Systematic in vivo dosimetry for quality assurance using diodes 2: Assessing radiotherapy techniques and developing an appropriate action protocol

Abstract

Purpose: Having previously reviewed the implementation of systematic in vivo dosimetry at the Norfolk and Norwich Hospital this paper examines the results of entrance dose measurements for specific sites/techniques and determines whether different action/alert protocols are required for these different categories. Methods and materials: Entrance dose measurements using p-type diodes were analysed for the following treatment categories: Breast, head and neck in beam direction shell, abdomino-pelvic and intrathoracic. A 4% tolerance was applied. Results: Mean deviations from expected dose and proportion of measurements exceeding tolerance were: Breast: ϩ1.15% Ϯ 3.04% (1SD), 238/1073 Ն 4%; Head and neck: ϩ0.35% Ϯ 2.20% (1SD), 21/326 Ն 4%; Abdomino-pelvic: ϩ0.52% Ϯ 2.75% (1SD), 93/712 Ն 4%; Intrathoracic: Ϫ0.01% Ϯ 2.75% (1SD), 22/119 Ն 4%. Significant improvements in results for breast patients were noted following the introduction of a commercial breast board. The results for abdomino-pelvic patients confirmed a substantial variation in diode response under short FSD, wedged fields at 16 MV (that had not been corrected for). The statistical uncertainty in dose measurement for each treatment category was calculated in order to assist determination of appropriate tolerance levels. Conclusions: A blanket tolerance of 4% was generally too low given the extent of measurement uncertainty. The relatively high number of readings outside tolerance where identification of errors was difficult/impossible resulted in inconsistent application of the action protocol. Some widening of tolerances is likely to improve quality of procedure and treatment. Appropriate action levels are recommended for each treatment category. This value could be reduced if a variation in diode response under wedged fields at 16 MV was corrected for. § These values are larger mainly as a consequence of a relatively higher proportion of FSD inaccuracies for this category. Improvements in this respect would reduce these figures to the level seen for isocentric abdomino-pelvic techniques.

A method to correct for temperature dependence and measure simultaneously dose and temperature using a plastic scintillation detector

“…Another catheter could be used, but this would preclude the possibility of measuring temperature fluctuations in the immediate vicinity of the detector and would give only a general idea of the local temperature. As a second illustration, we consider skin dose measurements, which are currently the most widely practiced form of in vivo dosimetry (Fiorino et al, 2000, Bloemen-van Gurp et al, 2003, Appleyard et al, 2005). Determining the absolute temperature and fluctuations is problematic.…”

Systematic in vivo dosimetry for quality assurance using diodes. Part 1: Experiences and results of the implementation of entrance dose measurements

“…30 However it plays a potentially vital role in an overall QA programme through the identification of individual errors as well as facilitating evaluation of the overall radiotherapy process and thus informing a long-term improvement strategy.…”

“…30 Most authors have agreed that an increase in diode response with respect to ionisation chamber for short FSD and the presence of beam modification can be attributed mainly to increased scatter reaching the surface of the patient and influencing the region of dose build up and so although variations will be equipment dependent an effect should be seen. However in retrospect this assessment was inadequate as it did not fully evaluate diode response with respect to wedges and FSD per se but concentrated on the effect of dose rate.The significance of this in the context of the results for certain techniques is discussed in a subsequent paper.…”

“…30 Where the action protocol was followed and MU checks were performed, no dose calculation errors were identified. These issues are discussed further in a subsequent paper in the context of tolerance levels and action protocols.…”

“…30 The reason for this small mean deviation was investigated through exploration of the measurement results and procedures for different treatment sites/techniques and could be identified as being related to dose measurement rather than dose delivery.…”