Evaluation of a diode array for QA measurements on a helical tomotherapy unit

Abstract

A helical tomotherapy system is used in our clinic to deliver intensity-modulated radiation therapy (IMRT) treatments. Since this machine is designed to deliver IMRT treatments, the traditional field flatness requirements are no longer applicable. This allows the unit to operate without a field flatness filter and consequently the 400 mm wide fan beam is highly inhomogeneous in intensity. The shape of this beam profile is mapped during machine commissioning and for quality assurance purposes the shape of the beam profile needs to be monitored. The use of a commercial diode array for quality assurance measurements is investigated. Central axis beam profiles were acquired at different depths using solid water built-up material. These profiles were compared with ion chamber scans taken in a water tank to test the accuracy of the diode array measurements. The sensitivity of the diode array to variations in the beam profile was checked. Over a seven week period, beam profiles were repeatedly measured. The observed variations are compared with those observed with an on-board beam profile monitor. The diode measurements were in agreement with the ion chamber scans. In the high dose, low gradient region the average ratio between the diode and ion chamber readings was 1.000 +/- 0.005 (+/- 1 standard deviation). In the penumbra region the agreement was poorer but all diodes passed the distance to agreement (DTA) requirement of 2 mm. The trend in the beam profile variations that was measured with the diode array device was in agreement with the on-board monitor. While the calculated amount of variation differs between the devices, both were sensitive to subtle variations in the beam profile. The diode array is a valuable tool to quickly and accurately monitor the beam profile on a helical tomotherapy unit.

Stability of the Helical TomoTherapy Hi·Art II detector for treatment beam irradiations

“…The ratios were obtained by dividing each beam profile by the profile measured 11 days after X‐ray target replacement. The shape modification is similar to findings by other authors (21) . The long‐term ${\text{CV}}_{\mathrm{i}}$ calculated respectively during the first and the second period of time were well within 1%.…”

“…It was shown by other authors, (21) who used a different measurement device, that the beam profile is changing a few percent in the beam edges as the X‐ray target is close to its end‐of‐life. We analyzed measurements of the periods before and after the X‐ray target replacement separately.…”

QA for helical tomotherapy: Report of the AAPM Task Group 148a)

“…Analysis of the detector data is hence a tool to test the consistency of the conebeam profiles. 5,12 Third-party vendors have created suitable diode arrays for cone profile measurements. 12 Finally, film can be used to monitor profile consistency.…”

Measurements to predict the time of target replacement of a helical tomotherapy

“…These techniques usually need more beam‐on time than open field treatments, which puts a higher burden onto the acceleration machine. It is widely known that the tungsten target of the TomoTherapy Hi·Art system (TomoTherapy, Madison, WI, USA, ( 1 ) ) suffers from wear effects ( 2 – 5 ) and needs periodic replacement. This replacement normally takes a couple of days, including the time for delivering the spare parts.…”