Personalization of Breast Radiotherapy According to Loco-regional Recurrence Risk and Toxicity Probability

Breast cancer is the most common cancer in women in the world and remains a major public health burden with 25% of all cancer cases and 15% of all cancer deaths among females. The Incidence has increased with the introduction of mammography screening and continues to rise, mainly due to population aging; meanwhile, breast cancer survival has significantly improved over the past decades.

Adjuvant radiotherapy (RT) is an essential component of the treatment. After breast surgery for invasive carcinoma, RT is commonly used and delivered without considering the different tumor subtypes, unless the node involvement risk, because it decreases the rate of local recurrence and by this way, specific mortality. The " one size fits all " approach is widely applied, with two main options: breast or chest wall only radiotherapy or breast or chest wall plus nodes radiotherapy. Rare are the centers that discuss IMRT use, external partial breast irradiation, adaptive breast necessity, etc….

1. Breast cancer and loco-regional recurrence risk Ten-year cancer specific survival exceeds 70%, with 89% survival for local and 62% for regional disease. The risk of recurrence is high during the first two years after the initial diagnosis for patients with hormonal receptor (HR)-negative breast cancer, but rapidly decreases below the recurrence risk of HR-positive tumors. Loco-regional recurrence (LRR) occurs in 5-15% of cases treated with breast conservative surgery (BCS) plus adjuvant radiotherapy (RT) or mastectomy and is considered an independent poor prognostic factor6. The management of LRR requires a multidisciplinary approach. Total mastectomy is the standard of care for isolated LRR after BCS. However, secondary BCS ± RT is an alternative approach that could be discussed case by case. Parameters to predict the risk of recurrence are those included in the NHS UK updated PREDICT score such as age, node status, tumor grade, proliferation index, Her2 and hormone receptor expression.
2. Adjuvant breast radiotherapy and subcutaneous toxicity probability Severe but also moderate toxicities after curative-intent radiotherapy (RT), such as fibrosis, retraction or telangiectasia with poor cosmetic outcome can have a negative impact on quality of life following breast cancer and a marked effect on subsequent psychological outcome. Multiple factors are known to increase the risk of radiation toxicity including individual radiosensitivity9. While the toxicity risks for patients are well-established, determining an individual's normal tissue radiosensitivity is rarely possible before treatment. Nevertheless, current practice standards often prescribe radiation dose and volume without regard to individual radiosensitivity. In that context, a normal tissue radiosensitivity test that includes a rapid (72h) radiosensitivity assay based on flow cytometric assessment of radiation-induced CD8 (cytotoxic T cells 8) T-lymphocyte apoptosis (RILA) was developed by Ozsahin et al. A prospective study was conducted to determine whether the RILA assay could help identifying patients at high risk of severe toxicities. The RILA assay was performed in 399 patients with different cancer types before RT. Findings confirmed that patients who developed severe toxicities displayed a compromised apoptotic response. More recently, the RILA assay was validated in a prospective multicenter trial with more than 500 patients with breast cancer. The negative predictive value for grade ≥2 breast fibrosis was 91% for RILA scores ≥20% and the positive predictive value was 22% for RILA scores <12% (the overall prevalence of grade ≥2 breast fibrosis was estimated at 14%). For the first time, RILA as a continuous variable was significantly associated with toxicities. Recently, the French results have been validated by a prospective European trial with similar predictive values of the RILA. In the CO-HO-RT trial, the RILA assay was used as the main stratification factor and no late toxicity occurred in patients with a RILA score >16%. In 2022, the RILA assay was included in the French recommendation (RECORAD) as the only validated predictive test for post-radiotherapy toxicity in a large, multi-center, prospective study. Based on these data, the NovaGray RILA Breast® test has been developed and combine both a biological analysis (radio-induced lymphocyte apoptosis) and an algorithmic analysis. The NovaGray RILA Breast is an vitro diagnostic medical device (IVMD) pertaining CE-mark. This prognostic test that assesses the risk of developing grade 2+ breast fibrosis at 36 months following radiotherapy. The test is performed on a blood draw and results are provided within a week, which does not delay the beginning of the radiotherapy.
3. Matrix approach according to tumor recurrence and toxicity risk When using a predictive radiosensitivity test in clinical practice, an alternative treatment must be available to the patient in case of an unfavourable result, i.e. an adjustment in the radiotherapy protocol (change in fractionation or total dose), the addition or elimination of a concomitant treatment (e.g. chemotherapy) or the absolute contraindication to radiotherapy in highly radiosensitive patients for whom the treatment-related risk may be greater than the expected benefit.

Based on tumor control probability (TCP) and normal tissue complication probability (NTCP) we can group the patients into four situations described below.

1. Clinical situation #1 : High TCP(Tissue Complication Probability) / Low NTCP (Normal Tissue Complication Probability) : COHORT A This circumstance is the optimum situation: a high local control probability and a low risk of developing toxicity. In this situation, the total dose of radiotherapy could be limited to the strict necessity since the probability of tumor control is already high. However, alternative fractionations such as moderate hypofractionated can be proposed to shorten the duration of treatment. In breast cancer, a regimen delivering 42.5 Gy in 16 fractions or 40 Gy in 15 fractions can be offered according to the Canadian or UK (united kingdom) regimen,. Recently, hypofractionated regimen are becoming more extreme with shorten fractionation regimen over 5 weeks or over 1 week . From a radiobiological and economic standpoint, these short regimens can provide a benefit in terms of quality of life with a lower cost of treatment. All of them are now included in the recommendations of the French National Society of Radiation Oncology and the French National Cancer Institute (INCa, Cancers du sein - Recommendations et outils d'aide à la pratique (e-cancer.fr)). All are considered as an option in daily clinical practice.
2. Clinical situation #2 : High TCP / High NTCP : COHORT B In this situation, the patient is at risk of developing toxicity, but the probability of tumor control is high). In those cases, possible alternatives to radiotherapy may be discussed such as surgical treatment alone. In breast cancer, this can mean offering total mastectomy for a small tumor (possibly with immediate breast reconstruction) to postpone the indication of adjuvant radiotherapy. For a tumor with a very favorable prognosis, namely after 65 years old, avoiding radiotherapy after breast surgery may also be an option. In that case, treatment with partial irradiation of the breast (using external technique or brachytherapy) can also be proposed, to improve the long-term cosmetic results, while ensuring good local control. All of them are now included in the recommendations of the French National Society of Radiation Oncology. All are considered as an option in daily clinical practice.
3. Clinical situation #3 : Low TCP / Low NTCP : COHORT C In that situation, the patient is at low risk of developing toxicity but the probability of tumor control is low. One of the possibilities is to increase the total dose of radiotherapy, namely with a localized boost within the tumor bed in invasive disease 5 and nodes irradiation according to the HypoG trial. This treatment is also included in the recommendations of the French National Society of Radiation Oncology.
4. Clinical situation #4 : Low TCP / High NTCP : COHORT D This is the most unfavourable situation, with a radiation-resistant tumor and a patient with a high risk of developing toxicity. Dose de-escalation cannot be proposed to decrease toxicity in such cases, as the risk of tumor recurrence may be too high. Alternative fractionations, such as hyperfractionation (moderate or high depending on the organs at risk) could however be useful by decreasing the risk of toxicity while retaining a satisfactory level of tumor control. These regimens have not been validated and discussions should be carried out on a case-by-case basis. However, the impact on the decrease of toxicity, especially late-onset toxicity, has yet to be demonstrated. If adjusted radiotherapy regimens are not feasible and standard oncological treatment administered, the radioprotection of normal tissue should be discussed using adaptive techniques as it could decrease the risk of toxicity while maintaining good tumor control.

In this specific COHORT, inclusion in a new clinical trial evaluating adaptive treatment will be proposed to the patient. In case of patient refusal or technique unavailable, a standard treatment available in the center for this indication will be delivered.