Background High-quality intensity-modulated radiation therapy (IMRT) / volumetric modulated arc therapy (VMAT) is necessary to drive positive patient outcomes, yet gaps in staff training hinder its implementation in low-to-middle-income countries (LMICs). We aimed to evaluate a large-scale remote training curriculum for medical physicists and clinicians with existing IMRT capabilities in LMICs. Methods We conducted a virtual course on "Mastering IMRT/VMAT for Medical Physicists." The program comprised 15 weekly 1-hour live video conferencing sessions with interactive didactics and case-based learning taught by expert volunteer medical physicists. Enrollment was free and open to LMICs globally for 1 month on a first-come basis with a limit of 500, plus few exceptions. We distributed an electronic survey before and after the course to assess participants' confidence (on a 1-5 Likert scale) in 7 domains of IMRT/VMAT and knowledge through 11 multiple choice questions. We tracked attendance hours for course credit. We also gave participants the option to develop an IMRT or VMAT plan for identical, standardized bilateral Head & Neck cancer cases before and after the course, earning additional credit hours upon submission. After the program, 8 medical physics volunteers independently graded treatment plans using a rubric for qualitative domains. Furthermore, we gave plans a quantitative score using an automated scorecard with dose-volume histogram metrics. We utilized the ProKnow DS platform for file-sharing and grading. Two-tailed paired t-test analyses compared pre- vs. post-course survey and treatment planning scores for medical physicists, medical physics residents, and dosimetrists. Results A total of 514 participants from 63 countries and 342 unique centers enrolled, including 448 (87.1%) medical physicists, 49 (9.5%) medical physics residents, 5 (1%) dosimetrists, and 12 (2.4%) other clinicians. Of these, 240 medical physicists, medical physics residents, and dosimetrists responded to both the pre- and post-course surveys. Mean confidence scores increased from 3.00/5 (SD: 1.04) to 3.80/5 (0.87) (p < 0.001). Knowledge scores improved from 4.16/11 (SD: 1.77) to 5.98/11 (SD: 2.11) (p < 0.001). Additionally, 33 participants completed both the pre-course and post-course treatment planning assignments. Automated scorecard performance significantly improved from 12.64/25 (SD: 7.50) to 17.74/25 (SD: 6.74) (p = 0.0068). Grading rubric scores did not significantly change, from 9.15/14 (SD: 3.33) to 9.76/14 (SD: 2.65) (p = 0.4). Discussion A virtual curriculum on IMRT/VMAT improved participant's treatment planning performance, knowledge, and confidence. A distributed team of volunteers enabled this to be a low-cost, scalable intervention to help clinics in LMICs. High-quality intensity-modulated radiation therapy (IMRT) / volumetric modulated arc therapy (VMAT) is necessary to drive positive patient outcomes, yet gaps in staff training hinder its implementation in low-to-middle-income countries (LMICs). We aimed to evaluate a large-scale remote training curriculum for medical physicists and clinicians with existing IMRT capabilities in LMICs. We conducted a virtual course on "Mastering IMRT/VMAT for Medical Physicists." The program comprised 15 weekly 1-hour live video conferencing sessions with interactive didactics and case-based learning taught by expert volunteer medical physicists. Enrollment was free and open to LMICs globally for 1 month on a first-come basis with a limit of 500, plus few exceptions. We distributed an electronic survey before and after the course to assess participants' confidence (on a 1-5 Likert scale) in 7 domains of IMRT/VMAT and knowledge through 11 multiple choice questions. We tracked attendance hours for course credit. We also gave participants the option to develop an IMRT or VMAT plan for identical, standardized bilateral Head & Neck cancer cases before and after the course, earning additional credit hours upon submission. After the program, 8 medical physics volunteers independently graded treatment plans using a rubric for qualitative domains. Furthermore, we gave plans a quantitative score using an automated scorecard with dose-volume histogram metrics. We utilized the ProKnow DS platform for file-sharing and grading. Two-tailed paired t-test analyses compared pre- vs. post-course survey and treatment planning scores for medical physicists, medical physics residents, and dosimetrists. A total of 514 participants from 63 countries and 342 unique centers enrolled, including 448 (87.1%) medical physicists, 49 (9.5%) medical physics residents, 5 (1%) dosimetrists, and 12 (2.4%) other clinicians. Of these, 240 medical physicists, medical physics residents, and dosimetrists responded to both the pre- and post-course surveys. Mean confidence scores increased from 3.00/5 (SD: 1.04) to 3.80/5 (0.87) (p < 0.001). Knowledge scores improved from 4.16/11 (SD: 1.77) to 5.98/11 (SD: 2.11) (p < 0.001). Additionally, 33 participants completed both the pre-course and post-course treatment planning assignments. Automated scorecard performance significantly improved from 12.64/25 (SD: 7.50) to 17.74/25 (SD: 6.74) (p = 0.0068). Grading rubric scores did not significantly change, from 9.15/14 (SD: 3.33) to 9.76/14 (SD: 2.65) (p = 0.4). A virtual curriculum on IMRT/VMAT improved participant's treatment planning performance, knowledge, and confidence. A distributed team of volunteers enabled this to be a low-cost, scalable intervention to help clinics in LMICs.