Feasibility Study Into Adjustment of the Radiation Beam to Account for Prostate Motion During Radiotherapy.

Prostate cancer now accounts for one third of all new cancer diagnoses in men and approximately 30% of men will have external beam radiotherapy as their primary local therapy. Prostate motion during radiotherapy can be divided into interfraction and intrafraction motion. Interfraction motion has been well established and has been largely overcome by daily online image verification with either ultrasound, online CT or implanted fiducial markers, however motion during the radiation beam on time (intrafraction motion) is not corrected and can be the cause of significant errors in radiation dose delivery.

The most common technology utilised in 2012 to allow prostate gating is the Calypso system. The Calypso system consists of implantable electromagnetic transponders, an array that contains source and receiver coils, computers for data analysis and display purposes, and an infrared camera system to localise the electromagnetic array in the treatment room. The array is placed over the patient, and the source coil in the array emit an electromagnetic signal that excites the transponders. Once the transponders are excited, the source coils are turned off and the receiver coils detect the signal emitted from the excited transponders. This process is repeated at a rate of 10 Hz, providing a realtime radiofrequency localisation of the prostate triangulating three implanted beacons. The current study will investigate using the continuous prostate positioning data from Calypso to integrate with the treatment beam delivery and allow real-time adaptation based on the prostate motion. This is called Realtime Dynamic Multileaf Collimator (DMLC) tracking. In this technique the multileaf collimator motion is altered in the gantry head in real time during beam delivery to account for the measured prostate motion.

The proposed study is examining the dosimetric impact of accounting for intrafraction motion with Calypso and DMLC tracking. We hypothesise the improvements in delivered prostate dose with DMLC tracking will be even greater than gating. This improved treatment delivery will ensure that the prostate cancer receives the appropriate dose and that normal tissues are spared from extra radiation.

Inclusion criteria

• Patients undergoing external beam radiotherapy at Northern Sydney Cancer Centre
• Histologically proven prostate adenocarcinoma
• Prostate Specific Antigen (PSA) obtained within 3 months prior to enrolment.
• Patient must be able to have Varian Calypso beacons placed in the prostate (if on anticoagulants, must be cleared by Local Medical Officer or cardiologist).
• ECOG performance status 0-2
• Ability to understand and the willingness to sign a written informed consent document.
• Body habitus enabling Calypso tracking (as per Calypso Determining a Patient's Localisation Designation & Orientation before implantation)
• Prostate dimension that allows leaf span with tracking margin of ±8mm

Exclusion criteria

• Previous pelvic radiotherapy
• Prior total prostatectomy
• Pacemaker
• Implantable defibrillator
• Insulin infusion pump
• Hip prosthesis
• Unwilling or unable to give informed consent
• Unwilling or unable to complete quality of life questionnaires.