Authors (including presenting author) :
Nelson Lam, Wicger Wong, Jessica Lai, Raymond Tang, Eric Lam, Tracy Shum
Affiliation :
Oncology Department, Princess Margaret Hospital
Introduction :
Cancer is the top killer in Hong Kong. Ageing population with high life expectancy creates a rising trend in cancer incidence. Demands of radiation therapy increase significantly and create huge service burden to our linear accelerator (Linac) in Oncology Department. Advanced radiation treatment delivery technology has to be updated in order to minimize patient on-couch time and increase throughput of Linac.
Objectives :
Majority of our Head & Neck cancer cases and some prostate cases are treated with Intensity Modulated Radiation Treatment (IMRT) technique. IMRT is a discrete (step-shoot) beam angle delivery method. Moreover, Volumetric Modulated Arc Therapy (VMAT) is a continuous arc therapy and it reduces beam-on time to 20% when compared to IMRT. The objective of this study is to change the delivery technique from IMRT to VMAT and increase the throughput of our linear accelerator whilst the planning quality is maintained or even improved.
Methodology :
The hurdle of changing to VMAT is lens dose limit. Among all HA Oncology centres, our limit is most stringent, maximum allowable lens dose < 6Gy. After aligning with criteria of other centres worldwide and considering the risk and benefit of patients, lens dose limit has been updated to 10 Gy maximum. The risk is cataract developed after few years of treatment. Nevertheless, cataract is common in ageing population and success rate of cataract surgery is very high. Benefit is VMAT can significantly reduce beam-on time of our Linac and more patients can be treated in Oncology. Apparently, benefit is much more than the risk involved.
Result & Outcome :
On average, beam-on time for IMRT is 25 mins. per fraction whereas VMAT is 5 mins per fraction. Typically, a complete treatment plan consists of 33 to 35 fractions, one fraction per day. As a result, it can save up to 11-hour machine time for one patient (20 mins x 33 fr). Planning quality of VMAT is similar to nine-beam IMRT in terms of doses to both Planning Target Volume (PTV) and Organs-At-Risk (OARs). In 2022, there were 153 IMRT plans and 172 VMAT plans, 325 patients were treatment with modulated radiotherapy. In 2024, almost all IMRT plans were migrated to VMAT, there were only 5 IMRT plans and 476 VMAT plans, 481 patients were treated. Thus, the number of patient received radiation treatment in Linac machine was increased by 48% by implementing VMAT technique.
