Accelerated Partial Breast Irradiation Using External Beam Volumetric Modulated Arc Therapy (VMAT): a Randomised Non-inferiority Trial of 30 Gy Versus 26 Gy in Five Fractions Investigating Patient-reported Outcomes

For women undergoing radiotherapy following surgery for early-stage breast cancer, breast-related quality of life (BrQoL) is an important consideration. Treating only the part of the breast by radiation where the cancer has been surgically removed (partial breast irradiation or "PBI") rather than the whole breast (whole breast irradiation) can reduce the toxic effects of radiotherapy. This trial aims to evaluate whether there is a difference in patient-reported BrQoL between two total doses of radiation given in five treatments using PBI. If BrQOL for the higher dose of PBI is no worse than the lower dose, using the higher dose would be advised as best practice, given that it is more likely to be more effective in reducing the chance of cancer coming back in the breast than the lower dose.

Single-blind, phase III, multisite, randomised non-inferiority trial. PBI will be planned and treated as per the protocol using VMAT. The prescribed dose will be 30 Gy in 5 daily fractions (Arm A) or 26 Gy in 5 daily fractions (Arm B). Participants will be blinded to their treatment allocation.

Follow-up will be at the following time points:

* Eight weeks following the end of radiation
* Every six months post-randomisation for two years following randomisation
* Three years post-randomisation (final visit)

Assessments to be conducted at each time point are:

* Clinical assessment
* Completion of Patient-Reported Outcome Measures (PROMs)
* Mammogram and ultrasound (baseline and annually)
* Documentation of IBTR - classified as a true recurrence or Elsewhere
* Documentation of any other type of recurrence (regional, distant, opposite breast)

Recruitment is planned for three years with a three-year follow-up period for all patients.

Eligibility Criteria:
Inclusion Criteria:

* Aged greater than or equal to 50 years old
* Histologically confirmed Infiltrating ductal carcinoma (IDC) or pure DCIS, less than or equal to 20mm maximum size.
* Lobular carcinoma in situ (LCIS) is permitted.
* Histologic grade I or II
* Estrogen receptor (ER) +/- progesterone receptor (PR) positive in greater than or equal to 10% of cells and HER2 receptor-negative.
* Tumour bed identifiable on imaging via surgical clips
* Clear surgical margins
* Sentinel nodes negative (at least one node taken if invasive and no isolated tumour cells)
* No evidence of distant metastasis

Exclusion Criteria:

* Ink on surgical margins or positive histological margins
* Lymphatic vessel invasion (LVI)
* Bilateral breast cancer
* Invasive lobular carcinoma
* Pleomorphic LCIS
* Multifocal or multicentric invasive cancer
* Invasive carcinoma with associated DCIS greater than or equal to 30mm.
* Patients receiving neoadjuvant chemotherapy, anti-HER2 agents or endocrine therapy
* Patients receiving adjuvant chemotherapy or anti-HER2 agents.
* Previous Hodgkin's lymphoma requiring mantle radiation
* Prior radiation therapy to the ipsilateral breast
* Triple-negative breast cancer
* Documented mutation of BRCA1, BRCA2 or TP53, or at high genetic risk of breast cancer
* Known inflammatory conditions associated with higher complications after RT, such as active scleroderma, systemic lupus erythematosus (requiring steroids or immune suppressive therapy)
* Oncoplastic surgery where the primary tumour site is difficult to delineate
* No previous cancer (except BCC or SCC of the skin) unless in remission beyond five years of diagnosis
* People who are pregnant or planning to become pregnant
* People who are unable or unwilling to comply with protocol requirements.