Treatment Isodose Research Articles

Local control and recurrence patterns after stereotactic irradiation delivered in more than 4 fractions for hepatocellular carcinomas and liver metastases: a retrospective study

BackgroundStereotactic Body Radiation Therapy (SBRT) is a safe and effective treatment for liver metastases or hepatocellular carcinoma (HCC) with a dose–response relationship for local control (LC). Proximity to organs at risk (OAR) often requires dose de-escalation. This study evaluated LC and recurrence patterns in patients administered hepatic SBRT in more than 4 fractions due to dosimetric constraints.MethodsThis retrospective study included 33 patients treated with SBRT (Cyberknife®) in more than 4 fractions for HCC or liver metastases, between January 2011 and December 2019. Patients were ineligible for treatment in 3 or 4 fractions due to OAR proximity. Recurrence patterns were analysed according to the volume shared between recurrence and initial target or treatment isodose volumes.ResultsThe primary dose ranged from 35 to 50 Gy delivered in 5 to 7 fractions for the treatment of HCC (39%) or liver metastases (61%) mainly secondary to colorectal cancer (40%).LC rate was 64%, with 12 patients showing recurrence volume overlap with the initial target volume or treatment isodose. In-field recurrence occurred in only 12.5% of patients with most relapses being out-of-field. No grade ≥ 3 events were reported.ConclusionDespite dose reductions to spare OAR, SBRT showed satisfactory LC with low toxicity. Out-of-field recurrence remains the most common pattern identified and likely related to underlying disease. Prospective data are necessary to determine whether preserving dose while reducing planning target volume (PTV) coverage could enhance LC.Trial registrationAll patients were retrospectively registered, and this study is registered at the Health Data Hub site (number HDH414).

Interpreting the patterns of local failure following postoperative volumetric-modulated arctherapy in oral cavity and oropharynx cancers: Impact of the different methods of analysis

PurposeIntensity-modulated radiation therapy or volumetric-modulated arctherapy is nowadays the recommended radiation technique for the treatment of head and neck cancers. However, by providing a significant dose gradient between target volumes and organs at risk, there is a risk of target missing and thus recurrence in case of inadequate delineation. It is therefore necessary to determine the origin of these recurrences to improve clinical practice. Over the past years, different methods have been described for the analysis of recurrences. Using the patterns of failure of patients with oral cavity and oropharynx carcinoma, treated with postoperative volumetric-modulated arctherapy in our institution, the purpose of this work was to analyse the sites of local recurrences and to evaluate the disparity in the classification of recurrences when different methods were used. Material and methodsBetween 2011 and 2019, 167 patients who underwent postoperative volumetric-modulated arctherapy for oral cavity or oropharyngeal cancers were included (60 and 40 % respectively). Two or three dose levels were prescribed (54Gy, 59.4/60Gy±66Gy). Local recurrence occurred in 17 patients (10.2 %). We assessed the patterns of local recurrences according to four methods: 1/ volume-based method using the volume overlap between the recurrence volume and initial target volumes; 2/ volume-based method of overlap between the recurrence volume and the 95 % treatment isodose; 3/ point-based method using the position of the barycentre of the recurrence volume; 4/ combined centroid method classifying recurrences according to both the initial target volumes and dose distribution. Each case was reviewed to make a clinical judgment on these classifications and assessed them as “appropriate”, “possible”, or “inappropriate”. ResultsFor the volume-based method using overlap between the recurrence volume and the initial clinical target volume, this classification was clinically judged as inappropriate in 11 out of 17 cases (65 %). For the volume-based method using overlap between the recurrence volume and the 95 % prescribed isodose, this classification was clinically judged as appropriate in 15 out of 17 cases (88 %). For the point-based method, this classification was clinically judged as appropriate in 14 out of 17 cases (82 %). Thirteen out of 17 local recurrences had the same classification between this point-based method and the volume-based method of overlap between the recurrence volume and the 95 % prescribed isodose. For the combined centroid method, among 17 local recurrences nine were classified as type A, two as type B, two as type C, three as type D and one as type E. This classification was clinically judged as appropriate in 15 out of 17 cases (88 %). Only five out of 17 of the local recurrences were classified the same way according to the four different methods (29 %). ConclusionRecurrences that are “marginal” or “outfield” represent a major challenge for intensity-modulated radiation therapy/volumetric-modulated arctherapy quality assurance and improvement of delineation recommendations. To date, there are no published methods that give complete satisfaction.

Ophthalmic applicator displacement as a method of treating large diffuse uveal melanomas.

The presentation of results of an ophthalmic plaque displacement as a brachytherapy treatment method of large diffuse uveal melanomas. This was a retrospective analysis of treatment results of 9 patients with large diffuse uveal melanomas using ophthalmic plaque displacement. Patients were treated with this method in our center between 2012 and 2021 (last follow-up visit in 2023). To achieve appropriate radiation dose distribution for large tumors with a base greater than 18 mm, brachytherapy (106Ru in 7 patients and 125I in 2 patients) with applicator displacement was used as primary treatment. Median follow-up was 2.9 years, and for patients with positive primary treatment results, it was 1.7 months. Median time to local relapse was 2.3 years. In 5 patients, a positive result of local treatment was obtained, out of whom, one patient underwent enucleation due to complications. In the next 4 cases, local recurrence developed. In all tumors, the use of applicator displacement method caused that planning target volume (PTV) was effectively covered with treatment isodose. Brachytherapy with ocular applicator displacement allows for the treatment of tumors with base measurements larger than 18 mm. The application of this method may be considered as an alternative for eye enucleation in particular cases of large diffuse tumors, such as a neoplasm of the eye with vison, or when a patient does not consent to enucleation.

Patient and Dosimetric Predictors of Genitourinary and Gastrointestinal Toxicity After Prostate SBRT: A Secondary Analysis of a Prospective, Multi-Institutional Phase II Trial

Stereotactic body radiation therapy (SBRT) is an emerging treatment option for localized prostate cancer. However, optimal patient selection & normal tissue tolerances for prostate SBRT are not well defined. We therefore analyzed patient factors & dosimetric correlates for genitourinary (GU) and gastrointestinal (GI) toxicity on a multi-institutional prospective trial.Patients (pts) with low- and intermediate-risk prostate cancer were enrolled on a phase II trial from 2011-2020. A dose of 36.25 Gy in 5 fractions to the PTV and 40 Gy to the prostate gland was administered on non-consecutive days. No androgen deprivation therapy or rectal spacer were used. Pts received a CT simulation with Foley catheter for urethral delineation. The clinical target volume was prostate (low-risk) or prostate plus 2cm of proximal seminal vesicles (intermediate-risk). Multiple dosimetric measures for bladder, urethra, & rectum were prospectively recorded. The maximum acute & late GU and GI toxicities using CTCAE v.3.0 were assessed at protocol-specific time points. Linear logistic regression was used to assess factors associated with Grade 1+ GI & GU toxicities. A multivariate (MV) model was constructed using all covariates with P-values < 0.2 on univariate (UV) analysis.A total of 160 evaluable men were enrolled. Median age was 67.9 years (IQR 62.6-73.2), 60 men (37.5%) were low risk, and 100 (62.5%) were intermediate risk. Median follow-up was 2.2 years. No patient factor or dosimetric correlate predicted acute Grade 1+ GU toxicity (all tested covariates P > 0.2). Treatment isodose line (P = 0.065) & age (P = 0.098) reached borderline significance for acute Grade 1+ GI toxicity on UV analysis, though neither were significant on MV analysis (P = 0.057 and P = 0.103, respectively). For late Grade 1+ GU toxicity, prior TURP, bladder V37(cc), bladder D10%, baseline AUA, & treatment isodose line reached borderline significance on UV analysis. On MV analysis, treatment isodose line trended towards significance at P = 0.055. For late Grade 1+ GI toxicity, rectum D20%, rectum D10%, rectum D5%, & history of coronary artery disease reached borderline significance on UV analysis, & a history of coronary artery disease predicted Grade 1+ GI toxicity on MV analysis (odds ratio 4.7, 95% CI 1.37-15.9, P = 0.009).No patient factor or dosimetric correlate predicted acute GI or GU toxicity after prostate SBRT. Late GU toxicity is possibly attributable to dose inhomogeneity. A history of coronary artery disease predicts for late GI toxicity, possibly due to anticoagulation.

Bacterial Nanocellulose Membrane As Bolus in Radiotherapy: “Proof of Concept”

Bolus characterize materials with an electromagnetic radiation attenuation coefficient similar to biological tissue and which are positioned on the skin of patients during radiotherapy treatment. Such devices are responsible for the superficialization of the maximum dose of high energy photons and electrons, restricting its penetration and improving the coverage of the treatment isodose. The development of new materials, mainly from sustainable biotechnological routes, will contribute to increasing efficiency and expanding the use of these technologies. The objective of this research was to develop the “proof of concept” regarding the use of the bacterial nanocellulose membrane (BNCm) as a bolus for radiotherapy procedures. For this purpose, 1.4 cm thick BNCm were produced from static cultivation of the K. rhaeticus, purified and subjected to the physical‐chemical characterization process by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT‐IR). The samples were then subjected to the determination of the radiological density (RD), by computed tomography, and the radiation attenuation potential (%), after being irradiated with 6MV photons in a linear accelerator using ionization and phantom chamber at doses of 100 and 50 UM. The results were compared with data from the analyzes in a commercial bolus (CB) with same thickness. The physical‐chemical analysis demonstrated the constitution of a pure, highly hydrated, homogeneous and nanostructured network of cellulose fibers. The mean RD of BNCm was 1.1 and 5 Hounsfield units respectively, demonstrating high water (0HU) equivalence from BNCm. The values corresponding to the radiation attenuation at the dose of 100 MU were 5.1 ± 0.1% and 4.6 ± 0.2%, for CB and BNCm respectively. The values for the dose of 50 MU were 3.8 ± 0.1% and 3.0 ± 0.2%. In both doses, the attenuation of BNCm was similar to the CB. It stands out the superiority of the BNCm moldability and the possibility to change its composition to improve the radiation attenuation. The results support the concept of use the BNCm as an effective biotechnological device, align to the sustainability idea, as a bolus for radiotherapy treatment.Support or Funding InformationI.P.F.B. Indústria de Produtos Farm. e Biotecnológicos Ltda.

SU‐C‐BRB‐01: Automated Dose Deformation for Re‐Irradiation

Purpose:An objective of retreatment planning is to minimize dose to previously irradiated tissues. Conventional retreatment planning is based largely on best‐guess superposition of the previous treatment's isodose lines. In this study, we report a rigorous, automated retreatment planning process to minimize dose to previously irradiated organs at risk (OAR).Methods:Data for representative patients previously treated using helical tomotherapy and later retreated in the vicinity of the original disease site were retrospectively analyzed in an automated fashion using a prototype treatment planning system equipped with a retreatment planning module (Accuray, Inc.). The initial plan's CT, structures, and planned dose were input along with the retreatment CT and structure set. Using a deformable registration algorithm implemented in the module, the initially planned dose and structures were warped onto the retreatment CT. An integrated third‐party sourced software (MIM, Inc.) was used to evaluate registration quality and to contour overlapping regions between isodose lines and OARs, providing additional constraints during retreatment planning. The resulting plan and the conventionally generated retreatment plan were compared.Results:Jacobian maps showed good quality registration between the initial plan and retreatment CTs. For a right orbit case, the dose deformation facilitated delineating the regions of the eyes and optic chiasm originally receiving 13 to 42 Gy. Using these regions as dose constraints, the new retreatment plan resulted in V50 reduction of 28% for the right eye and 8% for the optic chiasm, relative to the conventional plan. Meanwhile, differences in the PTV dose coverage were clinically insignificant.Conclusion:Automated retreatment planning with dose deformation and definition of previously‐irradiated regions allowed for additional planning constraints to be defined to minimize re‐irradiation of OARs. For serial organs that do not recover from radiation damage, this method provides a more precise and quantitative means to limit cumulative dose.This research is partially supported by Accuray, Inc

The use of isodose levels to interpret radiation induced lung injury: a quantitative analysis of computed tomography changes.

Patients treated with stereotactic body radiation therapy (SBRT) for lung cancer are often found to have radiation-induced lung injury (RILI) surrounding the treated tumor. We investigated whether treatment isodose levels could predict RILI. Thirty-seven lung lesions in 32 patients were treated with SBRT and received post-treatment follow up (FU) computed tomography (CT). Each CT was fused with the original simulation CT and treatment isodose levels were overlaid. The RILI surrounding the treated lesion was contoured. The RILI extension index [fibrosis extension index (FEI)] was defined as the volume of RILI extending outside a given isodose level relative to the total volume of RILI and was expressed as a percentage. Univariate analysis revealed that the planning target volume (PTV) was positively correlated with RILI volume at FU: correlation coefficient (CC) =0.628 and P<0.0001 at 1(st) FU; CE =0.401 and P=0.021 at 2(nd) FU; CE =0.265 and P=0.306 at 3(rd) FU. FEI -40 Gy at 1(st) FU was significantly positively correlated with FEI -40 Gy at subsequent FU's (CC =0.689 and P=6.5×10(-5) comparing 1(st) and 2(nd) FU; 0.901 and P=0.020 comparing 2(nd) and 3(rd) FU. Ninety-six percent of the RILI was found within the 20 Gy isodose line. Sixty-five percent of patients were found to have a decrease in RILI on the second 2(nd) CT. We have shown that RILI evolves over time and 1(st) CT correlates well with subsequent CTs. Ninety-six percent of the RILI can be found to occur within the 20 Gy isodose lines, which may prove beneficial to radiologists attempting to distinguish recurrence vs. RILI.

Distinguishing Pseudoprogression From True Progression by FLAIR Volumetric Characteristics Compared to 45 Gy Isodose Volumes in Treated Glioblastoma Patients

remain dismal with a median survival of 12-14 months for glioblastoma multiforme and 36 months for anaplastic astrocytoma with failure most commonly occurring in the GTV. Large treatment volumes may also result in unnecessary toxicity without a dramatic change in overall survival. Given these factors, smaller GTV to CTV and CTV to PTV margins have been for the past five years. Materials/Methods: A retrospective review was conducted of all adult patients who received definitive treatment for high-grade glioma at UIHC from 2006 to 2011. Results: One hundred thirty patients with histologically confirmed anaplastic astrocytoma (n Z 30) or glioblastoma multiforme (n Z 100) underwent definitive treatment at UIHC between January 1, 2006 and July 1, 2011. Of these, 77 (59.2%) were male with a median age of 56 years (range, 18-84). Forty-two underwent biopsy alone (32%), while 51 underwent subtotal resection (39.3%), and 37 (28.5%) underwent apparent gross total resection. Following surgery, 95 (73%) patients completed adjuvant radiation therapy to 59.4-61.2 Gy, 29 (22.3%) patients received doses between 45 and 59.3 Gy and 6 (4.5%) patients discontinued treatment early at a dose <45 Gy. Stereotactic biteblock localization was used in approximately one half of patients (n Z 67, 51.5%). The vast majority were treated with GTV to CTV and CTV to PTV margins ranging from 6 to 8 mm (total margin of 1.2-1.6 cm). The majority of patients also received concurrent (89.2%) and adjuvant (71.5%) chemotherapy. At the time of last follow-up, 71.5% of patients had died with a median overall survival of 18.9 months for GBM and 29.7 months for AA. Progression free survival was 12.6 months and 18.9 months for GBM and AA, respectively. In multivariate analysis, younger age, grade III histology, gross total resection and a total dose of 59.4-61.2 Gy were found to be independent predictors of survival. Mean follow-up was 24.7 months. Conclusions: Similar to published literature, age, grade, and extent of tumor resection were found to be independent prognostic factors. Overall survival and progression-free survival for patients treated at UIHC were also found to be comparable to the published literature. Smaller GTV to CTVand CTV to PTV margins may thus be considered without a dramatic reduction in survival outcomes. Author Disclosure: A. Morris: None. M. TenNapel: None. C. Anderson: None. M. Smith: None. J. Buatti: E. Research Grant; Siemens HX4, NCI. Q. Leadership; UpToDate Editorial Board.

Prone Positioning for Successful Gamma Knife Radiosurgical Treatment of Far Anterior Skull Base Lesions: A Technical Note

Gamma Knife targeting is geometrically limited by the physical dimensions of the head, helmet, headframe, and the radiation target position. All four aspects need to be considered for accurate treatment planning. Within the available space, delivery of the treatment isodose is based on the target position, which must be brought into the center of the collimator. In cases of anatomically challenging target positions in far eccentric locations of the head, careful treatment planning needs to be performed that does not exceed the limits of the system. We describe a case of a challenging far anterior ethmoid sinus hemangiopericytoma in an 80-year-old woman. The tumor was treated successfully by positioning the patient prone on a Model C Gamma unit. The physical restrictions of the Gamma Knife surgical system and the rationale for improved anterior targeting using the prone position are discussed. A thoughtful approach to positioning in challenging anterior lesions extends the targeting capabilities of certain Gamma units.

HYPOFRACTIONATED CYBERKNIFE RADIOSURGERY FOR PERICHIASMATIC PITUITARY ADENOMAS

Radiation therapy is recommended for pituitary tumors that are refractory to surgical and medical therapies. The efficacy of single-fraction radiosurgery is established for these lesions, but lesions within 3 mm of the optic pathway cannot be safely treated with doses higher than 8 to 10 Gy. We hypothesized that the optic nerve will tolerate 5 consecutive daily radiosurgery fractions of 500 cGy with effective tumor control. We reviewed our first 20 patients with recurrent or residual pituitary adenomas within 3 mm of the optic chiasm treated with the CyberKnife radiosurgery system (Accuray, Inc., Sunnyvale, CA). Tumors were treated with a mean coverage of 97 +/- 2.2% (range, 89.8-99.7%), a mean conformity index of 1.3 +/- 0.2 (range, 1.1-1.6), and a mean treatment isodose line of 74.5 +/- 6.6% (range, 60-86%). The primary end point was an interim analysis of visual preservation, and secondary end points were radiographic and endocrinological tumor control. The mean follow-up period for visual field testing was 26.6 +/- 10.5 months (range, 10.6-41 months). The vision of all 14 patients with intact preoperative vision remained intact. Of the 5 patients with impaired vision, 2 remained stable, and 3 improved. No patient's vision deteriorated. The mean radiographic follow-up was 29.3 +/- 8.6 months (range, 10.2-40.5 months). On magnetic resonance imaging, 12 tumors were stable, 8 were smaller, and none enlarged. This preliminary study establishes that the optic nerve and chiasm tolerate CyberKnife hypofractionated radiosurgery of 5 x 500 cGy to perichiasmatic pituitary adenomas. Early data suggest that this dosing paradigm may achieve satisfactory radiographic and endocrinological tumor control for these challenging lesions, but longer follow-up is necessary to confirm these results.

Optimisation in brachytherapy

Advances in technology have initiated a process of redefinition of planning and evaluation of brachytherapy. The new approach involves conformal positioning of a source configuration for which the treatment isodose contour is adapted to the shape of the target volume which is defined by 3D dimensional imaging. A good implant aims for a balance between good coverage of the target volume, a high dose uniformity inside the target volume and a steep dose fall-off outside the target volume. Computer hardware and software is now available to obtain a high level of sophistication, for instance in treatment planning. In addition, new after loading equipment became available allowing a large variety in source placement in the catheters. To facilitate optimisation procedures, new parameters for the judgement of the quality of the implant have been suggested.

Radiosurgery in patients with renal cell carcinoma metastasis to the brain: long-term outcomes and prognostic factors influencing survival and local tumor control.

Renal cell carcinoma is a leading cause of death from cancer and its incidence is increasing. In many patients with renal cell cancer, metastasis to the brain develops at some time during the course of the disease. Corticosteroid therapy, radiotherapy, and resection have been the mainstays of treatment. Nonetheless, the median survival in patients with renal cell carcinoma metastasis is approximately 3 to 6 months. In this study the authors examined the efficacy of gamma knife surgery in treating renal cell carcinoma metastases to the brain and evaluated factors affecting long-term survival. The authors conducted a retrospective review of 69 patients undergoing stereotactic radiosurgery for a total of 146 renal cell cancer metastases. Clinical and radiographic data encompassing a 14-year treatment interval were collected. Multivariate analyses were used to determine significant prognostic factors influencing survival. The overall median length of survival was 15 months (range 1-65 months) from the diagnosis of brain metastasis. After radiosurgery, the median survival was 13 months in patients without and 5 months in those with active extracranial disease. In a multivariate analysis, factors significantly affecting the rate of survival included the following: 1) younger patient age (p = 0.0076); 2) preoperative Karnofsky Performance Scale score (p = 0.0012); 3) time from initial cancer diagnosis to brain metastasis diagnosis (p = 0.0017); 4) treatment dose to the tumor margin (p = 0.0252); 5) maximal treatment dose (p = 0.0127); and 6) treatment isodose (p = 0.0354). Prior tumor resection, chemotherapy, immunotherapy, or whole-brain radiation therapy did not correlate with extended survival. Postradiosurgical imaging of the brain demonstrated that 63% of the metastases had decreased, 33% remained stable, and 4% eventually increased in size. Two patients (2.9%) later underwent a craniotomy and resection for a tumor refractory to radiosurgery or a new symptomatic metastasis. Eighty-three percent of patients died of progression of extracranial disease. Stereotactic radiosurgery for treatment of renal cell carcinoma metastases to the brain provides effective local tumor control in approximately 96% of patients and a median length of survival of 15 months. Early detection of brain metastases, aggressive treatment of systemic disease, and a therapeutic strategy including radiosurgery can offer patients an extended survival.

Radiosurgery for non-small cell lung carcinoma metastatic to the brain: long-term outcomes and prognostic factors influencing patient survival time and local tumor control.

Lung carcinoma is the leading cause of death from cancer. More than 25% of those patients with lung cancer develop a brain metastasis at some time during the course of their disease. Corticosteroid therapy, radiotherapy, and resection have been the mainstays of treatment. Nonetheless, the median survival for patients with lung carcinoma metastasis is approximately 3 to 6 months. The authors examine the efficacy of gamma knife radiosurgery (GKS) for treating non-small cell lung carcinoma (NSCLC) metastases to the brain and evaluate factors affecting long-term patient survival. A retrospective review of 273 patients who had undergone GKS to treat a total of 627 NSCLC metastases was performed. Clinical and neuroimaging data encompassing a 14-year treatment interval were collected. Univariate and multivariate analyses were performed to determine significant prognostic factors influencing patient survival. The overall median patient survival time was 15 months (range 1-116 months) from the diagnosis of brain metastases. The median survival was 10 months from GKS treatment in those patients with adenocarcinoma and 7 months for those with other histological tumor types. In patients with no active extracranial disease at the time of GKS, the median survival time was 16 months. In multivariate analyses, factors significantly affecting survival included: 1) female sex (p = 0.014); 2) preoperative Karnofsky Performance Scale score (p < 0.0001); 3) adenocarcinoma histological subtype (p = 0.0028); 4) active systemic disease (p = 0.0001); and 5) time from lung cancer diagnosis to the development of brain metastasis (p = 0.0074). Prior tumor resection or whole-brain radiation therapy did not correlate with extended patient survival time. Postradiosurgical imaging of brain metastases revealed that 60% decreased, 24% remained stable, and 16% eventually increased in size. Factors affecting local tumor control included tumor volume (p = 0.042) and treatment isodose (p = 0.015). Fourteen patients (5.1%) later underwent craniotomy and tumor resection for tumor refractory to GKS or a new symptomatic metastasis. Gamma knife surgery for NSCLC metastases affords effective local tumor control in approximately 84% of patients. Early detection of brain metastases, aggressive treatment of systemic disease, and a therapeutic strategy including GKS can afford patients an extended survival time.

Analytic characterization of linear accelerator radiosurgery dose distributions for fast optimization

Linear accelerator (linac) radiosurgery utilizes non-coplanar arc therapy delivered through circular collimators. Generally, spherically symmetric arc sets are used, resulting in nominally spherical dose distributions. Various treatment planning parameters may be manipulated to provide dose conformation to irregular lesions. Iterative manipulation of these variables can be a difficult and time-consuming task, because (a) understanding the effect of these parameters is complicated and (b) three-dimensional (3D) dose calculations are computationally expensive. This manipulation can be simplified, however, because the prescription isodose surface for all single isocentre distributions can be approximated by conic sections. In this study, the effects of treatment planning parameter manipulation on the dimensions of the treatment isodose surface were determined empirically. These dimensions were then fitted to analytic functions, assuming that the dose distributions were characterized as conic sections. These analytic functions allowed real-time approximation of the 3D isodose surface. Iterative plan optimization, either manual or automated, is achieved more efficiently using this real time approximation of the dose matrix. Subsequent to iterative plan optimization, the analytic function is related back to the appropriate plan parameters, and the dose distribution is determined using conventional dosimetry calculations. This provides a pseudo-inverse approach to radiosurgery optimization, based solely on geometric considerations.

Clinical commissioning of Laitinen Stereoadapter for fractionated stereotactic radiotherapy

The Laitinen Stereoadapter 5000 from Sandstroem Trade and Technology was acceptance tested and commissioned for clinical use in a Fractionated Stereotactic Radiotherapy Program at our facility. The frame was implemented to function as a localization device for target delineation rather than as an immobilization device. The frame is of non-invasive nature utilizing ear plugs and a nasion bridge adapter as the connecting points with the patient’s head. The reproducibility of the head frame position with respect to external skull reference points was tested. CT and MRI imaging studies were performed on a patient phantom with the stereoadapter in place. The target was delineated and target coordinates were calculated for two implanted targets. The phantom was positioned according to the target coordinates on a Siemens MXE Linear Accelerator by aid of the target positioning lasers. Radiographic port film images were taken with the circular fields typically used in stereotactic radiosurgery. A complete treatment isodose plan was performed and dosimetric accuracy was tested by positioning a small volume ionization chamber at the center of the target volume in the head phantom. The results of these tests were found to be clinically acceptable.

The need for individualized dosimetry for tangential breast treatment

We have examined the isodose distributions of 119 intact breast patients treated on a 6 MV linae to determine if a library of treatment plans could be used instead of individualized computer plans for patient treatments without compromising the quality of those treatments. The parameters studied were: field width, baseline separation, central axis separation, wedge angle, and isodose coverage. At least two wedges were used in the computer plans for each patient and the best plan was then chosen. In order to construct a library of plans, the choice of wedge, treatment isodose, and dose uniformity should be predictable. Our results show that for 90 out of 119 plans (76%), the 30° wedge was best. In the other 29 cases, either the 15° or the 45° wedge yielded better plans. On average, the improvement in dose homogeneity due to choice of wedge was about 2% (range 0–7%) for these cases. Although grouping like-patient parameters generally restricted the isodose variation to ± 2.5%, there were five patients for which up to a 7% underdosage would not have been predicted. For the set of plans using a 30° wedge, a significant correlation was found for the ratio of the baseline to central axis separation vs. treatment isodose. The average isodose which covered the target area was 97% (range 90–100%) and 102 out of 107 patient plans using the 30° wedge fell between 94 and 100%. We conclude from these results that the variation in dose distribution found with seemingly similar sized breasts is due to the variation in breast shape and symmetry. The use of a library plan with a single wedge and a standardized isodose line for tangential field treatment of intact breast could cause up to a 7% dose difference compared to the actual dosimetry for that patient.

Respiration-induced motion of the kidneys in whole abdominal radiotherapy: implications for treatment planning and late toxicity

Purpose: Whole abdominal radiotherapy (WAR) has potential utility in the management of several malignancies. The limited radiation tolerance of the kidneys is an important consideration in the design of WAR fields. Although renal blocking is standard for WAR, few guidelines exist in the literature to factor respiration-induced kidney motion into the design of these blocks. Methods: Radiographs were obtained to measure kidney excursion under forced respiratory conditions in eight patients (14 visualized kidneys). Intravenous contrast was administered and AP films were obtained at maximum inspiration and expiration. Renal excursion was measured relative to a horizontal reference line at the bottom of the L3 vertebral body. The kidney position on the actual treatment simulation film was also determined using this technique. Treatment isodose distributions through the kidneys were obtained for a sample patient using phantom measurements and two blocking schemes: AP/PA and PA only. These provided quantification of the actual dose received by the kidney in a typical WAR treatment. Results: In the worst case scenario, the left kidney block required an additional 10 mm above and 15 mm below the renal silhouette on the simulation film in order to account for all phases of respiration. The corresponding values for the right kidney were 2 mm and 19 mm, respectively. The dose received by the kidney under the center of the block was 20% of prescribed using AP/PA blocks and 50% of prescribed using PA blocks only. However, portions of ‘blocked’ kidney received up to 90% of the prescribed dose with either technique. Conclusions: Although kidney motion under forced respiratory conditions is not representative of typical treatment conditions, the data highlight the possibility of renal movement during treatment. This is particularly important in light of the significant dose (20 to 50%) delivered to the kidney under the center of the kidney block in typical treatments. Given the potential for overdosage of this critical organ, careful prospective documentation of renal function is warranted in patients receiving WAR.

Stereotactic radiotherapy of irregular targets: a comparison between static conformal beams and non-coplanar arcs

Stereotactic radiotherapy using a linear accelerator is usually equated with the technique of delivery using multiple non-coplanar arcs, which achieves a spherical dose distribution. As the majority of intracranial lesions are not spherical, a range of schematized tumour shapes were planned to assess the role of static conformal beams in the treatment of irregular lesions. A sphere and 2 ellipsoids, ranging from 20 to 50 mm maximum diameter located intracranially were planned using 3, 4, and 6 non-coplanar static beams with conformal blocks and were compared with four 120 non-coplanar arcs. Comparison of the plans was made by the relative sparing of normal tissue outside the target volume using three-dimensional dose-volume distributions. Non-coplanar arcs spared more normal tissue at low isodoses and achieved the best high dose sparing for spherical targets. For the majority of irregular targets, 3 and 4 static beams spared more tissue at doses ≥ 50% and ≥ 80% than the arc technique. For all irregular volumes, maximum sparing of normal tissue to isodoses ≥ 50% and ≥ 80% of the treatment isodose was obtained with 6 static conformal beams. We conclude that irregularly shaped tumours suitable for stereotactic radiotherapy with a linear accelerator are better treated with conformal static non-coplanar beams rather than with the multiple arc technique.

A CT-based evaluation method for permanent implants: application to prostate

A computerized tomography-based 3-dimensional evaluation scheme, using semi-automatic seed localization from transverse computerized tomography-images, has been developed for permanent implants. The treatment isodose contour is generated at each scan plane and overlaid on the corresponding target contour for coverage visualization. Volume and surface dose histograms are generated for the target and neighboring normal structures. Dose inhomogeneity within the target is also estimated from the full-width at half maximum of the differential volume dose histogram. The matched peripheral dose is calculated from the ellipsoidal volume approximation for the target. The present evaluation method has been applied here to 10 actual permanent 1–125 prostatic implantations. Coverages by 150 Gy and 100 Gy levels are found to range from 78–96% and 91–99% of the target volume, respectively. The average matched peripheral dose is found to be about two times the average peripheral dose (the dose encompassing 99% target) and about three times the average minimum peripheral dose (the dose encompassing 100% target). The dose inhomogeneities within the target volume, in these 10 cases, range from 220–420 Gy.

A CT-compatible version of the fletcher system intracavitary applicator: clinical application and 3-dimensional treatment planning

A new acrylic version of the familiar Fletcher intracavitary applicator, the Ann Arbor (AA) applicator, has been developed. This new device eliminates the problem of “streak” artifacts on CT images, but unlike other plastic applicators the ability to shield portions of the bladder and rectum is retained through the use of tungsten alloy shields which are afterloaded with the radioactive sources. To minimize changes in placement geometry and to take advantage of the wide clinical experience with the Fletcher system, the new applicator nearly duplicates the physical dimensions of the Fletcher applicator. With the Ann Arbor applicator in place, dummy sources are easier to locate on standard radiographic simulations. CT scans are free of artifact and provide clear, detailed visualizations of cross-sectional anatomy. The new applicator thus allows CI' images to be used to their potential in evaluating crucial anatomic relationships and in performing 3-D dosimetry with dose volume analysis. Using a treatment planning system with 3-D capabilities, solid surface graphic display of applicator, cervix, rectum, bladder, and treatment isodose volume has been performed. In addition, dose volume histograms can be generated to obtain precise measurements of the volume of cervix, rectum, or bladder receiving specified doses.