 The risk of lung cancer increases with the mean dose to the whole lung.52 A systematic review of modern radiation doses to the lung in BC radiotherapy showed an estimated absolute 30-year risk for lung cancer of 4 % for long-term continuing smokers and 0.3 % for non-smokers.9
The radiation dose to the lung increases with lymph node irradiation and the use of IMRT techniques, and decreases with breathing adaptations and prone/lateral decubitus positioning.52 Younger women (< 40 years of age) with an absorbed radiation dose > 1.0 Gy to the contralateral breast have an elevated long-term risk of developing a second primary contralateral BC.52
A study by Bhatnagar
Different strategies in BC radiotherapy exist to lower the dose to organs at risk. Approaches could be further divided according to patient or breast positioning modification, breathing adaptation, and treatment volume reduction.
The prone positioning setup demonstrated to be an excellent strategy to spare the ipsilateral lung in 100 % and heart dose in 85–87 % of the patients, independently of their BMI or breast size12,55,56,57,58, but a particular benefit was observed in large-breasted women (CTV > 1000 cm3).12,13,57 Similar findings were confirmed in a study by Formenti
3D-CRT lateral isocentric decubitus position was recently described as a treatment planning solution. Long-term toxicity results were published by the Institut Curie group.14,60 Women with a median BMI of 26.3 were treated with different types of fractionation. Acute dermatitis of any grade was present in 93 % of the patients and G3 dermatitis in only 2.8 %. In a 1-year follow-up, 94.1 % of cases had no skin reaction, making this technique feasible with excellent toxicity rates, but the results need to be confirmed with a longer follow-up. In a multivariate analysis, the cup size and the fractionation had a significant influence on acute dermatitis.14
The deep inspiration breath-hold (DIBH) technique helps to minimise the “trade-off” between the target and OAR, a compromise often required, and is less resource-intensive than the IMRT technique. It reduces the low-dose irradiation to the heart, left anterior descending coronary artery (LADCA), and lung, ultimately benefiting women of all breast sizes.61 DIBH can be accurately clinically implemented with an acceptable reproducibility and stability in both supine and prone position.61,62 In a group of women with a volume of the treated breast > 750 cm3, supine voluntary DIBH enabled a cardiac sparing and reproducibility superior to that of free-breathing prone position.63
Another strategy to lower the absorbed radiation dose to the heart is partial breast irradiation. Patients may benefit in terms of lower mean whole heart doses with moderate HF using 3D-CRT and the accelerated partial breast irradiation technique (APBI) with an external beam or interstitial brahcytherapy.64,65,66,67 Meszaros
A thermoplastic bra which helps to raise the lateral breast border is also an option to lower the dose to thoracic organs in BC radiotherapy. Piroth
Many different treatment planning options are available in the modern treatment era: 3D-CRT with or without wedged filters, forward-planned IMRT (F-IMRT), inverse-planned IMRT (INV-IMRT), Helical Tomotherapy (HT), VMAT, and hybrid techniques (H-IMRT). The recommended first choice for WBRT varies across numerous treatment planning studies comparing different modalities (
There are numerous publications comparing the dosimetric parameters of radiotherapy plans, mostly for patients with left-sided early BC. Some of them have been summarised in a review of treatment planning options by Balaji
For the purpose of this review, we have evaluated the treatment planning studies which included at least coronary arteries or cardiac chambers as organs at risk. Overall, we have found eight treatment planning studies comparing radiotherapy plans in free-breathing CTs.54,73,74,75,76,77,78,79 The studies indicate an improved dose homogeneity with the IMRT54,73,75,78,79 or VMAT73,74 techniques regardless of the PTV volume, but the number of CT study sets compared was relatively low (10–20). The sizes of the target volumes reported by investigators comparing treatment planning approaches were dissimilar, ranging from 296 cm3 (mean value) to 1160 cm3 (median value).74,79 A study by Tan
Besides the heart as a whole structure, authors typically delineated LADCA73,74,76,77,78, LV75,76,78,79, right ventricle (RV)76,78, left atrium (LA)76,78, right atrium (RA)76,78, great vessels78, and AMT.54,74,75 The delineation of heart substructures was not uniformly defined or was rarely guided by written instructions, making it difficult to compare the presented studies.54,74,75
Growing clinical data on BC adjuvant radiotherapy suggest that a smaller PTV and/or the use of the IMRT technique may be associated with a decreased rates of acute breast toxicity. Most studies evaluated women with small or medium-sized breasts, so maybe all of the results are not directly applicable in large-breasted woman. The clinical studies which reported a comparison of the clinical adverse events in regard to the three categories of breast sizes mostly used the CTCAE v.3.0 scoring system.7,10,23,25,27,28 The rates of ≥ G2 toxicity for the whole group of patients from selected studies used in Figure 1 ranged from 5.1 % to 70 %. The percentage of patients with ≥ G2 toxicity was the highest in the subcategory of large-breasted patients in all the studies, ranging from 7.4 % to 86.2 %.10,23,27,32 The differences in adverse event reporting could at least partly be attributed to different scoring systems,
Moderately hypofractionated schedules proved superiority over CF and conformal radiation therapy (3D-CRT or F-IMRT) over 2D-RT in terms of acute and late adverse effects in early BC WBRT.7,12,22,24,26,34,37 Clinical reports are confirming the long-term safety and feasibility of moderately hypofractionated schedules also in women with large breasts.11,17,32,36,38,42,80,81 Most of the investigators attributed higher toxicity rates in BC radiotherapy to dose inhomogeneity and a higher percentage of hot spots, irrespective of breast volume.7,12,17,31 V105–107 % of the prescribed dose (PD) was significantly related to increased desquamation, dermatitis, oedema, and pain12; and V105 % PD82,83 or V110 % PD83 to long-term breast pain. Significant reductions in hot spots can be achieved with 3D-CRT or F-IMRT treatment plans, also in patients with large/pendulous breasts (PTV >1000 cm3).20,26,28,84 An improvement in dose homogeneity was achieved with IMRT, and correlated with less acute toxicity rates in a study by Mulliez
However, some of the studies show that large breast volume seems to be a risk factor for acute or late adverse events independently of dose inhomogeneity and regardless of the conformal radiotherapy technique (3D-CRT
The data published support the hypothesis that every Gy of increase in the whole lung mean dose increases the risk for second lung cancer.9 In the long run, as far as the sub-population of continuing smokers is concerned, the second lung cancer risk is even greater and the benefits of adjuvant radiotherapy in early BC may be reduced to the point where long-term risks outweigh the benefits of adjuvant radiotherapy.9 At the same time smokers, portend lifelong cardiac mortality risks and smoking during BC radiotherapy significantly increase the risk of acute ≥ G2 dermatitis.32,52 Smoking cessation counselling may be provided to modify acute and late toxicity risks. Breathing-adapted radiotherapy in the prone or supine position in women with all breast sizes, and prone or lateral setup in medium or large-breasted patients (approximately ≥ 1000 cm3) have been shown to decrease the whole lung and heart dose parameters.12,13,52,57,88
In terms of the heart as an organ at risk, ideally, all treatment planning comparisons of WBRT in patients with left-sided BC should be done in DIBH. Treatment planning or retrospective dose evaluation studies often only include the heart as a whole structure, without separately delineated heart subvolumes, although the dose distribution in the heart itself in BC adjuvant radiotherapy is usually not homogeneous.89 In some patients, the dose to the LADCA can be significantly higher in the prone (without breath-hold) compared to the supine setup, which could also be attributed to the differences in contouring and treatment techniques.59 The routine use of delineation guidelines for thoracic organs at risk and dose reporting with clinical correlation could help us further understand normal tissue complication probability models, especially in the least known dose-response relationships,
Treatment planning studies usually compare a limited number of CT study sets and it is not known if small improvement in dosimetric metrics would translate into clinically meaningful lower rates of adverse events for the larger population. For example, one treatment planning study in the modern treatment era reported very little difference in dosimetric parameters between patients of different breast size regardless of the modality (static HT, IMRT, and 3D-CRT).21 Expected absolute differences in the rates of clinical adverse events (3D-CRT
In order to de-escalate radiotherapy, selected patients will be treated routinely with the moderate HF or accelerated partial breast irradiation techniques (treating only the tumour bed with a safety margin) in the near future, as emerging data confirm a similar 5-year cumulative incidence of loco-regional and distant relapse compared to WBRT.38,92,93 In a 5-year assessment, patients treated with partial radiotherapy approaches self-reported less moderate or marked skin (p = 0.051) or overall breast appearance change (p < 0.0001) compared to the WBRT group.88 APBI using interstitial brachytherapy was able to significantly reduce acute G3 skin toxicity (7 % in WBRT group
One of the important individual factors affecting the final cosmetic outcome of radiation therapy treatment is the size of the treated PTV. It seems that beside the target volume, inhomogeneity, and a higher percentage of the excessive radiation dose, bigger breast size itself is an independent risk factor for acute adverse effects regardless of the fractionation regimen or dose inhomogeneity, although the lower the excessive radiation dose, the lower the risk of ≥ G2 toxicity. While weighing the risk of BC relapse