Average D99 Research Articles

Predicting personalised and progressive adaptive dose escalation to gross tumour volume using knowledge-based planning models for inoperable advanced-stage non-small cell lung cancer patients treated with volumetric modulated arc therapy

Objectives. Increased radiation doses could improve local control and overall survival of lung cancer patients, however, this could be challenging without exceeding organs at risk (OAR) dose constraints, especially for patients with advanced-stage disease. Increasing OAR doses could reduce the therapeutic ratio and quality of life. It is therefore important to investigate methods to increase the dose to target volume without exceeding OAR dose constraints. Methods. Gross tumour volume (GTV) was contoured on synthetic computerised tomography (sCT) datasets produced using the Velocity adaptive radiotherapy software for eleven patients. The fractions where GTV volume decreased compared to that prior to radiotherapy (reference plan) were considered for personalised progressive dose escalation. The dose to the adapted GTV (GTVAdaptive) was increased until OAR doses were affected (as compared to the original clinical plan). Planning target volume (PTV) coverage was maintained for all plans. Doses were also escalated to the reference plan (GTVClinical) using the same method. Adapted, dose-escalated, plans were combined to estimate accumulated dose, D99 (dose to 99%) of GTVAdapted, PTV D99 and OAR doses and compared with those in the original clinical plans. Knowledge-based planning (KBP) model was developed to predict D99 of the adapted GTV with OAR doses and PTV coverage kept similar to the original clinical plans; prediction accuracy and model verification were performed using further data sets. Results. Compared to the original clinical plan, the dose to GTV was significantly increased without exceeding OAR doses. Adaptive dose-escalation increased the average D99 to GTVAdaptive by 15.1Gy and 8.7Gy compared to the clinical plans. The KBP models were verified and demonstrated prediction accuracy of 0.4% and 0.7% respectively. Conclusion. Progressive adaptive dose escalation can significantly increase the dose to GTV without increasing OAR doses or compromising the dose to microscopic disease. This may increase overall survival without increasing toxicities.

DOSIMETRIC ANALYSIS OF ADAPTIVE RADIOTHERAPY(ART) IN LOCALLY ADVANCED HEAD AND NECK CANCER

PURPOSE/OBJECTIVE: The objective of this study is to evaluate the dosimetric parameters in adaptive radiotherapy for locally advanced head and neck cancers METHODS AND MATERIALS: This is a Hospital-based Prospective study conducted in the period from Dec 2020 to March 2021. Histologically proven Head and Neck Carcinoma patients with Stage III to IV (locally advanced) were selected for the study. A total of 10 patients receiving denitive, conformal radiation therapy to the head and neck region were evaluated for the study. After the acquisition of CT images, target volumes, OARs were contoured in the planning CT. Images were again acquired midway during the planned course of radiation therapy. Body contours, target volumes, and organs at risk were redrawn on the new set of images. Two sets of additional treatment plans were generated: 1) a non-optimized plan (plan 2), which is an overlay of the original plan (plan 1) on the new set of contours, and 2) an optimized plan(plan 3) with the new set of contours. These 3 sets of plans were then compared for dosimetric differences. RESULTS: Four patients had locally advanced nasopharyngeal cancers, 4 patients had locally advanced oropharyngeal cancers, 2 patients had locally advanced hypopharyngeal cancer. The average reduction in gross tumour volume was 37.1 ml. The average changes in right and left parotid volume were 5.94 and 5.49 ml, respectively. With the non-optimized plan, the average increase in the maximum dose to the spinal cord was 9.8% (58.96-68.76; p= 0.156). With reoptimization, the maximum dose to the spinal cord decreased from 68.76% to 54.97% (mean difference, -13.79%, p=0.03). The average D99 for the planning target volume( dose received by 99% of the target volume) was 98.68% and 98.65% with the original and reoptimized plans, respectively. Most of the patients during radiation had Grade 2 skin toxicity and Grade 2 mucositis which was managed conservatively. CONCLUSIONS: This study demonstrates that during radiation there is gross changes of volumes in locally advanced head and neck cancers and thus adaptive radiation therapy plays a pivotal role in locally advanced head and neck cancer

SU-E-J-74: Dosimetric Advantages of Adaptive Radiotherapy for Head and Neck Cancer Are Confirmed with Weekly CBCT Images

Purpose: Our previous study showed that weekly dose monitoring using cone-beam CT (CBCT) images can guide the timing and need for adaptive re-planning during the treatment of head and neck (HN) cancer. Here we aim to confirm the dosimetric improvement of adaptive radiotherapy (ART) using weekly CBCTs. Methods: We randomly selected seven HN patients treated with ART due to noticeable anatomic changes. Twenty weekly images acquired during the second treatment course were included. These CBCTs were aligned with both the initial and re-planning simulation CTs according to the clinical shifts. Daily doses were re-calculated for both the initial and adaptive plans. Contours of the tumor and organs-at-risk (OARs) were manually delineated by a physician on the re-planning CT and then were transferred to the CBCTs for plan evaluation. Contour modifications were made based on the daily anatomic changes observed on CBCTs. All patients were treated with 70Gy to the primary tumor and 56Gy to the elective lymph nodes. Results: Volumetric changes of the tumor (range — 43.9%∼+15.9%) were observed. The average D99 to the primary tumor was (70.1±2.0)Gy (range 62.2∼72.5Gy) for the adaptive plan and (66.0±5.5)Gy (range 50.9∼70.7Gy) for the initial plan(p<<0.01). The average D99 to the elective neck was (56.3±1.3)Gy (range 52.8∼59.2Gy) for the adaptive plan and (52.4±7.0)Gy (range 37.7∼58.6Gy) for the initial plan(p=0.01). The parotid decreased in volume during the treatment course (range 7.3%∼42.2%). The average D_mean to the spared parotid decreased by 15.3% (p=0.002) for the adaptive plan when compared to the original. With ART, 4 out of 7 patients experienced better sparing of the spinal cord (D_max reduced by 2.5%∼10.2%) and the oral cavity (D_mean reduced by 3.5%∼20.1%). Conclusion: Weekly CBCT dosimetry confirms that ART is an effective method to accommodate on-treatment anatomic changes. In select patients, tumor coverage and OAR sparing may be improved with ART. Research is funded by Siemens.

Adaptive radiation therapy for pediatric head and neck malignancies: Dosimetric implications

PurposeThis study was designed to evaluate the role of adaptive radiation therapy for children receiving curative radiation therapy to the head and neck region. Methods and materialsTen children receiving definitive, conformal radiation therapy to the head and neck region were prospectively evaluated for anatomic changes during the course of radiation therapy. Images were acquired midway through the number of planned radiation therapy fractions during the planned course of radiation therapy. Body contours, target volumes, and organs at risk were redrawn on the new set of images. Two sets of additional treatment plans were generated: (1) a nonoptimized plan (plan 2), that is, an overlay of the original plan (plan 1) on the new set of contours, and (2) an optimized plan (plan 3) with the new set of contours. These 3 sets of plans were then compared for dosimetric differences. ResultsFive children had nasopharynx carcinoma, whereas the other 5 had embryonal rhabdomyosarcoma. The average reduction in gross tumor volume was 40% (mean volume, 41.87 mL; P = .005). The average changes in right and left parotid volumes were 2.72 and 1.92 mL, respectively. With nonoptimized plans, the average increase in maximum dose to the spinal cord was 15% (79.99%-94.99%; P = .013). With reoptimization, the maximum dose to the spinal cord decreased from 94.99% to 85.26% (mean difference, −9.73%; P = .02). Average D99 for the planning target volume (dose received by 99% of the target volume) was 88.66% and 86.89% with the original and reoptimized plans, respectively (P = .50). For the entire group, the mean conformation number index with nonoptimized plans was reduced from 0.734 to 0.628 (P = .013). This improved with reoptimization (P = .114). The homogeneity index improved with reoptimization from a mean value of 0.113 to 0.098 (P = 0.28). For nonoptimized plans, the average integral dose increased from 74.66 to 76.27 L-Gy (P = .486) compared with the original plans. Reoptimization resulted in a 5% average reduction in the integral dose, from 76.27 to 72.28 L-Gy (P = .007). Conclusions:This study demonstrates the usefulness of adaptive radiation therapy for children receiving radiation therapy to the head and neck region.

Treatment simulations with a statistical deformable motion model to evaluate margins for multiple targets in radiotherapy for high-risk prostate cancer

Background and purposeDeformation 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). Material and methodsThe 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. ResultsThe 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. ConclusionsTreatment 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.

Approach to dose definition to the gross tumor volume for lung cancer with respiratory tumor motion

The purpose of this study was to validate the dose prescription defined to the gross tumor volume (GTV) 3D and 4D dose distributions of stereotactic radiotherapy for lung cancer. Treatment plans for 94 patients were generated based on computed tomography (CT) under free breathing. A uniform margin of 8 mm was added to the internal target volume (ITV) to generate the planning target volume (PTV). A leaf margin of 2 mm was added to the PTV. The prescription dose was defined such that 99% of the GTV should receive 100% of the dose using the Monte Carlo calculation (iPlan RT DoseTM) for 6-MV photon beams. The 3D dose distribution was determined using CT under free breathing. The 4D dose distribution plan was recalculated to investigate the effect of tumor motion using the same monitor units as those used for the 3D dose distribution plan. D99 (99% of the GTV) in the 4D plan was defined as the average D99 in each of the four breathing phases (0%, 25%, 50% and 75%). The dose difference between maximum and minimum at D99 of the GTV in 4D calculations was 0.6 ± 1.0% (range 0.2–4.6%). The average D99 of the GTV from 4D calculations in most patients was almost 100% (99.8 ± 1.0%). No significant difference was found in dose to the GTV between 3D and 4D dose calculations (P = 0.67). This study supports the clinical acceptability of treatment planning based on the dose prescription defined to the GTV.