Boost first, eliminate systematic error, and individualize CTV to PTV margin when treating lymph nodes in high-risk prostate cancer
ABSTRACT The purpose of this report is to evaluate the movement of the planning target volume (PTV) in relation to the pelvic lymph nodes (PLNs) during treatment of high-risk prostate cancer.
We reviewed the daily treatment course of ten consecutively treated patients with high-risk prostate cancer. PLNs were included in the initial PTV for each patient. Daily on-board imaging of gold fiducial markers implanted in the prostate was used; daily couch shifts were made as needed and recorded. We analyzed how the daily couch shifts impacted the dose delivered to the PLN.
A PLN clinical target volume was identified in each man using CT-based treatment planning. At treatment planning, median minimum planned dose to the PLN was 95%, maximum 101%, and mean 97%. Daily couch shifting to prostate markers degraded the dose slightly; median minimum dose to the PLN was 92%, maximum, 101%, and mean delivered, 96%. We found two cases, where daily systematic shifts resulted in an underdosing of the PLN by 9% and 29%, respectively. In other cases, daily shifts were random and led to a mean 2.2% degradation of planned to delivered PLN dose.
We demonstrated degradation of the delivered dose to PLN PTV, which may occur if daily alignment only to the prostate is considered. To improve PLN PTV, it maybe preferable to deliver the prostate/boost treatment first, and adapt the PTV of the pelvic/nodal treatment to uncertainties documented during prostate/boost treatment.
SourceAvailable from: Mischa S Hoogeman[Show abstract] [Hide abstract]
ABSTRACT: Daily treatment plan selection from a plan library is a major adaptive radiotherapy strategy to account for individual internal anatomy variations. This strategy depends on the initial input images being representative for the variations observed later in the treatment course. Focusing on locally advanced prostate cancer, our aim was to evaluate if residual motion of the prostate (CTV-p) and the elective targets (CTV-sv, CTV-ln) can be prospectively accounted for with a statistical deformable model based on images acquired in the initial part of treatment. Methods. Thirteen patients with locally advanced prostate cancer, each with 9-10 repeat CT scans, were included. Displacement vectors fields (DVF) obtained from contour-based deformable registration of delineations in the repeat- and planning CT scans were used to create patient-specific statistical motion models using principal component analysis (PCA). For each patient and CTV, four PCA-models were created: one with all 9-10 DVF as input in addition to models with only four, five or six DVFs as input. Simulations of target shapes from each PCA-model were used to calculate iso-coverage levels, which were converted to contours. The levels were analyzed for sensitivity and precision. Results. A union of the simulated shapes was able to cover at least 97%, 97% and 95% of the volumes of the evaluated CTV shapes for PCA-models using six, five and four DVFs as input, respectively. There was a decrease in sensitivity with higher iso-coverage levels, with a sharper decline for greater target movements. Apart from having the steepest decline in sensitivity, CTV-sv also displayed the greatest influence on the number of geometries used in the PCA-model. Conclusions. PCA-based simulations of residual motion derived from four to six DVFs as input could account for the majority of the target shapes present during the latter part of the treatment. CTV-sv displayed the greatest range in both sensitivity and precision.Acta oncologica (Stockholm, Sweden) 08/2013; DOI:10.3109/0284186X.2013.818249 · 3.71 Impact Factor
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ABSTRACT: Deformation and correlated target motion remain challenges for margin recipes in radiotherapy (RT). This study presents a statistical deformable motion model for multiple targets and applies it to margin evaluations for locally advanced prostate cancer i.e. RT of the prostate (CTV-p), seminal vesicles (CTV-sv) and pelvic lymph nodes (CTV-ln). The 19 patients included in this study, all had 7-10 repeat CT-scans available that were rigidly aligned with the planning CT-scan using intra-prostatic implanted markers, followed by deformable registrations. The displacement vectors from the deformable registrations were used to create patient-specific statistical motion models. The models were applied in treatment simulations to determine probabilities for adequate target coverage, e.g. by establishing distributions of the accumulated dose to 99% of the target volumes (D99) for various CTV-PTV expansions in the planning-CTs. The method allowed for estimation of the expected accumulated dose and its variance of different DVH parameters for each patient. Simulations of inter-fractional motion resulted in 7, 10, and 18 patients with an average D99 >95% of the prescribed dose for CTV-p expansions of 3mm, 4mm and 5mm, respectively. For CTV-sv and CTV-ln, expansions of 3mm, 5mm and 7mm resulted in 1, 11 and 15 vs. 8, 18 and 18 patients respectively with an average D99 >95% of the prescription. Treatment simulations of target motion revealed large individual differences in accumulated dose mainly for CTV-sv, demanding the largest margins whereas those required for CTV-p and CTV-ln were comparable.Radiotherapy and Oncology 10/2013; 109(3). DOI:10.1016/j.radonc.2013.09.012 · 4.86 Impact Factor
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ABSTRACT: Objectives: There remains concern regarding the use of fiducial-based image guided radiotherapy (IGRT) in high risk prostate cancer patients also undergoing IMRT to pelvic nodes. We aim to ascertain the impact on lymph node PTV coverage with the use of fiducial-based IGRT, in a retrospective study. Methods: Thirty consecutive IMRT prostate and pelvic node plans were reviewed and dose was recalculated with 1mm increment movements in anterior, posterior, superior, inferior, right and left directions up to 10mm. All patients were treated with a full bladder after drinking 450-750mls water and empty rectum with the use of sodium citrate enemas daily. DVH parameters were recorded at each position, specifically nodal PTV V95%,V99% and V100%. A local IGRT database was used to identify the likelihood of a particular bony to fiducial off-set in all directions. The combined data was used to calculate the percentage risk of underdosing the lymph node PTV on any given fraction. Results: The likelihood of an off-set in the left, right, and anterior directions occurring, and resulting in a failure to cover the PTV was <0.25%. The likelihood of a posterior off-set occurring and resulting in inadequate coverage was slightly higher but remained <1%. Conclusions: This study confirms the safety of fiducial-based IG-IMRT with a strict bowel and bladder protocol, allowing a reduction of the CTV to PTV margin of the prostate volume and consequent reduction in rectal toxicity.The British journal of radiology 03/2014; 87(1037):20130696. DOI:10.1259/bjr.20130696 · 1.53 Impact Factor