A single-arm pilot study, to assess the efficacy of cerebellar IMRT combined with cerebral SRS in patients with brain metastases that are predominantly in the posterior fossa - a novel treatment approach

This study includes patients with brain metastases, presenting with more than 5 cerebellar lesions and less than 10 cerebral lesions. This is a pilot study assessing the efficacy of a novel, combined treatment approach involving Intensity-modulated Radiation Therapy (IMRT)/ Volumetric Modulated Arc Therapy (VMAT), with or without integrated boost to metastases, administered to the posterior fossa, and Stereotactic Radiosurgery (SRS) administered to cerebral lesions. This is opposed to the current recommended treatment approach for such cases, being whole brain radiotherapy (WBRT). While an effective treatment with respect to lesion control, WBRT is not ideal as the total deliverable dose is limited to 30 Gy due to toxicity. In addition, this technique is responsible for major side effects, especially neurocognitive deterioration.

There are select cases of multiple brain metastases which are predominantly found in the posterior fossa. Rectal cancer is one such notorious example, however this type of spread is not limited to one region.

The investigators hypothesize that utilizing the novel combinational treatment approach of IMRT and SRS in cerebral and cerebellar brain metastases will improve long term brain control, maintain the patients' cognitive function and potentially improve overall survival, as the need for potent WBRT will become obsolete.

Patients with multiple brain metastases present in the cerebrum and cerebellum will be treated with SRS and IMRT respectively, simultaneously (within 3 weeks of one another). Brain MRI scans will be performed before commencement of the novel treatment approach, at two month after RT, and then every 3 months, or as indicated clinically, after treatment start. Concurrently at each MRI scan time point, patients will be assessed based on brain and whole-body metastatic progression by RECIST. Patients will also be assessed for central nervous system (CNS) - progression free survival (PFS) and body-PFS, cognitive function, quality of life and overall survival status via standardized follow-up tests.

Inclusion criteria

1. Presence of multiple brain metastasis, predominantly in the posterior fossa: more than 5 metastases in the cerebellum, and less than 10 metastases in the cerebrum, visible on MRI, regardless of tumor origin.

2. Provided written informed consent.

3. Be male or female and at least 18 years of age on the day of signing informed consent.

4. Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2.

5. A minimum life expectancy of at least 3 months

6. Female patients:

   1. Willing to use adequate contraceptive measures until 6 weeks after the final dose of study treatment
   2. Not breast feeding
   3. Have a negative pregnancy test prior to the start of dosing if of childbearing potential or have evidence of non-childbearing potential by fulfilling one of the following criteria at screening:
   4. i. Post-menopausal defined as aged more than 50 years and amenorrheic for at least 12 months following cessation of all exogenous hormonal treatments ii. Women under 50 years old would be considered postmenopausal if they have been amenorrheic for 12 months or more following cessation of exogenous hormonal treatments and with Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH) levels in the post-menopausal range for the institution iii. Documentation of irreversible surgical sterilization by hysterectomy, bilateral oophorectomy or bilateral salpingectomy but not tubal ligation

7. Male patients who are willing to use barrier contraception (i.e. condoms) until 4 months after the final dose of study treatment.

Exclusion criteria

1. Prior treatment with Whole brain radiation (WBRT) (previous SRS is allowed for limited, up to 4 metastases, six month or more prior to the study treatment, and the index metastases should be all new)

2. An investigational drug within five half-lives of the compound.

3. Spinal cord compression unless asymptomatic and stable.

4. Leptomeningeal disease.

5. Moderate or severe symptomatic brain metastases defined as per Radiation therapy Oncology Group acute morbidity grade 3 to 4.

   Note: Grade 3 refers to neurological findings requiring hospitalization for initial management. Grade 4 refers to serious neurological impairment including paralysis, coma or seizures more than three times per week despite medication and requires hospitalization.

6. Any evidence of severe or uncontrolled systemic diseases, including uncontrolled hypertension and active bleeding diatheses, which in the investigator's opinion makes it undesirable for the patient to participate in the trial or which would jeopardize compliance with the protocol, or active infection including hepatitis B, hepatitis C and human immunodeficiency virus (HIV). Screening for chronic conditions is not required.

7. Involvement in the planning and conduct of the study

8. Judgement by the investigator that the patient should not participate in the study if the patient is unlikely to comply with study procedures, restrictions and requirements.