Stereotactic radiosurgery, X: clinical isodosimetry of gamma knife versus linear accelerator X-knife for pituitary and acoustic tumours

Inverse planning in Gamma Knife radiosurgery: A comparative planning study

PURPOSE

To determine the advantages of inverse planning using a prerelease version of Leksell Gamma Knife® (LGK) Lightning (Elekta AB, Sweden) compared to manual forward planning.

METHODS

Thirty-eight patients with metastases (MET, n = 15), vestibular schwannomas (VS, n = 11) and meningiomas (MEN, n = 12), treated with LGK Icon™ at our institution, were analyzed retrospectively. For each case, an inverse (inv) and inverse full coverage (fc) treatment plan was generated using LGK Lightning and compared to the clinical plans. Several dosimetry and efficiency characteristics were compared for each indication. The mean, median difference and interquartile range were reported and the significance was assessed with a paired-sample Wilcoxon test (significance level < 0.05). Further, the inter operator variability was analyzed for multiple users.

RESULTS

Inv and fc treatment plans show improved target coverage (up to 3.6%) for all analyzed paradigms. For inv plans, the selectivity is enhanced (MET: 2.9%; VS: 1.8%; MEN: 1%) and the organ at risk doses are significantly reduced (VS: up to 4.5%; MEN: up to 17.5%). For inv and fc plans, the beam on time (BOT) is shortened (MET: up to 7.9%; benign tumors: 49.5%). The inter operator variability analysis shows similar treatment plan quality with small differences in plan efficiency (difference in BOT: 1-3.3 min).

CONCLUSIONS

LGK Lightning allows to generate improved LGK treatment plans regarding plan quality with reduced BOT compared to manual forward plans. The inter operator variability showed that multiple users with different experiences can generate similar treatment plan quality using LGK Lightning.

Cochlea-sparing acoustic neuroma treatment with 4π radiation therapy

Purpose

This study investigates whether 4π noncoplanar radiation therapy can spare the cochleae and consequently potentially improve hearing preservation in patients with acoustic neuroma who are treated with radiation therapy.

Methods and materials

Clinical radiation therapy plans for 30 patients with acoustic neuroma were included (14 stereotactic radiation surgery [SRS], 6 stereotactic radiation therapy [SRT], and 10 intensity modulated radiation therapy [IMRT]). The 4π plans were created for each patient with 20 optimal beams selected using a greedy column generation method and subsequently recalculated in Eclipse for comparison. Organ-at-risk (OAR) doses, homogeneity index, conformity, and tumor control probability (TCP) were compared. Normal tissue complication probability (NTCP) was calculated for sensorineural hearing loss (SNHL) at 3 and 5 years posttreatment. The dose for each plan was then escalated to achieve 99.5% TCP.

Results

4π significantly reduced the mean dose to both cochleae by 2.0 Gy (32%) for SRS, 3.2 Gy (29%) for SRT, and 10.0 Gy (32%) for IMRT. The maximum dose to both cochleae was also reduced with 4π by 1.6 Gy (20%), 2.2 Gy (15%), and 7.1 Gy (18%) for SRS, SRT, and IMRT plans, respectively. The reductions in mean/maximum brainstem dose with 4π were also statistically significant. Mean doses to other OARs were reduced by 19% to 56% on average. 4π plans had a similar CN and TCP, with a significantly higher average homogeneity index (0.93 vs 0.92) and significantly lower average NTCP for SNHL at both 3 years (30.8% vs 40.8%) and 5 years (43.3% vs 61.7%). An average dose escalation of approximately 116% of the prescription dose achieved 99.5% TCP, which resulted in 32.6% and 43.4% NTCP for SNHL at 3 years and 46.4% and 64.7% at 5 years for 4π and clinical plans, respectively.

Conclusions

Compared with clinical planning methods, optimized 4π radiation therapy enables statistically significant sparing of the cochleae in acoustic neuroma treatment as well as lowering of other OAR doses, potentially reducing the risk of hearing loss.

Clinical outcomes following salvage Gamma Knife radiosurgery for recurrent glioblastoma

Glioblastoma multiforme (GBM) is the most common malignant primary brain tumor with a survival prognosis of 14-16 mo for the highest functioning patients. Despite aggressive, multimodal upfront therapies, the majority of GBMs will recur in approximately six months. Salvage therapy options for recurrent GBM (rGBM) are an area of intense research. This study compares recent survival and quality of life outcomes following Gamma Knife radiosurgery (GKRS) salvage therapy. Following a PubMed search for studies using GKRS as salvage therapy for malignant gliomas, nine articles from 2005 to July 2013 were identified which evaluated rGBM treatment. In this review, we compare Overall survival following diagnosis, Overall survival following salvage treatment, Progression-free survival, Time to recurrence, Local tumor control, and adverse radiation effects. This report discusses results for rGBM patient populations alone, not for mixed populations with other tumor histology grades. All nine studies reported median overall survival rates (from diagnosis, range: 16.7-33.2 mo; from salvage, range: 9-17.9 mo). Three studies identified median progression-free survival (range: 4.6-14.9 mo). Two showed median time to recurrence of GBM. Two discussed local tumor control. Six studies reported adverse radiation effects (range: 0%-46% of patients). The greatest survival advantages were seen in patients who received GKRS salvage along with other treatments, like resection or bevacizumab, suggesting that appropriately tailored multimodal therapy should be considered with each rGBM patient. However, there needs to be a randomized clinical trial to test GKRS for rGBM before the possibility of selection bias can be dismissed.

Comparative clinical dosimetry with X-knife and gamma knife

X-knife and gamma knife techniques are well-established for cranial stereotactic radiosurgery (SRS). Due to differences in their radiation delivery methods, some of the dosimetric parameters of these two techniques differ which may have clinical significance. There are many dosimetric studies comparing linear accelerator based techniques such as X-knife with gamma knife but generally from different institutions. We carried out a retrospective comparative study of the dosimetric parameters of the SRS treatments performed at our centre with X-knife (circular cones) and gamma knife. Our results indicate that the dose conformity and dose fall-off in the vicinity of the target volumes were better for patients treated with gamma knife as compared to X-knife. However, the dose fall-off pattern shows a reversal at a larger distance from the target. It was better for the X-knife as compared to gamma knife in the low dose region.

Helical TomoTherapy versus sterotactic Gamma Knife radiosurgery in the treatment of single and multiple brain tumors: a dosimetric comparison

The objective was to compare the dosimetry of Helical TomoTherapy (TOMO) and Gamma Knife (GK) treatment plans for tumor and normal brain in the treatment of single and multiple brain tumors. An anthropomorphic Rando Head phantom was used to compare the dosimetry of TOMO and GK. Eight brain tumors of various shapes, sizes and locations were used to generate 10 plans. The radiation dose was 20 Gy prescribed to the 100% isodose line for TOMO plans and to the 50% for the GK plans. Dose Volume Histograms for tumor and brain were compared. Equivalent Uniform Dose (gEUD), Tumor Control Probability (TCP) and Normal Tissue Complication Probability (NTCP) were performed and used for plan comparisons. Average minimum, mean, median and maximum tumor doses were 19.93, 27.83, 27.38, 39.60 Gy for GK and 20.17, 20.60, 20.59, 20.90 Gy for TOMO. Average gEUD values for tumor and normal brain were 25.0 and 7.2 Gy for GK and 20.7 and 8.1 Gy for TOMO. Conformity indices (CI) were similar for both modalities. Gradient indices (GI) were greater for TOMO. A combination plan was also generated using all eight tumors. TOMO was able to target all eight tumors simultaneously resulting in mean tumor and brain doses of 20.5 and 9.35 Gy, respectively. Due to the maximum limit of 50 beams per plan, GK was unable to provide a treatment plan for all eight tumors. GK provides an advantage for all tumor sizes with respect to tumor and normal brain dose. Clinical studies are needed to correlate these dosimetric findings with patient outcomes.

PACS number: 87.55‐x

Radiotherapy of nonfunctioning and gonadotroph adenomas

Transsphenoidal surgery is the treatment of choice for NFPA but is seldom curative. The management of patients in whom residual tumor is detected after surgery is not clear-cut. Radiation therapy is effective in controlling tumor mass in the majority of patients, but is associated with long term complications that call for restriction of its use to patients at high risk for tumor growth. New radiation techniques may prove to be safer while retaining the effectiveness of conventional radiotherapy, however longer follow-up is necessary to confirm this assumption. For now, it appears to be safe to withhold radiation and carefully follow patients with small tumor remnants, whereas large remnants from invasive tumors should be considered for radiotherapy. Nevertheless, there are no prospective controlled studies that support this empirical approach.

Gamma surgery in the treatment of nonsecretory pituitary macroadenoma

Object

The authors report on a retrospective analysis of the imaging and clinical outcomes following gamma surgery in 100 patients with nonsecretory pituitary macroadenoma.

Methods

Between June 1989 and March 2004, 100 consecutive patients with nonsecretory pituitary macroadenoma were treated at the Lars Leksell Center for Gamma Surgery, University of Virginia Health System (Charlottesville, VA). Ninety-two patients had residual or recurrent macroadenoma following one or more surgical procedures. In eight patients, gamma surgery was the primary treatment. Ten patients received conventional fractionated radiotherapy before the gamma surgery. Sixty-nine patients required hormone replacement therapy for one or more deficits before gamma knife treatment. Peripheral doses between 5 and 25 Gy (mean 18.5 Gy) were administered.

Imaging and endocrinological follow-up evaluations were performed in 90 patients; these studies ranged from 6 to 142 months (mean 44.9 months) and 6 to 127 months (mean 47.9 months), respectively. Tumor volume decreased in 59 patients (65.6%), remained unchanged in 24 (26.7%), and increased in seven (7.8%). The minimal effective peripheral dose was 12 Gy; peripheral doses greater than 20 Gy did not seem to provide additional benefit. Of 61 patients with a partially or fully functioning pituitary gland and follow-up data, 12 (19.7%) suffered new hormone deficits following gamma surgery. In patients with endocrinological follow-up data that had been collected over more than 2 years, the rate of new deficits was 25%. No neurological morbidity or death was related to treatment.

Conclusions

Current experience suggests that gamma surgery is an appropriate means of managing recurrent or residual nonsecretory pituitary macroadenoma following microsurgery and a primary treatment in selected patients. To evaluate definite rates of recurrence and new endocrine deficiencies, long-term follow-up studies are needed.

Is there still a role for radiotherapy in acromegaly?

External radiotherapy (ideally 3-field radiotherapy with a daily fractional dose no higher than 1.8 Gy or conformal irradiation) has been used extensively in the treatment of acromegaly, and virtually all studies have documented a predictable but slow reduction in growth hormone (GH) excess, which is at its maximum in the first year after treatment (30-50%) and continues at an average rate of 10-15% thereafter in the long term. Therefore, achievement of 'safe' GH concentrations in an acceptable time interval after radiotherapy will be realized only in those patients who have lower GH concentrations prior to irradiation either as a result of mild disease or previous surgery. Recent studies have demonstrated the value of stereotactic radiotherapy (either as multiple arc X-irradiation or as 'gamma knife' therapy) in the post-surgical treatment of acromegaly or as salvage therapy for disease persisting after conventional external irradiation. The development of potent medical therapies for acromegaly (somatostatin analogues and the GH receptor antagonist) has called into question the role of radiotherapy in the treatment of this disease. However, even if the concept of primary, open-ended medical therapy for selected patients is accepted, reference to the success rates of surgery and response rates to somatostatin analogues indicates that approximately 10-20% of all patients with acromegaly will require consideration of radiotherapy for hormonal or tumour mass control. For these reasons, radiotherapy (both conventional external and stereotactic irradiation) continues to have a major role in controlling acromegaly in selected patients.

Comparison of radiosurgery planning modalities for acoustic neuroma with regard to conformity and mean target dose

PURPOSE

To evaluate dose conformity and mean target dose in light of previous comparative studies and state-of-the-art radiosurgery delivery modalities.

MATERIALS AND METHODS

Seven patients with acoustic neuromas deemed clinically suitable for linear accelerator or Gamma Knife radiosurgery were planned such that the minimum doses for any plan were equal. Gamma Knife plans were prepared in three ways: by altering the prescription of previously published data, by hand and with the assistance of an automatic planning algorithm (wizard). The linear accelerator plans were prepared utilizing a micro-multileaf collimator in both static and dynamic modes. The dose volume histogram analyses lead to a measure of conformity and the mean and minimum target dose for each plan. Statistical significance was calculated as each planning modality was compared with every other.

RESULTS

All Gamma Knife plans demonstrated a statistically significantly better conformity when compared with fixed field linear accelerator techniques. When compared to linear accelerator techniques the wizard-assisted Gamma Knife plans demonstrated significantly better conformity. The mean target dose for all the Gamma Knife plans was significantly higher than that of the linear accelerator plans (19.2 Gy vs. 13.4 Gy).

CONCLUSIONS

Conformity of the prescription isodose to the target shape is of major importance in radiosurgery. The modalities compared represent commercially available and widely accepted systems. Gamma Knife plans derived using the 'wizard' option and finalized by hand yield the best conformity.

Stereotactic radiosurgery XVI: Isodosimetric comparison of photon stereotactic radiosurgery techniques (gamma knife vs. micromultileaf collimator linear accelerator) for acoustic neuroma—and potential clinical importance

PURPOSE

Two stereotactic photon radiation therapy methods are currently in practice for the treatment of acoustic neuroma. In the 1990s, our data and those of others demonstrated isodosimetric advantages for gamma knife technology over linear accelerator methodology. Since then, the introduction of micromultileaf collimator technology has improved the conformity of the linear accelerator method such that the isodosimetric differences between the two techniques have narrowed.

MATERIALS AND METHODS

In this study, modern gamma knife isodosimetry was compared to that of modern linac technology (conformal fixed fields and dynamic arcs) for the therapy of acoustic neuroma. This is an unusual target in that a special sensory nerve (holding the key to hearing preservation) frequently runs through the targeted volume, unlike the majority of other stereotactic radiation therapy targets. This was a single-dose prescription comparison; the perceived extra benefit of fractionation (a technique not routinely available to the gamma knife) was thereby abrogated.

RESULTS

Although the gamma knife technique maintained a slight, but statistically significant, advantage with regard to dose conformity (p < 0.02) (at the debatable cost of a lower minimum target dose), the much higher internal dose gradient (high maximum dose to prescription dose [MD:PD] ratio) could be interpreted as a disadvantage with respect to hearing preservation, although advantageous with regard to tumor ablation. Of the two linac methods, the dynamic arc method gave a statistically significant advantage over the fixed-field method as regards conformity (p < 0.05), at the expense of a slightly higher brainstem dose (an average of 12.4 Gy, cf. 11.7 Gy for fixed fields), but this result was not statistically significant. No significant difference was seen in the MD:PD ratio for the two single-isocenter linac techniques.

CONCLUSIONS

Gamma knife methodology remains well validated, with very good isodosimetry, but when hearing preservation is important, the improving linac technologies will compete with the gamma knife for optimal therapy. In these circumstances, the minor differences in isodosimetry between the two techniques will become important.

Growth hormone receptor antagonists: discovery, development, and use in patients with acromegaly

An understanding of the events that occur during GH receptor (GHR) signaling has facilitated the development of a GHR antagonist (pegvisomant) for use in humans. This molecule has been designed to compete with native GH for the GHR and to prevent its proper or functional dimerization-a process that is critical for GH signal transduction and IGF-I synthesis and secretion. Clinical trials in patients with acromegaly show GHR blockade to be an exciting new mode of therapy for this condition, and pegvisomant may have a therapeutic role in diseases, such as diabetes and malignancy, in which abnormalities of the GH/IGF-I axis have been observed. This review charts the discovery and development of GHR antagonists and details the experience gained in patients with acromegaly.

Stereotactic radiosurgery for pituitary adenomas

Pituitary adenoma is a radiosensitive disease and postoperative radiotherapy reduces the chance of relapse. Non-irradiated patients, followed in the modern era, suffer up to 20% five-year and up to 44% ten-year relapse. To some extent, predictors of relapse are available at the time of presentation or after surgery. Although conventionally fractionated radiotherapy has a very good track record with regard to controlling disease and safety in the modern age, there is considerable contemporary interest in the technique of radiosurgery (highly concentrated radiation therapy using stereotactic mapping). The usefulness of this technique in the treatment of pituitary adenoma is discussed in this review.

Stereotactically delivered cranial radiation therapy: a ten-year experience of linac-based radiosurgery in the UK

In 1989, linear accelerator (linac)-based cranial stereotactic radiation therapy ('radiosurgery') was introduced in the UK at St Bartholomew's Hospital; a new, relocatable stereotactic frame was first used at the same time, allowing fractionated stereotactic radiotherapy. In the first decade of clinical practice using this technology, some 200 patients with blood vessel tumours/malformations have been treated, together with another 200 suffering from other conditions. The usefulness of this technique for cerebral arteriovenous malformations (AVM) has been demonstrated, and also a significant cure rate for AVM of >3 cm diameter (which is larger than for those previously reported after treatment on the gamma unit), albeit attended by a higher complication rate. The epilepsy associated with AVM is much improved by successful radiotherapy. The usefulness of radiosurgery for glomus tumours has been confirmed and new data published on the efficacy of the technique for haemangioblastoma, with new radiation therapy strategies designed for patients with von Hippel-Lindau disease. The acoustic neuroma treatment results have included improvements in hearing (a result not reported in the gamma unit literature), which are ascribed to the lower internal dose gradient within the target volume. Fractionation will, it is argued, also lead to sparing of the special sensory cochlear nerve. The risks of radiosurgery to the brainstem for chordoma of the mid-clivus are reduced by using a 'spacer' technique for the prepontine space. For meningiomas involving the cavernous sinus, conventionally fractionated radiotherapy is recommended when the meningeal base diameter exceeds 3.0 cm and radiosurgery (utilizing fractionation where appropriate) is advised for smaller lesions. Thus far, radiosurgery indications for pituitary adenomas have been restricted to recurrences after conventional radiotherapy, usually those in the cavernous sinus. In therapy for recurrent craniopharyngioma, it is argued that fractionation delivered via the relocatable frame will be important, particularly when the disease envelops the optic chiasma. For semicystic/semisolid craniopharyngiomas, the stereotactic delivery of colloidal yttrium-90 into a cystic element is useful, while stereotactic radiosurgery is delivered to the solid component. Staff at this centre consider that radiosurgery for low-grade gliomas, perhaps as boost therapy after conventional fractionation, is worthy of more research. We have been extremely selective in the use of radiosurgery for brain metastases (2% of patients, compared with about 30% in some Gamma Knife units), but future indications may become broader, probably using it as a booster technique after whole-brain conventionally-fractionated radiotherapy. Positron emission tomography scanning, co-registered with magnetic resonance imaging, allows the 'boost' concept in radiosurgery to become a sophisticated and accurate reality. Post-radiosurgical sequelae have been placed within a standard framework classification. New observations are being made with regard to subacute reactions: late-responding intrinsic and extra-axial tumours may swell in the subacute period, prior to shrinkage, and be attended by symptomatic surrounding brain oedema.