Total Reference Air Kerma (TRAK) is Associated with Dosimetric Parameters in Template-Based High Dose-Rate (HDR) Interstitial Brachytherapy in Advanced Gynecologic Cancers

PURPOSE/OBJECTIVE(S)

To study TRAK and its association with dosimetric parameters in template-based high dose-rate interstitial brachytherapy in advanced gynecologic cancers.

MATERIALS/METHODS

Brachytherapy treatment plans of 53 patients treated between 2012 and 2022 at our institution with template-based Iridium-192 HDR brachytherapy, post-external beam RT, for locally advanced cancers of the cervix and vagina were retrospectively reviewed. Brachytherapy dose ranged from 25 to 30-Gy delivered in 4 to 6 fractions. The median number of flexi-guide catheters implanted was 18 (range 10-30). Clinical Target Volume (CTV) values were mean (±SD): 72.2 (±40.4) cm3 (high-risk, HR) and 182.2 (±73.7) cm3 (intermediate-risk, IR) respectively. TRAK per fraction (cGy at 1m), dose-volume information for the implant, target, and organ-at-risk (OAR) were recorded. Indices for dose coverage (CI), homogeneity (DHI), non-uniformity (DNR), overdose volume (ODI) were computed. Regression and correlation tests were used to study the TRAK relationship with various dosimetric parameters. The false discovery rate at a 5% level was corrected using the Benjamini-Hochberg procedure.

RESULTS

The average TRAK per fraction was 0.365 (±0.12) cGy. Mean and range values of plan quality indices were - CI 0.92 (0.7- 1.0), DHI 0.57 (0.41 - 0.77), DNR 0.43 (0.23 - 0.59) and ODI 0.22 (0.11 - 0.38), respectively Correlation results for TRAK with various dosimetric indices are presented in Table 1. TRAK showed a weak correlation with the number of flexi-guide catheters implanted (r = 0.35, p = 0.013). TRAK correlated strongly with target volumes (CTV_HR and CTV_IR and CTV_HR V100%) and with isodose volumes at both high (V300, V200, V150), and low dose levels (V90, V85, V50) (p<0.00001). TRAK correlated moderately with OAR 2-cm3 doses (p<0.00001). A weak correlation was observed between TRAK and plan quality indices.

CONCLUSION

TRAK correlates positively with target volume and volumes enclosed by isodoses at various dose levels in interstitial HDR brachytherapy of advanced Gynecologic Cancers. Interestingly, our study observed a comparatively stronger positive correlation between TRAK and Sigmoid 2-cm3 dose, equated to TRAK correlation with bladder, rectum, and small bowel 2-cm3 doses. This finding could interest future studies utilizing TRAK as a surrogate for treatment outcome and toxicity.

Pulsed Reduced Dose Rate Re-Irradiation for Recurrent Grade 4 Gliomas: A Retrospective Analysis of Safety and Efficacy

PURPOSE/OBJECTIVE(S)

Despite maximal treatment, nearly all patients with grade 4 gliomas develop recurrent disease. Treatment options for these patients are limited and overall survival is poor. Re-irradiation may be considered in certain patients, though risk of side effects often limits the effective dose able to be delivered. Pulsed reduced dose rate (PRDR) radiation is a treatment technique that reduces effective dose rate and increases treatment time allowing for intrafraction repair. Here, we report safety and efficacy of PRDR re-irradiation for recurrent grade 4 gliomas.

MATERIALS/METHODS

We performed a retrospective review of patients treated with PRDR between 2001 and 2022. Patients were treated with reduced dose rate radiation delivered in 0.2 Gy pulses every 3 minutes in 2 Gy daily fractions. Both 3D conformal and step and shoot IMRT radiation plans were utilized. Toxicities were evaluated based on Common Terminology Criteria for Adverse Events (CTCAE) v5.0 criteria. Kaplan Meier analysis was used to calculate overall survival (OS). Cox regression analysis was performed for multivariate analysis.

RESULTS

A total of 168 grade 4 glioma patients treated with PRDR re-irradiation were identified. The median age was 55 years old. The median initial radiation dose was 60 Gy (range 36 Gy - 72 Gy) and the median PRDR dose was 54 Gy (range 37.5 - 60 Gy). Seventy percent of patients received systemic therapy for recurrent disease prior to PRDR, while 30% received PRDR as first treatment for recurrent disease (or following re-resection without other treatment). The median survival following PRDR was 6.3 months. Multivariate analysis showed time since initial radiation of 14+ months (HR 0.66, p = 0.005, 95% CI 0.44 - 0.98), pre-PRDR use of steroids (HR 1.78, p = 0.005, 95% CI 1.2 - 2.66), and Karnofsky performance status of 70 or greater to be a significant predictor of survival (HR = 0.6, p = 0.008, 95% CI 0.44 - 0.98). No grade 4 or 5 toxicity was noted. Grade 3 new onset seizures was noted in 6% of patients, all subsequently controlled with medication. The most common grade 1-2 side effect after treatment was fatigue.

CONCLUSION

In this large, retrospective cohort, PRDR re-irradiation for recurrent grade 4 gliomas was well tolerated with low rates of grade 3 toxicity. Overall survival outcomes were encouraging, especially in heavily pre-treated patients. Prospective studies are ongoing to further evaluate the efficacy of PRDR for recurrent glioma treatment.

Clinical Experience with Commissioning a GRID Collimator for Spatially Fractionated Radiation Therapy

PURPOSE/OBJECTIVE(S)

In this study, we report our physics commissioning experience for clinically implementing a GRID collimator based Spatially Fractionated Radiation Therapy (SFRT).

MATERIALS/METHODS

For SFRT commissioning, we utilized a commercially available brass GRID collimator (DotDecimal, Sanford, FL) that is mounted on a blocking tray for positioning into the accessory mount of the Linac. The brass GRID is 7.62 cm thick and weighs 15.8 kg. The GRID has a total of 149 divergent holes arranged in a hexagonal pattern (1.43 cm in diameter and hole-centers spaced at 2.11 cm when projected at the isocenter distance). The GRID encompasses a maximum field size of 25 × 25 cm at the isocenter. A commercial 48 × 48 × 48 cm water phantom scanning system was used to collect GRID output factors (GRID field to open field ratio), depth dose and beam profile data. Output measurements were performed using a 0.13 cm3 active volume ion-chamber and beam scans were obtained with a diode detector. Data was collected for both flattening and flattening-free beams of nominal energies 6 MV and 10 MV photons. The measurement depths were at dmax (1.5 cm for 6 MV and 2.5 cm for 10 MV), 5-cm and 10-cm respectively. For each energy and depth of measurement, collimator settings were varied from 5 × 5 cm to 28 × 28 cm. From scan profiles at different depths, the valley (lowest) to peak (highest) dose ratios (VPDR) were calculated. A commercial treatment planning system (TPS) was used to test the accuracy of dose calculations with GRID. This was accomplished by importing vendor generated DICOM RT file into the TPS. A block transmission factor of 7% for 6 MV and 10.2% for 10 MV energy beams were applied. All measured data were compared with corresponding TPS calculated data. Test patient treatment plans with GRID were created in TPS and planned distributions were verified using a commercial detector array with 2.5 mm detector spacing.

RESULTS

The VPDR, expressed as %, are presented in Table 1. Measured and TPS calculated output factors agreed within 2% for 6 MV and within 3% for 10 MV photon beams. Percent depth doses were lower in magnitude for GRID field compared to open field for all energies studied (for e.g., 6 MV depth dose at 10 cm depth for 10 cm x10cm field 62% for GRID field vs. 66% for open field). Measured and calculated GRID beam profiles agreed within 5% dose difference and 1 mm distance-to agreement. For all test cases, the planned vs. measured dose distributions passed at an average gamma passing rate of 96.5% using a 3% dose difference/ 3 mm distance to agreement and 10% threshold criteria.

CONCLUSION

The Dot Decimal GRID collimator provides a simple way of achieving SFRT in the clinic, albeit heavy to use and has an irradiation field size limitation of 25 cm × 25 cm.

A dosimetric analysis of tomotherapy based intensity modulated radiation therapy with and without bone marrow sparing in gynecologic malignancies

Whole pelvic radiotherapy with concurrent chemotherapy is the standard of care for locally advanced cervical carcinoma. Published literature reports that the pelvic bone marrow (BM) dosimetric parameters of V10 > 90% and V20 > 80% are associated with higher rates of hematologic toxicities using this approach. Here, we investigate the ability of Tomotherapy based intensity modulated radiation therapy (IMRT) to reduce dose to pelvic BM while evaluating dose distribution to critical structures and planning target volume (PTV) coverage. Ten patients were selected for analysis. Normal structures, whole pelvic BM, PTV contours, and IMRT objects were standardized. Two whole pelvis Tomotherapy plans were created for each patient, one standard plan, and one with the addition of a BM sparing (BMS) constraint (V10 <85%, V20 < 80%). Data were calculated from multiple points with regard to BM dose, normal structure dose, and PTV coverage. Differences in dose distributions between the two sets of plans were analyzed using a paired t-test. The addition of a BMS planning constraint resulted in significant decreases in pelvic BM dose at the following dosimetric points: V5, V10, V15, V20, V30, V40, V50, and mean dose (p < 0.05 for all points). There were no significant differences in dose to small bowel, bladder or rectum, with the exception of one data point (small bowel V30, p = 0.004) between the two sets of plans. There was no sacrifice of PTV coverage or loss of homogeneity with the addition of a BMS planning constraint. BMS-IMRT significantly reduces radiation dose to the pelvic BM while maintaining the ability to spare dose to the small bowel, bladder and rectum. The planning constraints were met without violation of study criteria, and without sacrifice of PTV coverage. Further investigation is warranted to determine if rates of hematologic toxicity improve with utilization of Tomotherapy based BMS-IMRT.

Dose escalated, hypofractionated radiotherapy using helical tomotherapy for inoperable non-small cell lung cancer: preliminary results of a risk-stratified phase I dose escalation study

To improve local control for inoperable non-small cell lung cancer (NSCLC), a phase I dose escalation study for locally advanced and medically inoperable patients was devised to escalate tumor dose while limiting the dose to organs at risk including the esophagus, spinal cord, and residual lung. Helical tomotherapy provided image-guided IMRT, delivered in a 5-week hypofractionated schedule to minimize the effect of accelerated repopulation. Forty-six patients judged not to be surgical candidates with Stage I-IV NSCLC were treated. Concurrent chemotherapy was not allowed. Radiotherapy was delivered via helical tomotherapy and limited to the primary site and clinically proven or suspicious nodal regions without elective nodal irradiation. Patients were placed in 1 of 5 dose bins, all treated for 25 fractions, with dose per fraction ranging from 2.28 to 3.22 Gy. The bin doses of 57 to 80.5 Gy result in 2 Gy/fraction normalized tissue dose (NTD) equivalents of 60 to 100 Gy. In each bin, the starting dose was determined by the relative normalized tissue mean dose modeled to cause < 20% Grade 2 pneumonitis. Dose constraints included spinal cord maximum NTD of 50 Gy, esophageal maximum NTD < 64 Gy to < or = 0.5 cc volume, and esophageal effective volume of 30%. No grade 3 RTOG acute pneumonitis (NCI-CTC v.3) or esophageal toxicities (CTCAE v.3.0 and RTOG) were observed at median follow-up of 8.1 months. Pneumonitis rates were 70% grade 1 and 13% grade 2. Multivariate analysis identified lung NTD(mean) (p=0.012) and administration of adjuvant chemotherapy following radiotherapy (p=0.015) to be independent risk factors for grade 2 pneumonitis. Only seven patients (15%) required narcotic analgesics (RTOG grade 2 toxicity) for esophagitis, with only 2.3% average weight loss during treatment. Best in-field gross response rates were 17% complete response, 43% partial response, 26% stable disease, and 6.5% in-field thoracic progression. The out-of-field thoracic failure rate was 13%, and distal failure rate was 28%. The median survival was 18 months with 2-year overall survival of 46.8% +/- 9.7% for this cohort, 50% of whom were stage IIIB and 30% stage IIIA. Dose escalation can be safely achieved in NSCLC with lower than expected rates of pneumonitis and esophagitis using hypofractionated image-guided IMRT. The maximum tolerated dose has yet to be reached.

Acute radiation esophagitis in a phase I dose-escalation trial evaluating helical tomotherapy for inoperable non-small cell lung cancer (NSCLC)

7694

Background: In an attempt to improve the poor local control in patients with locally advanced and medically inoperable lung cancer we investigated helical tomotherapy as a means to decrease radiation dose to critical normal structures while escalating tumor dose. Methods: Thirty-six patients with Stage I-IV NSCLC requiring definitive radiotherapy and judged not to be surgical candidates due to medical comorbidities or advanced tumor stage were enrolled in this phase I trial. Chemotherapy was given sequentially but not allowed concurrently. Pts were placed in 1 of 4 dose-per-fraction bins, all treated for 25 fractions, with dose per fraction ranging from 2.28 to 3.22 Gy. Bins were calculated to yield normalized total dose (NTD) equivalents in 2 Gy fractions of 60–100 Gy. Dose escalation limiting constraints included esophageal maximum NTD, lung NTD mean, and esophageal effective volume (Veff). Radiotherapy was limited to the primary site and clinically proven or suspicious nodal regions. Elective nodal irradiation (ENI) was not performed. A graded-response logistic regression analysis was performed to identify dosimetric and clinical factors associated with esophagitis. Results: No grade 3 (CTCAE v3.0 and RTOG) acute esophageal toxicities were observed. Only 6 patients (16.7%) required narcotic analgesia (RTOG gr. 2 toxicity). Average mean NTD dose to the esophagus (Dmean)=15.3 Gy. Average volume of the esophagus receiving NTD 55 Gy (V55)=3.65%. Both Dmean and V55 were significantly associated with the grade of esophagitis, with P- values of 0.018 and 0.010, respectively. In this limited series, there was no significant effect of sequential chemotherapy, in agreement with previous studies. Conclusions: 1) Acute esophageal toxicity was minimal despite dose escalation via helical tomotherapy for inoperable NSCLC cancer. 2) The observed significant associations between V55, Dmean, and grade of esophagitis validate improved dose distribution as a strategy to permit radiation dose escalation. Pending acceptable rates of radiation pneumonitis the addition of concurrent chemotherapy is anticipated. Supported by NIH Grant CA-88960.

No significant financial relationships to disclose.

Tumor volume changes on serial imaging with megavoltage CT for non-small cell lung cancer during conformal radiotherapy

17041

Background: Adaptive radiotherapy allows treatment plan modification based on data obtained during treatment. Assessing volume changes during treatment is now possible with intra-treatment imaging capabilities on radiotherapy devices. This study assesses non-small cell lung cancer (NSCLC) volume changes during treatment with conformal intensity-modulated radiotherapy by evaluating serial megavoltage CT (MVCT) scans. Methods: MVCTs were retrospectively reviewed for 25 patients treated with the TomoTherapy Hi-Art system at the University of Wisconsin. Twenty-one patients received definitive radiotherapy, 4 with extracranial stereotactic radioablation (60 Gy in 5 fractions) and 17 on a dose-per-fraction escalation protocol (57–80.5 Gy in 25 fractions). Four patients were treated palliatively (22–30 Gy in 8–10 fractions). Gross tumor volumes (GTVs) were contoured on serial MVCTs at weekly intervals, by individuals other than the treating physician, to minimize bias. Each patient had 3–25 scans including one at the beginning, mid-way, and one at the end of treatment. Initial GTVs ranged from 1.4 - 565.5 cm3 (mean = 70 cm3). Results: At completion of treatment, no patient demonstrated a complete response (CR). Partial response (PR) defined as a >65% decrease in tumor volume occurred in 3 (12%) and marginal response (MR) defined as a 35 - 65% reduction in tumor volume was noted in 5 (20%). The remaining 17 patients (68%) showed stable disease (SD) defined as <35% reduction or <40% increase in size. The minimum “scorable threshold” for volume discrepancy between scans to account for inter-scan assessment variability was set at >25% volume change; 10 patients (40%) had >25% tumor regression. None of the patients treated ablatively or palliatively showed tumor regression during treatment. Conclusions: Although gross tumor regression during treatment may be objectively measured using MVCTs, substantial volumetric decrease occurs only in a minority. The clinical significance of this regression is questionable, as there is no way to document histologic tumor clearance and therefore field reductions during radiotherapy cannot be recommended.

[Table: see text]

Daily set up variation in patients receiving radiation with tomotherapy for lung tumors

17005

Background: The on-board megavoltage (MV) computed tomography (CT) capabilities of a Tomotherapy unit were used to obtain the daily MVCT images of lung cancer patients. For daily patient alignment, differences between the MVCT scan and planning CT were resolved by calculating the necessary couch shifts in the X = mediolateral, Y = craniocaudal, and Z = anteriorposterior directions. Daily shifts were analyzed. Methods: 583 alignments from 36 patients with lung cancer were available for analysis. The systematic (Σ) and random (σ) errors were calculated and a covariate analysis was performed with tumor size, Karnofsky Performance Score (KPS), and presence of atelectasis. Two error minimization strategies were applied to the data - 1) shifts from fraction 1 were subtracted from subsequent shifts, and 2) the average of shifts 1–3 were subtracted from shifts 4 onward. Σ and σ were calculated for each of the 3 data sets and applied to van Herk’s margin recipe 2.5 Σ + 0.7σ. The mean, standard deviation, and standard error of the magnitude shifts for 13 patients who each received 23 fractions were analyzed by Spearman’s rank correlation test for the relationship between shift magnitude and fraction number. Results: The presence of atelectasis was significantly related to a smaller σ in millimeters, 2.8 ± 0.08 vs. 3.5 ± 0.09 (p = 1.1 × 10−8). The other covariates were not significantly related to set-up error. The 2nd error minimization strategy decreased Σ in the X, Y, and Z directions from 4.7 ± 0.6, 5.8 ± 0.9, 4.9 ± 0.6 to 2.1 ± 0.1, 4.2 ± 0.5, 3.4 ± 0.3 (p = 2.0 × 10−5, 0.13, 0.02) respectively. Calculated margins from van Herk’s equation for all data reported as (x, y, z) in mm were (13.8, 19.6, and 15.9). For strategies 1 and 2 respectively, calculated margins were reduced by (27.2%, 11.5%, 10.6%) and (46.7%, 21.5%, 23.2%). The mean magnitude of isocenter shift and the standard deviation were found to increase with fraction number (p = 1.0 × 10−6 and 5.0 × 10−5 respectively). Conclusion: The error correction strategies significantly reduced Σ but did not reduce the margins dramatically. Drift in accuracy during a long treatment course and an inability to identify subgroups of patients based on our covariates who may not need daily imaging suggests that daily image verification + correction will help reduce error and margins.

No significant financial relationships to disclose.

Feasibility report of stereotactic body radiotherapy with tomotherapy for early stage medically inoperable lung cancer using extreme hypofractionation

17097

Background: This is a report on the technical feasibility, dosimetric aspects, and daily image-guidance capability with megavoltage computed tomography (MvCT) imaging of stereotactic body radiotherapy (SBRT) using tomotherapy for patients with medically inoperable early stage non-small cell lung cancer (NSCLC). Methods: Treatment planning was performed using 4D PET-CT in a BodyFix device. A pilot group of 4 patients with <5 cm T1/2, N0 tumors, was treated using IMRT (tomotherapy), utilizing daily image guidance with MvCT. Patients received 60 Gy within 10 days, delivered in five 12Gy fractions. The primary endpoint of this study was evaluation of acute and sub-acute treatment related toxicities. The secondary endpoint was radiographic evaluation for objective tumor response. Results: All 20 fractions were successfully delivered. MvCT provided excellent tumor visualization for daily image guidance. No significant tumor regression was observed in any patient during the 2 weeks of therapy, using volumetric contouring on MvCT images. The relevant dosimetric aspects, measured as median normalized total dose (NTD) were as follows: tumor = 116.5 Gy10; whole lung = 6.8 Gy3. Maximum fraction-size equivalent dose (FED) demonstrated median values as follows: esophagus = 4.04 Gy3; spinal cord = 6.53 Gy3. All patients are alive at a median follow-up of 6 months, with no grade 2 or higher acute pulmonary toxicities. Early radiographic follow-up demonstrates all 4 patients experienced a partial response. The mean tumor regression is 79% (range 64–85%). Conclusions: 4D PET-CT permits precise target delineation. In this, the first reported clinical experience, IMRT delivery of SBRT using tomotherapy is feasible, safe and free of major acute toxicities. The conformality which is obtained by helical delivery of the therapeutic dose makes it well-suited for dose escalation through extreme hypofractionation, with extremely low mean NTD to normal tissue. The ability to obtain on-treatment megavoltage CT scans to confirm tumor localization is a substantial advantage. Further studies and longer follow-up are warranted to evaluate the efficacy of this treatment modality.

[Table: see text]

A high-precision system for conformal intracranial radiotherapy

PURPOSE

Currently, optimally precise delivery of intracranial radiotherapy is possible with stereotactic radiosurgery and fractionated stereotactic radiotherapy. We report on an optimally precise optically guided system for three-dimensional (3D) conformal radiotherapy using multiple noncoplanar fixed fields.

METHODS AND MATERIALS

The optically guided system detects infrared light emitting diodes (IRLEDs) attached to a custom bite plate linked to the patient's maxillary dentition. The IRLEDs are monitored by a commercially available stereo camera system, which is interfaced to a personal computer. An IRLED reference is established with the patient at the selected stereotactic isocenter, and the computer reports the patient's current position based on the location of the IRLEDs relative to this reference position. Using this readout from the computer, the patient may be dialed directly to the desired position in stereotactic space. The patient is localized on the first day and a reference file is established for 5 different couch positions. The patient's image data are then imported into a commercial convolution-based 3D radiotherapy planning system. The previously established isocenter and couch positions are then used as a template upon which to design a conformal 3D plan with maximum beam separation.

RESULTS

The use of the optically guided system in conjunction with noncoplanar radiotherapy treatment planning using fixed fields allows the generation of highly conformal treatment plans that exhibit a high degree of dose homogeneity and a steep dose gradient. To date, this approach has been used to treat 28 patients.

CONCLUSION

Because IRLED technology improves the accuracy of patient localization relative to the linac isocenter and allows real-time monitoring of patient position, one can choose treatment-field margins that only account for beam penumbra and image resolution without adding margin to account for larger and poorly defined setup uncertainty. This approach enhances the normal tissue sparing, high degree of conformality, and homogeneity characteristics possible with 3D conformal radiotherapy.