ABSTRACT
A retrospective study was performed to explore the use of dose volume histogram (DVH) metrics in a patient-specific quality assurance protocol for volumetric modulated arc therapy (VMAT). Fourteen head and neck (HN) and ten brain patients treated with VMAT at the Launceston General Hospital were retrospectively analyzed using the new protocol to identify cases where patient dose errors exceed the established action levels that were not originally detected by either point dose and/or gamma index methods. The Sun Nuclear 3DVH software was used to estimate the dose delivered to the patient volume in terms of DVH dose errors. Thus, three different pretreatment verification methods were used to assess if a plan was considered acceptable. In two particular cases, the dose difference determined with point dose was above the established threshold, although it was found that this was due to the placement of the chamber in the phantom. In all cases, 3DVH confirmed that the dose delivered to target volumes (planning target volume - D50%) and to relevant organs at risk was within prescribed dose tolerances. This study has demonstrated the integration of DVH metrics into a VMAT PSQA protocol to provide clinically meaningful results that complement point dose and gamma index measurements. 3DVH should be regarded as an investigative tool that may be useful in diagnosing the cause of failed plans since it allows dose errors to be related to the patient anatomy.
ABSTRACT
PURPOSE: To develop an in vivo dosimetry based investigative action level relevant for a corrective protocol for HDR brachytherapy boost treatment. METHODS AND MATERIALS: The dose delivered to points within the urethra and rectum was measured using TLD in vivo dosimetry in 56 patients. Comparisons between the urethral and rectal measurements and TPS calculations showed differences, which are related to the relative position of the implant and TLD trains, and allowed shifts of implant position relative to the prostate to be estimated. RESULTS AND CONCLUSIONS: Analysis of rectal dose measurements is consistent with implant movement, which was previously only identified with the urethral data. Shift corrected doses were compared with results from the TPS. Comparison of peak doses to the urethra and rectum has been assessed against the proposed corrective protocol to limit overdosing these critical structures. An initial investigative level of 20% difference between measured and TPS peak dose was established, which corresponds to 1/3 of patients which was practical for the caseload. These patients were assessed resulting in corrective action being applied for one patient. Multiple triggering for selective investigative action is outlined. The use of a single in vivo measurement in the first fraction optimizes patient benefit at acceptable cost.
Subject(s)
Brachytherapy/methods , Prostatic Neoplasms/radiotherapy , Rectum/radiation effects , Thermoluminescent Dosimetry/methods , Urethra/radiation effects , Brachytherapy/standards , Humans , Male , Radiotherapy DosageABSTRACT
The purpose of this study is to identify factors predicting morbidity in patients undergoing high dose rate (HDR) brachytherapy boost with external beam irradiation for prostate cancer. Acute and late morbidity data were collected for 104 prostate cancer patients treated with an HDR boost together with external beam radiotherapy. Significant urinary and rectal morbidity were correlated with urethral and rectal point doses, and the proportions of the target volume receiving 100%, 150% and 200% (V200) or more of the prescribed dose. Rectal or urethral point doses did not predict morbidity. By contrast, the V200 was significantly higher for patients experiencing either acute or late urinary morbidity. The cut-off V200 was 15% of the target volume. Although theoretically beneficial for tumour cell kill, the treatment of significant proportions of the prostate to high dose might be associated with increased morbidity, and should preferably be avoided.
