A Novel Compensator Device to Deliver IMRT With Cobalt-60 Units in Low- and Middle-Income Countries: A Comparative Dosimetric Analysis

Abstract

Radiation therapy is a critical and cost-effective component of cancer care that is vastly underutilized in low-and middle-income countries (LMICs). Technologies developed for industrialized countries are often ill-suited for sudden adoption in LMIC settings. We propose a novel device to efficiently deliver intensity modulated radiation therapy (IMRT) with lower cost and complexity. The purpose of this study is to demonstrate through dosimetric analysis that this device can create safe, clinically acceptable treatments with similar tumor coverage and organ at risk (OAR) sparing as Linac/MLC based IMRT plans. The device employs nine patient-specific physical compensators filled with reusable tungsten pellets arranged in a ring around the patient. The compensators are manufactured at a regional center, involve no moving parts, and can be retrofitted to existing Cobalt-60 or linear accelerator treatment units. Total treatment times are short (approximately 5 minutes with Cobalt-60) because compensators are extremely efficient at using dose and the ring arrangement eliminates the need to access the treatment vault. We commissioned a Cobalt-60 compensator-based machine in the Pinnacle Treatment Planning System (v.9.8, Philips Inc., Madison, WI) and benchmarked with Monte Carlo calculations (EGSnrc). We generated five head and neck and three pelvic plans using the Cobalt-60 compensator IMRT system and compared these to clinical IMRT plans using a MLC linear accelerator (6MV), and calculated median percent differences and used t test for OAR and PTV coverage. In the five head and neck cases the Cobalt-60 compensator plans resulted in a lower mean parotid gland dose (9% lower) and a lower maximum spinal cord dose (2.5% lower) while maintaining the PTV coverage, i.e. PTV V95 within 1% and no statistical difference between the D2 (P = 0.45) and D99 (P = 0.38). In the three pelvic cases the Cobalt-60 compensator plans had similar doses to femoral heads (max dose 7% difference), bladder (D35 1% difference), and rectum (D30 3% difference). The PTV coverage was similar between the two plans, with no statistical difference between the D2 (P = 0.17) and D99 (P = 0.34). The novel physical compensator device allows for efficient delivery of Cobalt-60 based IMRT without the use of an MLC. Our results suggest that tumor coverage and OAR sparing are similar to treatments using a MLC linear accelerator. A prototype system will be developed with commercial and clinical partners at an LMIC-site and NCI support.