PTV-based VMAT vs. robust IMPT for head-and-neck cancer: A probabilistic uncertainty analysis of clinical plan evaluation with the Dutch model-based selection

BACKGROUND AND PURPOSE

In the Netherlands, head-and-neck cancer (HNC) patients are referred for proton therapy (PT) through model-based selection (MBS). However, treatment errors may compromise adequate CTV dose. Our aims are: (i) to derive probabilistic plan evaluation metrics on the CTV consistent with clinical metrics; (ii) to evaluate plan consistency between photon (VMAT) and proton (IMPT) planning in terms of CTV dose iso-effectiveness and (iii) to assess the robustness of the OAR doses and of the risk toxicities involved in the MBS.

MATERIALS AND METHODS

Sixty HNC plans (30 IMPT/30 VMAT) were included. A robustness evaluation with 100,000 treatment scenarios per plan was performed using Polynomial Chaos Expansion (PCE). PCE was applied to determine scenario distributions of clinically relevant dosimetric parameters, which were compared between the 2 modalities. Finally, PCE-based probabilistic dose parameters were derived and compared to clinical PTV-based photon and voxel-wise proton evaluation metrics.

RESULTS

Probabilistic dose to near-minimum volume v = 99.8% for the CTV correlated best with clinical PTV-D98% and VWmin-D98%,CTV doses for VMAT and IMPT respectively. IMPT showed slightly higher nominal CTV doses, with an average increase of 0.8 GyRBE in the median of the D99.8%,CTV distribution. Most patients qualified for IMPT through the dysphagia grade II model, for which an average NTCP gain of 10.5 percentages points (%-point) was found. For all complications, uncertainties resulted in moderate NTCP spreads lower than 3 p.p. on average for both modalities.

CONCLUSION

Despite the differences between photon and proton planning, the comparison between PTV-based VMAT and robust IMPT is consistent. Treatment errors had a moderate impact on NTCPs, showing that the nominal plans are a good estimator to qualify patients for PT.

Neoadjuvant radiotherapy dose escalation for locally advanced rectal cancers in the new era of radiotherapy: A review of literature

BACKGROUND

The standard treatment of locally advanced rectal cancers (LARC) consists on neoadjuvant chemoradiotherapy followed by total mesorectal excision. Different data in literature showed a benefit on tumor downstaging and pathological complete response (pCR) rate using radiotherapy dose escalation, however there is shortage of studies regarding dose escalation using the innovative techniques for LARC (T3-4 or N1-2).

AIM

To analyze the role of neoadjuvant radiotherapy dose escalation for LARC using innovative radiotherapy techniques.

METHODS

In December 2020, we conducted a comprehensive literature search of the following electronic databases: PubMed, Web of Science, Scopus and Cochrane library. The limit period of research included articles published from January 2009 to December 2020. Screening by title and abstract was carried out to identify only studies using radiation doses equivalent dose 2 Gy fraction (EQD2) ≥ 54 Gy and Volumetric Modulated Arc Therapy (VMAT), intensity-modulated radiotherapy or image-guided radiotherapy (IGRT) techniques. The authors’ searches generated a total of 2287 results and, according to PRISMA Group (2009) screening process, 21 publications fulfil selection criteria and were included for the review.

RESULTS

The main radiotherapy technique used consisted in VMAT and IGRT modality. The mainly dose prescription was 55 Gy to high risk volume and 45 Gy as prophylactic volume in 25 fractions given with simultaneous integrated boosts technique (42.85%). The mean pCR was 28.2% with no correlation between dose prescribed and response rates (P value ≥ 0.5). The R0 margins and sphincter preservation rates were 98.88% and 76.03%, respectively. After a mean follow-up of 35 months local control was 92.29%. G3 or higher toxicity was 11.06% with no correlation between dose prescription and toxicities. Patients receiving EQD2 dose > 58.9 Gy and BED > 70.7 Gy had higher surgical complications rates compared to other group (P value = 0.047).

CONCLUSION

Dose escalation neoadjuvant radiotherapy using innovative techniques is safe for LARC achieving higher rates of pCR. EQD2 doses > 58.9 Gy is associated with higher rate of surgical complications.

Improving organ at risk sparing in oropharyngeal treatment planning by increasing target dose heterogeneity: A feasibility study

Target dose homogeneity has historically been a priority in radiotherapy treatment planning. However, in an era of more advanced modulated techniques, there is now greater flexibility in shaping dose distributions suggesting that allowing controlled target dose heterogeneity may consequently improve organ at risk (OAR) sparing. This study sought to determine the feasibility of allowing an increase in target dose heterogeneity in oropharyngeal VMAT plans, and to examine the dosimetric impact this has on target coverage and OARs such as the parotid glands, spinal cord, brainstem and mandible. Nineteen oropharyngeal patients' plans were created with homogeneous dose distributions specified in the London Cancer Head and Neck Radiotherapy Protocol. The upper dose constraint (UDC) objective of the primary planning target volumes (PTV) for each plan were increased in increments of 10% until a maximum of 150% of the prescribed dose was reached. These plans were dosimetrically compared to plans with a uniform dose distribution in terms of OAR sparing and target coverage. Minimal coverage was not compromised, with the largest median changes being a 0.81% decrease [98.6 to 97.8%] to the PTV_70Gy D98% and a 2.86% decrease [99.81 to 96.96%] to the PTV_54Gy D98% at a UDC of 150% of the prescription dose. An OAR sparing effect was observed for the parotid glands, spinal cord and oral cavity sub PTV. Mandible and brainstem Dmax values increased as the PTV UDC increased. Changes in brainstem dose were not statistically significant. All other differences were statistically significant for UDC's above 130%. Target coverage was not compromised as a result of increased target dose heterogeneity. The OAR sparing effect was promising for most organs, however further research with a larger dataset is necessary surrounding the effect on organs that overlap with the PTV.

Approximation of dose quality indicator values in multi-criteria optimized (MCO) volumetric modulated arc therapy (VMAT) treatment planning using trilinear dose interpolation

PURPOSE

To approximate dose-volume histogram (DVH) based quality indicators in volumetric modulated arc therapy (VMAT) planning using multi-criteria optimization (MCO) with a low number of composite optimization parameters.

METHODS

The solution space for VMAT optimization with a low number of composite optimization parameters is approximated by trilinear dose inter- polation and prediction of dose-volume-histogram (DVH) based plan quality indicator values. To assess the approximation quality a diverse dataset of 44 cranial and 18 spine patient geometries was chosen. Optimization results are governed by three composite parameters focusing on target-organ-at-risk- (OAR)-trade-off, overall healthy tissue sparing, and delivery/quality assurance complexity. 21,266 optimized dose distributions were pre-calculated and the numerical values for a choice of 10 DVH points, referred to as plan quality indicators, were stored to serve as ground truth. Using a subset of 8 and 27 pre-calculated optimization results, dose distributions for unknown parameter values were approximated by trilinear interpolation. The resulting quality indicator values were compared to the previously obtained exact solutions.

RESULTS

The magnitude of the deviation between exact and approximated values varied largely with respect to patient geometry and the criterion under investigation. Approximation with 27 pre-calculated results yielded lower deviations than approximation with 8 results, at the cost of a higher pre-calculation workload.

CONCLUSIONS

Solution space approximation via trilinear dose interpolation in VMAT treatment planning governed by composite optimization parameters is possible without further knowledge of the internal implementation of the underlying optimizer. Maximum average deviations between approxi- mation and actual values of characteristic dose quality indicators below 1% (cranial) and 8% (spine) allow for a quick qualitative assessment of the possible solution landscape.

Evaluation of geometrical uncertainties on localized prostate radiotherapy of patients with bilateral metallic hip prostheses using 3D-CRT, IMRT and VMAT: A planning study

OBJECTIVE

Since most radiation treatment plans are based on computed tomography (CT) images, which makes it difficult to define the targeted tumor volume located near a metal implant, this study aims to evaluate and compare three treatment plans in order to optimally reduce geometrical uncertainty in external radiation treatment of localized prostate cancer.

METHODS

Experimental subjects were three prostate patients with bilateral hip prosthesis who had undergone radical radiotherapy. The treatment plans were five-field three-dimensional conformal radiation therapy (3D-CRT), fixed 5-field intensity-modulated radiation therapy (IMRT) using similar gantry angles, and single-arc volumetric modulated arc therapy (VMAT). The monitor units (MUs), dose volume histograms (DVHs), the dose indices of planning target volume (PTV), clinical target volume (CTV) and rectum were compared among the three techniques. The geometrical uncertainties were evaluated by shifting the iso-center (2- 10 mm in the anterior, posterior, left, right, superior, and inferior directions). The CTV and rectum dose indexes with and without the iso-center shifts were compared in each plan.

RESULTS

The Conformity Index of PTV were 1.35 in 3D-CRT, 1.12 in IMRT, and 1.04 in VMAT, respectively. The rectum doses in 3D-CRT are also higher than those in IMRT and VMAT. The iso-center shift little affected the CTV dose when smaller than the margin size. The rectum dose increased especially after a posterior shift. Additionally, this dose increase was larger in the VMAT plan than in the 3D- CRT plan. However, the VMAT achieved a superior rectum DVH to that of 3D- CRT, and this effect clearly exceeded the rectum-dose increase elicited by the iso-center shift.

CONCLUSION

For radiotherapy treatment of localized prostate cancer in patients with hip prosthesis, the dose distribution was better in the VMAT and Metal Artifact Reduction (MAR)-CT image methods than the conventional methods. Because the anatomical structure of the male pelvic region is relatively constant among individuals, we consider that VMAT is a valid treatment plan despite analyzing just three cases.

A dosimetric comparison of Volumetric Modulated Arc Therapy (VMAT) and High Dose Rate (HDR) brachytherapy in localized cervical cancer radiotherapy

BACKGROUND

Cervical cancer radiotherapy is usually administrated through 3-Dimensional Conformal Radiation Therapy (3DCRT) followed by a brachytherapy (BT) boost.

PURPOSE

To investigate whether Volumetric Modulated Arc Therapy (VMAT) can replace High Dose Rate (HDR) intracavitary BT boost for patients undergoing cervical cancer radiotherapy.

MATERIALS AND METHODS

Computed Tomography (CT) images for ten patients with tandem and ovoids were included in this study. Target volumes, rectum, bladder, sigmoid, small bowel and both femoral heads were delineated. Two plans were carried out including (a) a BT plan optimized manually by modifying dwell time and Ir-192 source positions, (b) a VMAT plan generated using two partial arcs with 10 MV photon beam. The prescribed dose was 7 Gy. The relevant dose volume parameters (DVPs) of target volumes and OARs for the two plans were analyzed statistically using SPSS Wilcoxon Signed Rank test.

RESULTS

VMAT plan showed a significant reduction of 9.1%, 9.3%, 15.4%, 14.4% and 13.1% in rectum maximum dose, rectum D2cc, bladder maximum dose, bladder D2cc and sigmoid maximum dose (P < 0.05). VMAT and BT plans showed comparable D2cc of sigmoid and small bowel maximum doses (P = 0.333 and P = 0.646). On the other hand, VMAT showed significantly higher small bowel D2cc and maximum point dose for both femoral heads comparing to BT plan (P < 0.05). Also, VMAT plan yielded greater homogeneous target coverage compared to BT plan (P < 0.05).

CONCLUSION

The study demonstrated that VMAT plan achieves significant dose reduction of rectum, bladder and sigmoid, as well as superior homogeneous target coverage compared to BT plan. On the other hand, VMAT delivers more radiation exposures to small bowel and femoral heads.

A treatment planning study comparing tomotherapy, volumetric modulated arc therapy, Sliding Window and proton therapy for low-risk prostate carcinoma

BACKGROUND

Comparing radiation treatment plans to ascertain the optimal intensity-modulated radiation technique for low-risk prostate cancer.

METHODS

Treatment plans for 20 randomly selected patients were generated using the same dose objectives. A dosimetric comparison was performed between various intensity-modulated techniques, including protons. All treatment plans provided conventional treatment with 79.2Gy. Dosimetric indices for the target volume and organs at risk (OAR), including homogeneity index and four conformity indices were analyzed.

RESULTS

No statistically significant differences between techniques were observed for homogeneity values. Dose distributions showed significant differences at low-to-medium doses. At doses above 50Gy all techniques revealed a steep dose gradient outside the planning target volume (PTV). Protons demonstrated superior rectum sparing at low-to-higher doses (V10-V70, P < .05) and bladder sparing at low-to-medium doses (V10-V30, P < .05). Helical tomotherapy (HT) provided superior rectum sparing compared to Sliding Window (SW) and Rapid Arc (RA) (V10-V70, P < .05). SW displayed superior bladder sparing compared to HT and RA (V10-V50, P < .05). Protons generated significantly higher femoral heads exposure and HT had superior sparing of those.

CONCLUSION

All techniques are able to provide a homogeneous and highly conformal dose distribution. Protons demonstrated superior sparing of the rectum and bladder at a wide dose spectrum. The radiation technique itself as well as treatment planning algorithms result in different OAR sparing between HT, SW and RA, with superior rectum sparing by HT and superior bladder sparing by SW. Radiation plans can be further optimized by individual modification of dose objectives dependent on treatment plan strategy.

Is VMAT beneficial for patients undergoing radiotherapy to the head and neck?

Volumetric Intensity-Modulated Arc Therapy (VMAT) offers solutions to the time consuming, higher monitor unit (MU) delivery of Intensity-Modulated Radiotherapy (IMRT) for patients undergoing radiotherapy for head and neck cancer (HNC). Thus the aim was to determine whether VMAT would be beneficial for these patients. The majority of literature was planning comparative studies with only two studies on early clinical experience. Main themes arising from literature were conformity and homogeneity, organs at risk (OAR), MU and delivery time. VMAT provides plans with similar conformity, better homogeneity and results for improved sparing of OAR compared to IMRT. Thus it is clear that VMAT would be able to match the performance of IMRT plans, with the ability to treat patients in a short space of time and less MU. There is little clinical data on the side effects of VMAT, and more is needed to further evaluate the efficacy of VMAT. Overall these are encouraging results of VMAT for patients with HNC.

A treatment planning comparison of volumetric modulated arc therapy and proton therapy for a sample of breast cancer patients treated with post-mastectomy radiotherapy

Post-mastectomy radiotherapy (PMRT) has been shown to improve disease-free survival and overall survival for locally advanced breast cancer. However, long term survivors may develop life threatening acute and chronic treatment-related toxicities after radiotherapy, like cardiac toxicity and second cancers. The more advanced techniques like volumetric arc therapy (VMAT), and proton therapy have the potential to improve treatment outcome by constraining doses to radiosensitive organs, but evidence from outcome study will not be available until years or decades later. Furthermore, the literature is largely incomplete regarding systematic comparison of potential benefits of advanced technologies for PMRT. The purpose of this study was to compare proton therapy, both passively scattered (PSPT) and intensity modulated (IMPT), to VMAT and develop an evidence-based rationale for selecting a treatment modality for left sided post-mastectomy radiotherapy (PMRT) patients. Eight left-sided PMRT patients previously treated with VMAT were included in this study. Planning target volumes (PTV) included the chest wall and regional lymph nodes. PSPT and IMPT plans were created using a commercial proton treatment planning system. The resulting plans were compared to the corresponding VMAT on the basis of dosimetric and radiobiological endpoints. The uncertainties in risk from proton range, set-up errors, and dose-response models were also evaluated. All modalities produced clinically acceptable treatment plans with nearly 100% tumor control probability. Both proton techniques provided significantly lower normal tissue complication probability values for the heart (p < 0.02) and lung (p < 0.001). Patient-averaged second cancer risk for the contralateral breast and lungs were also significantly lower (p < 0.001) with protons compared to VMAT. The findings of this study were upheld by the uncertainty analysis. All three techniques provided acceptable PMRT treatment plans. Proton therapy showed significant advantages in terms of predicted normal tissue sparing compared to VMAT, taking into account possible uncertainties.

Treatment of brain oligometastases with hypofractionated stereotactic radiotherapy utilising volumetric modulated arc therapy

Stereotactic radiosurgery (SRS) is commonly used to treat brain metastases, particularly in the oligometastatic setting. This study analyses our initial experience in treating oligometastatic brain disease using Volumetric Modulated Arc Therapy (VMAT) to deliver hypofractionated stereotactic radiotherapy (HFSRT). Sixty-one patients were treated with HFSRT with a median dose of 24 Gy (range 22-40 Gy) in a median of three fractions (range 2-10 fractions). With a median follow-up of 23 months, the local control rate was 74 % for the entire cohort. Local control was 87 % for patients who had surgery with no radiological evidence of residual disease followed by HFSRT compared with 69 % in patients treated with HFSRT alone. The overall median time post radiotherapy to local failure was 8.6 months and to extracranial failure was 7.9 months. The mean time to distant brain failure was 9.9 months. Twenty-two patients (36 %) died during the study with median time to death of 4.4 months. Median overall survival (OS) from treatment was 21 months and 12 month OS was 60 %. Our experience with HFSRT using VMAT for oligometastatic brain metastases in the post-operative setting demonstrates comparable local control and survival rates compared with international published data. In the intact brain metastasis setting, local control using the dose levels and delivery in this cohort may be inferior to radio-surgical series. Local control is independent of histology. Careful selection of patients remains critical.