External beam patient dose verification based on the integral quality monitor (IQM®) output signals.

BACKGROUND: The Integral Quality Monitor (IQM®) can essentially measure the integral fluence through a segment and provide real-time information about the accuracy of radiation delivery based on comparisons of measured segment signals and pre-calculated reference values. However, the present IQM chamber cannot calculate the dose in the patient. AIM: This study aims to make use of IQM field output signals to calculate the number of monitor units (MUs) delivered through an arbitrary treatment field in order to convert Monte Carlo (MC)-generated dose distributions in a patient model into absolute dose. METHODS: XiO and Monaco treatment planning systems (TPSs) were used to define treatment beam portals for cervix and esophagus conformal radiotherapy as well as prostate intensity-modulated radiotherapy for the translation of patient and beam setup information from DICOM to DOSXYZnrc. The planned beams were simulated in a patient model built from actual patient CT images and each simulated integral field/segment was weighted with its MUs before summation to get the total dose in the plan. The segment beam weights (MUs) were calculated as the ratio of the open-field IQM measured signal and the calculated signal per MU extracted from chamber sensitivity maps. These are the actual MUs delivered not just MUs set. The beam weighting method was evaluated by comparing weighted MC doses with original planned doses using profile and isodose comparisons, dose difference maps, γ analysis and dose-volume histogram (DVH) data. RESULTS: γ pass rates of up to 98% were found, except for the esophagus plan where the γ pass rate was below 45%. DVH comparisons showed good agreement for most organs, with the largest differences observed in low-density lung. However, these discrepancies can result from differences in dose calculation algorithms or differences in MUs used for dose weighting planned by the TPS and MUs calculated using IQM field output signals. To test this, a 4-field box DOSXYZnrc MC simulation weighted with planned (XiO) MUs was compared with the same simulation weighted with IQM-based MUs. Dose differences of up to 5% were found on the isocentre slice. For XiO versus MC, up to 7% dose differences were found, indicating additional error due to limitations of XiO's superposition algorithm. Dose differences between MC Monaco and MC EGSnrc were less than 3%. CONCLUSIONS: The most valuable comparison was MC versus MC as it eliminated algorithm discrepancies and evaluated dose differences precisely according to beam weighting. For XiO TPS, care must be taken as dose differences may also arise due to limitations in XiO's planning software, not merely due to differences in MUs. Overall, the IQM was successfully used to compute beam dose weights to accurately reconstruct the patient dose using unweighted MC beams. Our technique can be used for pre-treatment QA provided each segment output is known and an accurate linac source model is available.

Integral quality monitor (IQM® ) signal correction factors for small fields to predict larger irregular segment output signals.

PURPOSE: To develop a method of correcting for the inaccuracies of small adjoined field segments in their contribution to larger fields in order to get a better match between their combined signals and the measured integral quality monitor (IQM) open field signals. This would enable the pre-calculation of known irregular segment output signals per monitor unit (MU), which would be later useful for patient-based dose calculations for treatment verification during pre-treatment treatment validation using the IQM output signal per MU. METHODS: Small fields exhibit source obscurity and loss of scatter, resulting in smaller signals being measured by the IQM and the subsequent underestimation of IQM output signals of larger segments obtained by combining small segment signals. Larger field segments were broken down into a set of smaller, regular, abutted segments, whose individual signals were added together to get the predicted output signal of the larger field. The signal/MU for each smaller constituent segment was extracted at its exact location from measured IQM response maps, generated by irradiating the IQM with small elementary segments ranging from 1 × 1 cm2 -5 × 5 cm2 , shifting each segment 1 cm at a time and measuring its corresponding output signal/MU throughout the entire IQM sensitive area. The predicted signal was weighed against the IQM-measured signal of the open field to calculate a signal correction factor (CF) of each elementary segment size. The CFs were applied to known signals of each set of elementary fields before summation in order to pre-calculate signals of larger irregular fields more accurately. The dependence of CFs on elementary segment size, location of the open field, and beam energy was investigated. RESULTS: CFs exhibited an exponential decrease with increase in elementary segment size. CFs were also invariant with beam energy, changing by ≤1% from 6-15 MV. Uncorrected signals for regular fields had relative errors of above 5% whilst signal correction reduced these errors down to ~0.4% (i.e., 99.6% accuracy). For irregular fields, signal correction reduced calculation errors from ~10% to well below 1.5%. Larger signal prediction errors were found when smaller segments were used to reconstruct the field. Open field size and location had a great impact on measured signals but virtually no significance on CFs. CONCLUSIONS: Results indicate that summation of small segment signals cannot sufficiently reproduce the same output given by an open field if individual elementary segment signals are not weighted with their respective CFs. This effect is particularly predominant for elementary segments 3 × 3 cm2 and for irregular fields. The method outlined enabled the calculation of signal CFs in order to match predicted signals with measured signals to 98.5% accuracy, thus enabling the pre-calculation of irregular segment output signals/MU for future patient dose calculations.

Dosimetry Effects Caused by Unilateral and Bilateral Hip Prostheses: A Monte Carlo Case Study in Megavoltage Photon Radiotherapy for Computed Tomography Data without Metal Artifacts.

BACKGROUND: Hip prostheses (HPs) are routinely used in hip augmentation to replace painful or dysfunctional hip joints. However, high-density and high-atomic-number (Z) inserts may cause dose perturbations in the target volume and interface regions. AIM: To evaluate the dosimetric influence of various HPs during megavoltage conformal radiotherapy (RT) of the prostate using Monte Carlo (MC) simulations. MATERIALS AND METHODS: BEAMnrc and DOSXYZnrc MC user-codes were respectively used to simulate the linac head and to calculate 3D absorbed dose distributions in a computed tomography (CT)-based phantom. A novel technique was used to synthetically introduce HPs into the raw patient CT dataset. The prosthesis materials evaluated were stainless steel (SS316L), titanium (Ti6Al4V), and ultra-high-molecular-weight polyethylene (UHMWPE). Four, five, and six conformal photon fields of 6-20 MV were used. RESULTS: The absorbed dose within and beyond metallic prostheses dropped significantly due to beam attenuation. For bilateral HPs, the target dose reduction ranged up to 23% and 17% for SS316L and Ti6Al4V, respectively. For unilateral HP, the respective dose reductions were 19% and 12%. Dose enhancement was always <1% for UHMWPE. The 6-field plan produced the best target coverage. Up to 38% dose increase was found at the bone-SS316L proximal interface. CONCLUSIONS: The novel technique used enabled the complete exclusion of metal artifacts in the CT dataset. High-energy plans with more oblique beams can help minimize dose attenuation through HPs. Shadowing and interface effects are density dependent and greatest for SS316L, while UHMWPE poses negligible dose perturbation.