Accurate IMRT fluence verification for prostate cancer patients using ‘in-vivo’ measured EPID images and in-room acquired kilovoltage cone-beam CT scans

Abstract

BackgroundTo investigate for prostate cancer patients the comparison of ‘in-vivo’ measured portal dose images (PDIs) with predictions based on a kilovoltage cone-beam CT scan (CBCT), acquired during the same treatment fraction, as an alternative for pre-treatment verification. For evaluation purposes, predictions were also performed using the patients’ planning CTs (pCT).MethodsTo get reliable CBCT electron densities for PDI predictions, Hounsfield units from the pCT were mapped onto the CBCT, while accounting for non-rigidity in patient anatomy in an approximate way. PDI prediction accuracy was first validated for an anatomical phantom, using IMRT treatment plans of ten prostate cancer patients. Clinical performance was studied using data acquired for 50 prostate cancer patients. For each patient, 4–5 CBCTs were available, resulting in a total of 1413 evaluated images. Measured and predicted PDIs were compared using γ-analyses with 3% global dose difference and 3 mm distance to agreement as reference criteria. Moreover, the pass rate for automated PDI comparison was assessed. To quantify improvements in IMRT fluence verification accuracy results from multiple fractions were combined by generating a γ-image with values halfway the minimum and median γ values, pixel by pixel.ResultsFor patients, CBCT-based PDI predictions showed a high agreement with measurements, with an average percentage of rejected pixels of 1.41% only. In spite of possible intra-fraction motion and anatomy changes, this was only slightly larger than for phantom measurements (0.86%). For pCT-based predictions, the agreement deteriorated (average percentage of rejected pixels 2.98%), due to an enhanced impact of anatomy variations. For predictions based on CBCT, combination of the first 2 fractions yielded gamma results in close agreement with pre-treatment analyses (average percentage of rejected pixels 0.63% versus 0.35%, percentage of rejected beams 0.6% versus 0%). For the pCT-based approach, only combination of the first 5 fractions resulted in acceptable agreement with pre-treatment results.ConclusionIn-room acquired CBCT scans can be used for high accuracy IMRT fluence verification based on in-vivo measured EPID images. Combination of γ results for the first 2 fractions can largely compensate for small accuracy reductions, with respect to pre-treatment verification, related to intra-fraction motion and anatomy changes.

Dosimetric properties of fluoroscopic EPID for transit dosimetry

“…However, in order to achieve electronic equilibrium in the EPID’s sensitive layer, and also to reduce the detection of high energy electrons generated in the absorber, we used an additional 1 mm thick stainless steel slab on the fluorescent screen. As recent publications state, image quality is affected by the additional layer so acquired images can be suitable for patient set-up and portal dose verification 10 , 13 , 14 , 18 …”

Investigating the effectiveness of monitoring relevant variations during IMRT and VMAT treatments by EPID-based 3D in vivo verification performed using planning CTs

“…Cone-beam computed tomography (CBCT), as online images acquired in the course of treatment, can effectively reflect the position errors and anatomical changes of patients. Thus, CBCT images seems to be the best choice for EPID-based in vivo verification, and a few studies have also proven the feasibility of CBCT selection [11, 12]. However, the image quality of CBCT is poor, and it needs to be calibrated before being used for dose calculation, which limits the application of CBCT to EPID-based in vivo dose verification.…”

Surface refraction of sound waves affects calibration of three-dimensional ultrasound

“…In image guided radiotherapy, the ability to improve tumor control and reduce toxicity [ 1 ], e.g. through dose escalation and hypo-fractionation [ 2 ], and through advanced techniques [ 3 , 4 ], relies on an accurate target concept [ 5 ] and precise positioning to achieve the desired dose distribution [ 6 , 7 ].…”