Authors (including presenting author) :
Lam KH (1), Wong KH (1)
Affiliation :
(1) Medical Physics Unit, 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 Therapy (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 better.
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 and considering the risk and benefit of patients, lens dose limit has been updated to 10 Gy maximum. The risk is cataract developed few years after treatment. Nevertheless, cataract is common in ageing population and it’s not life-threatening. Benefit is VMAT can greatly reduce beam-on time of our Linac and more patients can be treated through improved throughput. 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 33-35 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). Compared to same day on 21-December-22 one year ago, 13 IMRT plans have been migrated to VMAT, only one IMRT plan left. We have saved 13 x 20mins. = 260 mins. (4.33 hours) per day. It is equivalent to a half-day service time of a Linac. Thus, the capacity of a Linac was increased by 50% after implementing this strategy.
