Arshin Sheybani

The dosimetric impact of heterogeneity corrections in high-dose-rate 192Ir brachytherapy for cervical cancer: Investigation of both conventional Point-A and volume-optimized plans

PURPOSE To evaluate the dosimetric impact of heterogeneity corrections on both conventional and volume-optimized high-dose-rate (HDR) ¹⁹²Ir brachytherapy tandem-and-ovoid treatment plans. METHODS AND MATERIALS Both conventional and volume-optimized treatment plans were retrospectively created using eight unique CT data sets. In the volume-optimized plans, the clinical target volume (CTV) and organs-at-risk (rectum, bladder, and sigmoid) were contoured on the CT data sets by a single physician. For each plan, dose calculations representing homogeneous water medium were performed using the Task Group (TG-43) formalism and dose calculations with heterogeneity corrections were performed using a commercially available treatment planning system. RESULTS For the conventional plans, the change in dose between TG-43 and heterogeneity-corrected calculations was assessed for the following points: Point-A (left and right) and International Commission on Radiation Units and Measurements (ICRU) 38 defined rectum and bladder points. It was found that the dose to the ICRU bladder decreased the most (-2.2±0.9%), whereas ICRU rectum (-1.7±0.8%), Point-A right (-1.1±0.4%), and Point-A left (-1.0±0.3%) also showed decreases with heterogeneity-corrected calculations. For the volume-optimized plans, the change in dose between TG-43 and heterogeneity-corrected calculations was assessed for the following dose-volume histogram parameters: D(90) of the CTV and D(2cc) of the rectum, bladder, and sigmoid. It was found that D(90) of the CTV decreased by -1.9±0.7% and D(2cc) decreased by -2.6±1.4%, -1.0±0.4%, and -2.0±0.6% for the rectum, bladder and sigmoid, respectively, with heterogeneity-corrected calculations. CONCLUSIONS Heterogeneity corrections on high-dose rate plans were found to have only a small dosimetric impact over TG-43-based dose calculations for both conventional Point-A and volume-optimized plans.

Impact of spot size on plan quality of spot scanning proton radiosurgery for peripheral brain lesions.

PURPOSE To determine the plan quality of proton spot scanning (SS) radiosurgery as a function of spot size (in-air sigma) in comparison to x-ray radiosurgery for treating peripheral brain lesions. METHODS Single-field optimized (SFO) proton SS plans with sigma ranging from 1 to 8 mm, cone-based x-ray radiosurgery (Cone), and x-ray volumetric modulated arc therapy (VMAT) plans were generated for 11 patients. Plans were evaluated using secondary cancer risk and brain necrosis normal tissue complication probability (NTCP). RESULTS For all patients, secondary cancer is a negligible risk compared to brain necrosis NTCP. Secondary cancer risk was lower in proton SS plans than in photon plans regardless of spot size (p = 0.001). Brain necrosis NTCP increased monotonically from an average of 2.34/100 (range 0.42/100-4.49/100) to 6.05/100 (range 1.38/100-11.6/100) as sigma increased from 1 to 8 mm, compared to the average of 6.01/100 (range 0.82/100-11.5/100) for Cone and 5.22/100 (range 1.37/100-8.00/100) for VMAT. An in-air sigma less than 4.3 mm was required for proton SS plans to reduce NTCP over photon techniques for the cohort of patients studied with statistical significance (p = 0.0186). Proton SS plans with in-air sigma larger than 7.1 mm had significantly greater brain necrosis NTCP than photon techniques (p = 0.0322). CONCLUSIONS For treating peripheral brain lesions--where proton therapy would be expected to have the greatest depth-dose advantage over photon therapy--the lateral penumbra strongly impacts the SS plan quality relative to photon techniques: proton beamlet sigma at patient surface must be small (<7.1 mm for three-beam single-field optimized SS plans) in order to achieve comparable or smaller brain necrosis NTCP relative to photon radiosurgery techniques. Achieving such small in-air sigma values at low energy (<70 MeV) is a major technological challenge in commercially available proton therapy systems.

Disease outcomes for skull base and spinal chordomas: A single center experience ,

OBJECTIVE Chordomas carry significant morbidity due to their growth patterns and surgical constraints in resection. En bloc resection, when feasible, is the ideal treatment goal, but is associated with significant morbidity. We sought to elucidate the relationship between extent of surgery, location and radiotherapy in relation to overall disease and progression free survival (PFS). METHODS We reviewed case records for all patients with a primary histopathological diagnosis of clival and spinal chordomas that was presented to our institution between 1978 and 2010. RESULTS A total of 49 patients (location: n=30, skull base/clival; n=12 vertebral column; n=7 sacrum) were identified with mean follow-up period of 6.3 years (range 0.25 months-33 years). Improved 5 year and 10 year survival rates were noted following gross total resection (n=8, 5 year and 10 year survival=88%) as compared to patients that underwent subtotal resection (n=41, 55% and 31%, respectively), (p-value>0.05, GTR versus STR). Adjuvant high-dose stereotactic fractionated radiotherapy (HS-FSRT) significantly improved 5 year PFS in craniocervical chordoma patients (70%, n=13) as compared to standard dose radiation therapy (20%, n=16; p-value=0.03). Overall 10 year survival for craniocervical patients undergoing HD-FSRT (40%) was however not significantly different in comparison with conventional radiotherapy (45%). Sacral chordomas had the worst prognosis with 3 year survival of 28.6%. CONCLUSIONS GTR offers the best prognosis for improved long-term survival. Adjuvant HD FSRT for cranio-cervical/clival chordomas significantly improves disease free survival though the long-term benefits on survival have yet to be established. Sacral chordomas are associated with a worse prognosis and poor long-term survival.

Risk of Radiation-Induced Malignancy With Heterotopic Ossification Prophylaxis: A Case–Control Analysis

PURPOSE To determine the risk of radiation-induced malignancy after prophylactic treatment for heterotopic ossification (HO). METHODS AND MATERIALS A matched case-control study was conducted within a population-based cohort of 3489 patients treated either for acetabular fractures with acetabular open reduction internal fixation or who underwent total hip arthroplasty from 1990 to 2009. Record-linkage techniques identified patients who were diagnosed with a malignancy from our state health registry. Patients with a prior history of malignancy were excluded from the cohort. For each documented case of cancer, 2 controls were selected by stratified random sampling from the cohort that did not develop a malignancy. Matching factors were sex, age at time of hip treatment, and duration of follow-up. RESULTS A total of 243 patients were diagnosed with a malignancy after hip treatment. Five patients were excluded owing to inadequate follow-up time in the corresponding control cohort. A cohort of 238 cases (control, 476 patients) was included. Mean follow-up was 10 years, 12 years in the control group. In the cancer cohort, 4% of patients had radiation therapy (RT), compared with 7% in the control group. Of the 9 patients diagnosed with cancer after RT, none occurred within the field. The mean latency period was 5.9 years in the patients who received RT and 6.6 years in the patients who did not. Median (range) age at time of cancer diagnosis in patients who received RT was 62 (43-75) years, compared with 70 (32-92) years in the non-RT patients. An ad hoc analysis was subsequently performed in all 2749 patients who were not matched and found neither an increased incidence of malignancy nor a difference in distribution of type of malignancy. CONCLUSION We were unable to demonstrate an increased risk of malignancy in patients who were treated with RT for HO prophylaxis compared with those who were not.

SU-E-T-690: Radiosurgery of Peripheral Brain Lesions by Spot Scanning Proton Therapy

PURPOSE To determine how small the lateral width in air of proton spots must be in order for proton spot scanning (SS) plan quality to be superior to x-ray radiosurgery for peripheral brain lesions. METHODS Single-beam proton SS plans with sigma (lateral spot intensity standard deviation) values ranging from 1 to 14 mm, x-ray volumetric modulated arc therapy (VMAT) utilizing a 0.5 cm leaf thickness, and cone-based x-ray radiosurgery plans were generated for eleven patients with clinical target volumes ranging from 0.5 cm3 to 25 cm3. Plans were evaluated using brain necrosis normal tissue complication probability (NTCP), high dose conformity index, and integral dose metrics. RESULTS For proton SS plans, as sigma increased from 1 to 14 mm, integral dose relative to the cone plan increased from an average of 0.10 (range 0.05 to 0.17) to 0.63 (range 0.26 to 0.85) compared an average of 1.49 (range 0.90 to 3.00) for VMAT. As SS sigma increased from 1 mm to 14 mm, NTCP for healthy brain necrosis increased from an average of 0.02 (range 0.01 to 0.07) to 0.08 (range 0.01 to 0.18) compared to the averages of 0.06 (range 0.01 to 0.15) for cone and 0.05 (range 0.01 to 0.11) for VMAT plans. An in-air sigma of less than 6 mm was required for the SS plans to have a statistically significant (p < 0.05) NTCP reduction relative to the x-ray techniques. CONCLUSION When treating peripheral brain lesions, which are ideal cases for SS since only a single proton beam direction is needed, SS only provides a superior NTCP to x-ray radiosurgery techniques if the proton therapy system can produce an in-air sigma of 6 mm or less.

Comparative analysis of uptake for 18F-fluorodeoxyglucose (FDG) versus 18F-fluorodeoxythymidine (FLT) PET scans in rectal cancer.

657 Background: FLT PET is a novel metabolite for imaging cellular proliferation and has shown promise for treatment response monitoring. The utility of FLT PET imaging in rectal adenocarcinomas (RA) has not previously been established. We set out to compare FDG and FLT uptake in RA and correlate FLT response during treatment to tumor regression. Methods: We evaluated 6 patients with RA who were treated on an institutional prospective clinical trial (NCT01717391). Patients were treated neoadjuvantly with concurrent chemoradiation therapy per standard of care. Patients had both FDG and FLT PET scans prior to starting treatment and FLT scans after 1 and 2 weeks of radiotherapy. Tumor volumes and pelvic lymph nodes measuring greater than 6 mm were contoured on the simulation CT scan and adjusted for bladder and rectal filling based on the attenuation correction CT for each PET scan. FLT and FDG SUVs were collected from a total of 40 lymph nodes among the six patients. Pretreatment and change in tumor FLT SUV...

SU‐D‐137‐02: Evaluation of Integrated Equivalent Dose in 2 Gy (EQD2) DVH of EBRT and HDR Plans for Cervical Cancer

PURPOSE To evaluate equivalent dose in 2 Gy (EQD2) parameters from an integrated EQD2 DVH of EBRT and HDR plans and compare them with the estimated values per GEC-ESTRO recommendations. METHODS EBRT plans (Pinnacle, Phillips) consist of 45Gy (1.8Gy/fraction) and, if necessary, a 5.4- 9 Gy (1.8Gy/fraction) parametrial and/or paraaortic boost. An HDR brachytherapy plan (BrachyVision, Varian) was generated per each implant with 27.5Gy/5fractions - 28Gy/4fractions. Fifteen point A plans and six conformal (volume - optimization) plans were retrospectively analyzed. Rectum and bladder were contoured on T2-weighted MRI. An EQD2 DVH tool was developed in-house to convert initial dose maps into EQD2 maps for the generation of EQD2 DVHs. The integrated D2cc and D0.1cc EQD2 values were compared to those calculated per the recommendations: assuming that the OARs receive full prescription dose from the initial EBRT and paraaortic boost, but receive no additional dose from the parametrial boost due to its 4 cm central block. RESULTS All cases having a parametrial boost showed D2cc and D0.1cc EQD2 doses to the rectum and bladder that were higher than those calculated via the recommended method. The impact of this integrated analysis technique on the D2cc EQD2 values for the rectum and bladder was noted to be larger for conformal plans than for point A plans. Up to 28.8% (22.3%) and 22.9% (16.3%) higher D2cc EQD2 values were recorded for the rectum and bladder, respectively, for conformal plans (point A plans). The integrated D2cc EQD2 values for the rectum and bladder were, on average, 9.4 +- 10.1% (7.3 +- 4.7%) and 10.0 +- 6.8% (5.1 +- 4.7%) for conformal plans (point A plans). CONCLUSION The integrated maximum dose to OARs could be considerably underestimated for the rectum and bladder when GEC-ESTRO recommended methodology is made.

SU‐E‐T‐375: The Impact of Heterogeneity Corrections in High Dose Rate Brachytherapy

Purpose: To evaluate the dosimetric impact of heterogeneity corrections on both conventional and volume‐optimized high dose rate (HDR) tandem‐and‐ovoid (TO rectum, bladder, and sigmoid) were contoured on the CT datasets by a single physician. Dose calculations with and without heterogeneity corrections were then performed for both the conventional Point‐A based plans and the volume‐optimized plans using Brachyvision v8.9 (Varian) along with Acuros: a heterogeneity correction application available in Brachyvision. Results: For the conventional plans, the change in dose to Point‐A (left and right) and ICRU 38 defined rectum and bladder points was assessed after applying the heterogeneity correction with Acuros. It was found that the dose to the ICRU bladder decreased the most (− 2.2±0.9%) while ICRU rectum (−1.7±0.8%), Point‐A right (−1.1±0.4%), and Point‐A left (−1.0±0.3%) also showed decreases. For the volume‐optimized plans, dose‐volume‐histogram (DVH) parameters (D90 of CTV and D2cc of rectum, bladder, and sigmoid) were evaluated before and after applying the heterogeneity correction with Acuros. It was found that D90 of the CTV decreased by −1.9±0.7% and D2cc decreased by −2.6±1.4%, −1.0±0.4%, and −2.0±0.6% for the rectum, bladder and sigmoid, respectively. Conclusions: Heterogeneity corrections on T&O brachytherapy plans were found to have only a small dosimetric impact over TG‐43 based dose calculations for both conventional Point‐A and 3D imaging based volume‐optimization plans.