In vivo diode dosimetry for routine quality assurance in IMRT

Abstract

Due to the complexity of IMRT dosimetry, dose delivery evaluation is generally done using a treatment plan in which the optimized fluence distribution has been transferred to a test phantom for accessibility and simplicity of measurement. The actual patient doses may be reconstructed in vivo through the use of electronic portal imaging devices or films, but the assessment of absolute dose from these measurements is time-consuming and complicated. In our clinic we have instituted the use of routine diode dosimetry for IMRT patients following the same procedure used for standard radiation therapy patients in which each new treatment field is checked at the start of treatment. For standard cases the dose at dmax is calculated as part of the monitor unit calculation. For the IMRT cases, the dose contribution to the dmax depth for each field is taken from the treatment plan. We found that about 90% of the diode measurements agreed to within +/- 10% of the planned doses (45/51 fields) and 63% (32/51 fields) achieved +/- 5% agreement. By using this direct in vivo method to verify the clinical doses delivered, we have been able to make a uniform startup procedure for all patients while simplifying our IMRT QA process.

Practical guidelines for routine intensity-modulated radiotherapy verification: pre-treatment verification with portal dosimetry and treatment verification within vivodosimetry

“…Although the preparation of the small MLC fields may take up to 1 h in treatment planning, their use facilitates precise diode positioning during the first treatment session. As our results show a significant increase in accuracy over levels reported previously [33], we consider this to be a worthwhile investment in patient safety. …”

“…To date, there is little reported information on the use of diodes for IMRT treatment verification. Higgins et al [33] report 90% of diode measurements agreeing to within ¡10% of planned doses and 63% achieving ¡5%. The agreement reported here is better: 95% of all IMRT fields are within a standard treatment tolerance of ¡5% and 99% within ¡8%.…”

“…The agreement reported here is better: 95% of all IMRT fields are within a standard treatment tolerance of ¡5% and 99% within ¡8%. Higgins et al [33] discussed the potential complication of scatter conditions in the use of dynamic MLC IMRT techniques but found no significant differences in diode response between dynamic MLC techniques and step and shoot techniques. Although their results are for step and shoot IMRT, we have shown similar results for dynamic IMRT.…”

In vivo dose verification of IMRT treated head and neck cancer patients

“…Both the segmental (step and shoot) and dynamic (sliding window) IMRT techniques are associated with radiation fields of inhomogeneous fluences and sharp dose gradients. Although in vivo dosimetry by means of entrance dose measurements with diodes or TLD has proven to be an important method in conventional radiotherapy, it is difficult to use it with sufficient accuracy for intensity-modulated fields [5]. Burman et al (1997) verified the entrance dose using TLDs positioned on the central axis of the beam for selected IMRT fields [1].…”

Dosimetry tools and techniques for IMRT

“…Therefore, diode detectors are usually used for smallfield dose distribution measurements where there are relatively few low-energy photons. [32][33][34] Sometimes, low-energy filters, also known as shields, are included in the diode construction to minimize the low-energy sensitivity. These are termed energy-compensated diode detectors.…”