Cyclophosphamide is the name of a medicine given to prevent graft-versus-host-disease (GVHD) after half-matched transplant. This medicine is given on the 3rd and 4th day after stem cell transplant. The standard dose of this medicine is 50 mg per kg of the patient's weight given on the 3rd and the 4th day. However, using this medicine at this dose of 50 mg / kg for 2 days is associated with certain problems such as susceptibility to infections and delay in the recovery of the immune system after stem cell transplant. Therefore, several research groups across the world have tried to reduce the dose of cyclophosphamide that is used. These groups have tried reducing the dose from 50 mg per kg to 25-40 mg per kg. These studies have shown that the reduced dose cyclophosphamide is equally effective in preventing GVHD. However, these studies are carried out on small numbers of patients and further studies are essential to confirm whether reduced dose of cyclophosphamide is equally effective. In this study, we will use cyclophosphamide at a lower dose (25 mg/kg x 2 days) and see if the lower dose results in equal efficacy but lesser toxicities This is a single-arm study. All participants will receive the same treatment; there is no comparison group.

Participants will:

Adults (age ≥18) undergoing haploidentical stem cell transplant for blood cancers Receive low-dose cyclophosphamide (25 mg/kg/day) on Day 3 and Day 4 after transplant Also receive standard GVHD preventive medicines (calcineurin inhibitor and mycophenolate) Undergo regular blood tests, immune system monitoring, and GVHD assessments Have immune cell and cytokine profiles analyzed through blood samples