Jack Yang

Dosimetric evaluation of 4 different treatment modalities for curative-intent stereotactic body radiation therapy for isolated thoracic spinal metastases

To investigate the dosimetric characteristics of 4 SBRT-capable dose delivery systems, CyberKnife (CK), Helical TomoTherapy (HT), Volumetric Modulated Arc Therapy (VMAT) by Varian RapidArc (RA), and segmental step-and-shoot intensity-modulated radiation therapy (IMRT) by Elekta, on isolated thoracic spinal lesions. CK, HT, RA, and IMRT planning were performed simultaneously for 10 randomly selected patients with 6 body types and 6 body + pedicle types with isolated thoracic lesions. The prescription was set with curative intent and dose of either 33Gy in 3 fractions (3F) or 40Gy in 5F to cover at least 90% of the planning target volume (PTV), correspondingly. Different dosimetric indices, beam-on time, and monitor units (MUs) were evaluated to compare the advantages/disadvantages of each delivery modality. In ensuring the dose-volume constraints for cord and esophagus of the premise, CK, HT, and RA all achieved a sharp conformity index (CI) and a small penumbra volume compared to IMRT. RA achieved a CI comparable to those from CK, HT, and IMRT. CK had a heterogeneous dose distribution in the target as its radiosurgical nature with less dose uniformity inside the target. CK had the longest beam-on time and the largest MUs, followed by HT and RA. IMRT presented the shortest beam-on time and the least MUs delivery. For the body-type lesions, CK, HT, and RA satisfied the target coverage criterion in 6 cases, but the criterion was satisfied in only 3 (50%) cases with the IMRT technique. For the body + pedicle-type lesions, HT satisfied the criterion of the target coverage of ≥90% in 4 of the 6 cases, and reached a target coverage of 89.0% in another case. However, the criterion of the target coverage of ≥90% was reached in 2 cases by CK and RA, and only in 1 case by IMRT. For curative-intent SBRT of isolated thoracic spinal lesions, RA is the first choice for the body-type lesions owing to its delivery efficiency (time); the second choice is CK or HT; HT is the preferential choice for the body + pedicle-type lesions. This study suggests further clinical investigations with longer follow-up for these studied cases.

Oncentra brachytherapy planning system

In modern cancer management, treatment planning has progressed as a contemporary tool with all the advances in computing power in recent years. One of the advanced planning tools uses 3-dimensional (3D) data sets for accurate dose distributions in patient prescription. Among these planning processes, brachytherapy has been a very important part of a successful cancer management program, offering clinical benefits with specific or combined treatments with external beam therapy. In this chapter, we mainly discussed the Elekta Oncentra planning system, which is the main treatment planning tool for high-dose rate (HDR) modality in our facility and in many other facilities in the United States. HDR is a technically advanced form of brachytherapy; a high-intensity radiation source (3.6 mm in length) is delivered with step motor in submillimeter precision under computer guidance directly into the tumor areas while minimizing injury to surrounding normal healthy tissue. Oncentra planning is the key component to generate a deliverable brachytherapy procedure, which is executed on the microSelectron V3 remote afterloader treatment system. Creating a highly conformal plan can be a time-consuming task. The development of Oncentra software (version 4.5.3) offers a variety of useful tools that facilitate many of the clinical challenging tasks for planning, such as contouring and image reconstruction, as well as rapid planning calculations with dose and dose volume histogram analysis. Oncentra Brachy module creates workflow and optimizes the planning accuracy for wide varieties of clinical HDR treatments, such as skin, gynecologic (GYN), breast, prostate, and many other applications. The treatment file can also be transferred to the afterloader control station for speedy delivery. The design concept, calculation algorithms, and optimization modules presented some key characteristics to plan and treat the patients effectively and accurately. The dose distribution and accuracy of several clinical sample cases were discussed to illustrate the effectiveness and clinical efficacy. The American Association of Physicists in Medicine brachytherapy reports of TG-43 and TG-186 were also described and compared in evaluations of fundamental calculation methodologies.

SU‐E‐T‐453: Optimization of Dose Gradient for Gamma Knife Radiosurgery

PURPOSE The goals of stereotactic radiosurgery (SRS) are the ablation of target tissue and sparing of critical normal tissue. We develop tools to aid in the selection of collimation and prescription (Rx) isodose line to optimize the dose gradient for single isocenter intracranial stereotactic radiosurgery (SRS) with GammaKnife 4C utilizing the updated physics data in GammaPlan v10.1. METHODS Single isocenter intracranial SRS plans were created to treat the center of a solid water anthropomorphism head phantom for each GammaKnife collimator (4 mm, 8 mm, 14 mm, and 18 mm). The dose gradient, defined as the difference of effective radii of spheres equal to half and full Rx volumes, and Rx treatment volume was analyzed for isodoses from 99% to 20% of Rx. RESULTS The dosimetric data on Rx volume and dose gradient vs. Rx isodose for each collimator was compiled into an easy to read nomogram as well as plotted graphically. The 4, 8, 14, and 18 mm collimators have the sharpest dose gradient at the 64%, 70%, 76%, and 77% Rx isodose lines, respectively. This corresponds to treating 4.77 mm, 8.86 mm, 14.78 mm, and 18.77 mm diameter targets with dose gradients radii of 1.06 mm, 1.63 mm, 2.54 mm, and 3.17 mm, respectively. CONCLUSION We analyzed the dosimetric data for the most recent version of GammaPlan treatment planning software to develop tools that when applied clinically will aid in the selection of a collimator and Rx isodose line for optimal dose gradient and target coverage for single isocenter intracranial SRS with GammaKnife 4C.

Predictors of Acute Toxicity after HDR Brachytherapy and 3D-CRT for Localized Prostate Cancer: Updated Results of Phase I-II Dose Escalation Study

Purpose: To identify patient and dosimetric variables predicting for > grade 2 acute morbidity after HDR brachytherapy and 3D-CRT in the treatment of prostate cancer. Materials and Methods: 102 men were treated on a Phase I-II dose escalation trial for intermediate/unfavorable prostate cancer. Pts recieved 6–12 mos of androgen deprivation(AD). HDR consisted of 16.5 Gy Ir-192 in 3 fractions in 62 pts, with dose escalation to 18 Gy in 3 fractions in 40 pts. 3D-CRT consisted of 50–54 Gy using a 4–6 field conformal technique. Median f/u time was 12 mos. (range: 6–30 mos). Acute toxicity was assessed using the RTOG morbidity scale. Base-line and sequential IPSS scores, and dosimetric parameters were correlated with acute toxicity. Results: Grade 2 GU toxicity at 1, 3, 6, and 12 months for 55 patients with f/u > 12 months was 26%, 15%, 3% and 9%, respectively. One pt (2%) developed a urethral stricture (grade 3 GU). The max urethral dose and baseline IPSS score correlated with acute GU toxicity. The mean urethral dose among patients experiencing Grade 2 GU toxicity was 2.1 Gy vs 1.9 Gy for Grade 1 toxicity and 1.7 Gy for Grade 0. The mean pre-trt IPSS score in patients with grade 2 toxicity was 11 vs 6 for Grade 0/1. The IPSS post-trt score at 12 mos, normalized in 70%. The incidence of grade 2 rectal bleeding at 1, 3, 6, and 12 months for 55 patients were 7%, 7%, 2%, and 2% respectively. No Grade 3 GI toxicities occurred. Only Pre-trt IPSS score (p = 0.008) was predictive of GU toxicity. Age (p = 0.04), Max-Urethral Dose (p = 0.03), and lymph nodes treated(p = 0.02) were predictive of Grade 2 GI toxicity. In multivariate analysis, IPSS score predicted for GU morbidity, (p = 0.008). No difference was seen in GU or GI symptoms in the two dose levels (p = 0.09, and 0.16 respectively) Conclusion: Excellent acute tolerance was observed. The incidence of acute grade 2/3 morbidity correlated with max urethral dose from HDR-BRT and baseline IPSS scores. Dose escalation is feasible.