Between November 2017 and November 2021, 32 consecutive patients who received TBI and underwent HSCT were included in the study. HSCT was started in the Civil Service Hospital of Nepal in 2016. However, TBI was not used for conditioning due to a lack of radiotherapy facilities. In 2017, to start a TBI-based conditioning regimen, a proposal was made to do TBI at the Kathmandu Cancer Center, which is located in a suburb of Kathmandu Valley at a distance of 15 km from the Civil Service Hospital.

The main challenge of this program was to use resources from two different institutions. To coordinate treatment plans and patient transfers, a group was created using instant messaging software (Viber) that included hematologists, residents, transplant nurses, radiation oncologists, physicists, and treating technicians. A focal person was identified at both the hematology center and the radiotherapy center. Once the patient was scheduled for HSCT, s/he visited the radiation center for registration and any necessary dosimetry assessments. The patient was then admitted to the hematology center and started conditioning chemotherapy. Once a patient was started on chemotherapy, it was communicated through a Viber group, and the focal person was informed about the scheduled day of TBI.

Post-TBI, stem cell therapy with peripheral blood stem cells was administered the following day. High-dose cyclophosphamide was used as GVHD prophylaxis after transplantation.

Patients were called to visit the radiotherapy department to talk and be trained on how to pose during treatment. The patients’ information, such as name, sex, age, relevant medical diagnosis, and treatment schedule, was collected. Anthropometric measurements, including lateral separation (shoulder-to-shoulder distance), height, and weight of the patients were used to calculate the radiation dose.

In order to avoid office-hour traffic during patient transport, TBI was taken as the first case of the morning. Since registration was completed during the previous visits, the patient was immediately transferred from the ambulance to the radiotherapy vault after confirming their identity. For hygienic precautions, the radiotherapy vault was fumigated the night before treatment. All involved personnel used masks and gloves, and the treatment couch was disinfected before treatment. Standard hospital hygiene measures, including hand disinfection, were strictly followed.

Patients were positioned in a supine position at an extended source-to-skin distance (SSD) of 330 cm on a custom-made wooden couch with a lucite sheet of 1 cm on both sides. The couch was kept on top of the trolley to facilitate changing its direction. The treatment involved stationary beams with field sizes of the order of 70 × 200 cm2 encompassing the whole length of the patient. The collimator was rotated to 45° to get the field size, equal to the diagonal length of the square field size hence accommodating the longest possible side. Patients were kept at central, 75% of the light field size, which was considered a dosimetric field size to account for clipping of the collimator and penumbra. Sometimes patients needed to flex their knees to encompass the dosimetric field. To improve the uniformity of dose distribution along the long axis of the patients within ± 10%, the empty volume between the patients and custom-made couch lucite walls was filled with bags of rice (Fig. 1). It has been observed that rice exhibits the same attenuation characteristics as human tissue, and provides uniform thickness, thereby minimizing absorption variations and ensuring acceptable dose uniformity. Additionally, it acts as immobilization for the patient and simplifies the physical dosimetry.

Patient positioning in customized treatment couch for TBI. Rice bags are placed on both sides which act as missing tissue compensators

A bilateral, parallel opposed field technique with 6 MV photon energy was used, and the couch was rotated 180 degrees between the treatments. The prescribed dose for every patient was 3 Gy in a complete session, delivered to the mid-depth of the patient at the level of the umbilicus.

Shielding was not needed due to the low total dose. Beam profiles, percentage depth dose (PDD), and output calibration of the linear accelerator at the treatment position were measured before each TBI treatment. These measurements were necessary because the parameters used to characterize the X-ray beam at the standard treatment distance of 100 cm may not be valid for extended treatment.

A manual monitor unit (MU) calculation was done based on the data obtained from the phantom measurement in the extended SSD condition. For the patient with a lateral shift less than equal to 44 cm, the total MUs calculated was 6082, and for the patient with a lateral shift less than equal to 52 cm, it was 7140 MU. The dose rate for each treatment is 600 MU/min. Care was taken to avoid any metal objects on the patient's body that may perturb the dose distribution.

Patients were followed up closely during inpatient visits and monthly on an outpatient basis. Demographic data, treatment details, and major toxicities were analyzed with appropriate descriptive statistics.

This retrospective study by chart review was approved by the Nepal Health Research Council to analyze the results of HSCT using TBI as a conditioning regimen along with chemotherapy.